Model paper

Transcription

Model paper

ENTRAINMENT OF CIRCATIDAL ACTIVITY RHYTHMS IN
OVIGEROUS BLUE CRABS, CALLINECTES SAPIDUS, AND
EFFECTIVENESS OF SELECTIVE TIDAL-STREAM TRANSPORT
by
PATRICIA NATHALIE POCHELON
B.S., Florida Institute of Technology
A thesis submitted to the Department of Biological Sciences of Florida Institute of
Technology in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
in
BIOLOGICAL SCIENCES
Melbourne, Florida
May 2005
OVIGEROUS BLUE CRABS, CALLINECTES SAPIDUS, AND
EFFECTIVENESS OF SELECTIVE TIDAL-STREAM TRANSPORT
A THESIS
by
PATRICIA NATHALIE POCHELON
Approved as to style and content by:
________________________________________
Richard A. Tankersley, PhD., Chairperson
Associate Professor
Department of Biological Sciences
________________________________________
John G. Morris, PhD.
Associate Professor
________________________________________
Eric D. Thosteson, PhD, P.E.
Assistant Professor
Department of Marine and Environmental Systems
________________________________________
Gary N. Wells, PhD.
Professor and Head
May 2005
ABSTRACT
ENTRAINMENT OF CIRCATIDAL ACTIVITY RHYTHMS IN OVIGEROUS
BLUE CRABS, CALLINECTES SAPIDUS AND EFFECTIVENESS OF
SELECTIVE TIDAL-STREAM TRANSPORT
by Patricia Nathalie Pochelon, B.S., Florida Institute of Technology
Chairperson of Advisory Committee: Richard A. Tankersley, PhD.
Prior to larval release, ovigerous blue crabs Callinectes sapidus use
selective tidal-stream transport (STST) to migrate seaward from adult habitats in
estuaries to spawn near coastal areas. Crabs enter the water column during
nocturnal ebb-tides but remain near the bottom at other times. During periods of
flood tide, they vigorously pump their abdomen to aerate the egg mass. The timing
of both the migratory and abdominal pumping behaviors is controlled by an
internal clock. This study tested the hypothesis that these endogenous activity
rhythms can be entrained by changes in temperature. Ovigerous crabs were
collected near Sebastian Inlet, FL from June-November 2003 and 2004. First,
crabs were placed in plastic aquarium for 4 days under constant conditions to
determine if crabs from the collection site exhibited an endogenous circatidal clock
in migratory activity and pumping behavior. One crab displayed a circatidal
rhythm in both migratory activity and pumping behavior while another only
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exhibited a rhythm in pumping. The remaining three crabs were arrhythmic.
Another set of freshly collected crabs were then subjected to cyclic changes in
temperature for 96 h and subsequently placed under constant conditions for four
days. Only two of the five crabs tested displayed a rhythm in migratory
restlessness. During the entrainment phase, four crabs displayed rhythms in
pumping with peaks occurring around the time of maximum temperature. Under
constant conditions, peaks in pumping activity exhibited by one crab occurred near
the time of expected maximal temperature, whereas activity peaks displayed by the
other two crabs occurred 6 h later. The remaining two crabs were arrhythmic.
Thus, temperature did not serve as an entrainment cue for the circatidal behaviors
of ovigerous blue crabs.
A computer model was also used to assess the effects of the timing and
duration of vertical migratory activity associated with STST on net transport.
Previous modeling studies indicate that STST is most efficient when the migratory
behavior is perfectly synchronized with the dominant tidal constituent (e.g., M2;
period = 12.42 h). However, differences in the relative timing of proximal cues
underlying STST and the tides may cause vertical swimming activity to be out of
phase with tidal currents, thereby reducing net displacement. Small differences in
the phase relationship between vertical migratory behavior and tidal currents and
small reductions in the duration of swimming had relatively little effect on net
iv
displacement. Yet, transport declined my more than 30% when the initiation of
swimming was delayed until 1 h after slack water. The impact of diel differences
in activity (e.g., suppression of vertical migration during the day) on displacement
was also examined. When a diel cycle was added (D:N = 10:14), daily transport
depended upon the phase relationship between the tidal and day:night cycles,
causing variations in net displacement. A pattern in daily transport with a 15.5 day
periodicity that was independent of the spring neap cycle was observed. Transport
was greatest when the entire active phase took place at night (57% greater than
when the least of the active phase occurs at night). An increase in the duration of
the night phase resulted in an increase in net displacement since a greater
proportion of the active phase occurred in darkness. However, the 15.5 day cycle
in daily transport remained the same. Consequently, peaks in recruitment and
migration can be expected be related to the synchrony between the day:night and
the tidal cycles and not to the spring:neap cycle.
v
DEDICATION
This thesis is dedicated to my family and friends for their love and support.
vi
ACKNOWLEDGMENTS
Many people supported me during the completion of this thesis with criticism,
assistance, and encouragement. This thesis would not have been possible without
them. First of all, I wish to express my sincere gratitude to my advisor Richard
Tankersley, who guided this work and helped whenever I was in need and opened
my eyes to the fascinating world of invertebrates. He was a wonderful supervisor
whose assistance and motivation were greatly appreciated. I would also like to
thank the members of my committee, Dr. Eric Thosteson for his time and guidance
with statistics and the design of the model and Dr. John Morris for his support.
Their help and advice were greatly appreciated.
A special thanks to my roommates and friends, Luce Bassetti, William
Gosling, Anne Riquier, and Robert Robinson for their technical assistance but
especially their moral support.
I also would like to thank people from the Tankersley lab who quickly
became much more than just labmates, Khayree Butler, Brady Denger, Phillip
Gravinese, Paola Lopez, and Jackie Lorne. Those nights spent canoeing in the
cove and catching crabs on the boat will certainly not be forgotten.
I am grateful to my friends and fellow students for their field assistance;
Meghan Anderson, Patrick Connelly, Dwayne Edwards, Brian Guido, Fleur
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Lacharmoise, Sarah McMahon, Sébastien Moreau, Sarah Rhodes, Thomas
Samarco, and Kim Trosvik
Last but certainly not least, I would like to show my gratitude to my parents,
Guy and Beatrice Pochelon, for their love, encouragement, and financial support
throughout my studies. I would not even have started this thesis without them. I
will never forget my brother Olivier who even got his hands wet and came to help
me in the field.
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TABLE OF CONTENTS
ABSTRACT............................................................................................................. iii
DEDICATION ......................................................................................................... vi
ACKOWLEDGMENTS ......................................................................................... vii
TABLE OF CONTENTS......................................................................................... ix
LIST OF FIGURES ................................................................................................. xi
CHAPTER 1: ENTRAINMENT OF CIRCATIDAL ACTIVITY RHYTHMS IN
OVIGEROUS BLUE CRABS, CALLINECTES SAPIDUS, USING CYCLES IN
TEMPERATURE...................................................................................................... 1
INTRODUCTION ................................................................................................ 1
ENTRAINMENT OF CIRCATIDAL ACTIVITY RHYTHMS ...................... 4
RHYTHMIC ACTIVITY IN CALLINECTES SAPIDUS ................................. 5
OBJECTIVES AND HYPOTHESIS ................................................................ 8
MATERIAL AND METHODS .......................................................................... 10
COLLECTION AND MAINTENANCE OF ANIMALS .............................. 10
CIRCATIDAL RHYTHMS IN MIGRATORY BEHAVIOR AND
ABDOMINAL PUMPING ............................................................................. 11
ENTRAINMENT OF ACTIVITY RHYTHMS USING SIMULATED
TIDAL CHANGES IN TEMPERATURE ..................................................... 13
RESULTS ........................................................................................................... 17
ENDOGENOUS RHYTHMS IN MIGRATORY BEHAVIOR AND
ABDOMINAL PUMPING ............................................................................. 17
ENTRAINMENT OF ACTIVITY RHYTHMS USING SIMULATED
TIDAL CHANGES IN TEMPERATURE ..................................................... 34
DISCUSSION ..................................................................................................... 86
CHAPTER 2: IMPACT OF TIME OF INITIATION AND DURATION OF
VERTICAL MIGRATION ON THE EFFECTIVENESS OF SELECTIVE
TIDAL-STREAM TRANSPORT........................................................................... 94
INTRODUCTION .............................................................................................. 94
MATERIAL AND METHODS ........................................................................ 100
EFFECT OF SYNCHRONY BETWEEN VERTICAL MIGRATION
AND TIDAL CURRENTS ON TRANSPORT ............................................ 103
EFFECT OF TIMING OF INITIATION OF VERTICAL MIGRATION ON
TRANSPORT ............................................................................................... 105
EFFECT OF DIEL CYCLE ON TRANSPORT........................................... 106
ix
RESULTS ......................................................................................................... 109
EFFECT OF SYNCHRONY BETWEEN VERTICAL MIGRATION
AND TIDAL CURRENTS ON TRANSPORT ............................................ 109
EFFECT OF TIMING OF INITIATION OF VERTICAL MIGRATION
ON TRANSPORT......................................................................................... 112
EFFECT OF DIEL CYCLE ON TRANSPORT........................................... 114
DISCUSSION ................................................................................................... 123
LITTERATURE CITED....................................................................................... 132
x
LIST OF FIGURES
Figure 1. Diagrammatic representation of the apparatus used to subject crabs to
cycles in temperature. . ........................................................................ 14
Figure 2. Actographs of pumping activity (upper panel) and migratory
restlessness (lower panel) for Crab 1A under constant conditions.. ..... 19
Figure 3. Correlogram (top) and MESA spectrum (bottom) for the time series
of pumping activity for Crab 1A (Fig. 2).............................................. 20
of migratory restlessness for Crab 1A (Fig. 2)...................................... 21
restlessness (lower panel) for Crab 4A under constant conditions ....... 28
of pumping activity for Crab 4A (Fig. 11)............................................ 29
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of migratory restlessness for Crab 4A (Fig. 11).................................... 30
of pumping activity for Crab 5A (Fig. 14)............................................ 32
of migratory restlessness for Crab 5A (Fig. 14).................................... 33
Figure 17. Actographs of pumping activity (# pumps/30 min) and migratory
restlessness (% time spent active) for Crab 1B subjected to
simulated tidal changes in temperature for the first 96h and
subsequently maintained under constant conditions for an
additional 96 h....................................................................................... 36
Figure 18. Correlogram (top) and MESA spectrum (bottom) for the pumping
activity for Crab 1B (Fig. 17) subjected to a simulated tidal cycle in
temperature for 96 h (entrainment phase). ............................................ 37
activity for Crab 1B (Fig. 17) subjected to constant conditions for 96
h (post-entrainment phase).................................................................... 38
Figure 20. Correlogram (top) and MESA spectrum (bottom) for the migratory
restlessness for Crab 1B (Fig. 17) subjected to a simulated tidal
cycle in temperature for 96 h (entrainment phase)................................ 39
restlessness for Crab 1B (Fig. 17) subjected to constant conditions
for 96 h (post-entrainment phase). ........................................................ 40
additional 96 h....................................................................................... 41
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temperature for 96 h (entrainment phase).. ........................................... 42
additional 96 h....................................................................................... 46
for 96 h (post-entrainment phase).. ....................................................... 50
xiii
additional 96 h....................................................................................... 51
temperature for 96 h (entrainment phase) ............................................. 52
restlessness activity for Crab 4B (Fig. 32) subjected to a simulated
tidal cycle in temperature for 96 h (entrainment phase) ....................... 54
for 96 h (post-entrainment phase) ......................................................... 55
additional 96 h....................................................................................... 56
xiv
Figure 42. Phase relationship between pumping activity of Crab 2B and
realized (left panel) and expected (right panel) changes in
temperature............................................................................................ 62
Figure 43. Phase relationship between pumping activity of Crab 4B and
realized (left panel) and expected (right panel) changes in
temperature............................................................................................ 63
restlessness (% time spent active) for Crab 1C subjected constant
conditions for 192 h. ............................................................................. 66
activity for Crab 1C (Fig. 42) subjected to constant conditions for
the first 96 h of the experiment. ............................................................ 67
the last 96 h of the experiment. ............................................................. 68
restlessness for Crab 1C (Fig. 42) subjected to constant conditions
for the first 96 h of the experiment. ...................................................... 69
for the last 96 h of the experiment. ....................................................... 70
conditions for 192 h. ............................................................................. 71
xv
for the last 96 h of the experiment.. ...................................................... 75
conditions for 192 h. . .......................................................................... 76
the first 96 h of the experiment.. ........................................................... 77
the last 96 h of the experiment.. ............................................................ 78
for the first 96 h of the experiment.. ..................................................... 79
for the last 96 h of the experiment.. ...................................................... 80
conditions for 192 h.. ............................................................................ 81
xvi
for the last 96 h of the experiment. ....................................................... 85
Figure 64. Diagram representing tidal currents [U(t)], vertical migration pattern
[M(t)] and diel pattern [D(t)] used to calculate net transport resulting
from STST behaviors.. ........................................................................ 101
Figure 65. Diagram depicting the parameter [i.e., acrophase (φ), duration (Tw)
and time of initiation (Ti)] describing the timing and duration of
migration and their variations for the different model algorithms...... 104
Figure 66. Effect of acrophase between vertical migratory behavior and current
flow on net transport during STST (Migration-Flow Synchrony
Model) over ten tidal cycles.. .............................................................. 110
Figure 67. Effect of duration of the active phase and acrophase between vertical
migratory behavior and current flow on net transport during STST
(Migration Duration-Flow Synchrony Model) over ten tidal cycles... 111
Figure 68. Effect of the delay in the time of initiation of swimming on net
transport during STST (Initiation of Migration Model) over ten tidal
cycles .................................................................................................. 113
Figure 69: Daily horizontal displacement resulting from selective tidal vertical
migrations that are influenced by the combined effects of tidal and
diel cycles (Combined Diel and Migration-Flow Synchrony Model). 115
Figure 70. Net daily displacement resulting from cycles in vertical migration
that included both tidal and diel components (Combined Diel and
Migration-Flow Synchrony Model ; Fig. 69).. .................................... 117
xvii
Figure 71. Daily horizontal displacement resulting from tidal vertical
migration that is influenced by the combined effects of tidal and diel
cycles (Combined Diel and Initiation of Migration Model). .............. 119
Figure 72. Cumulative net transport after 14.79 days for simulated organisms in
the Combined Diel and Initiation of Migration Model. ...................... 120
Figure 73. Net daily displacement resulting from cycles in vertical migration
that included both tidal and diel components (Combined Diel and
Initiation of Migration Model; Fig. 71).. ............................................ 122
xviii
1
CHAPTER 1
OVIGEROUS BLUE CRABS, CALLINECTES SAPIDUS,
USING CYCLES IN TEMPERATURE
INTRODUCTION
Many organisms possess biological rhythms that are used to control
physiological, behavioral, and biochemical aspects of their lives. These cyclic
patterns synchronize biological processes with varying periodic events. Rhythms
that persist in the laboratory in the absence of environmental cues are considered to
be under the control of an endogenous timing mechanism or biological clock. The
period length for any single clock often varies and includes cycles lasting a year
(365 days), synodic month (29.5 days), lunar day (24.8 h), solar day (24 h), or
tidal interval (12.4 h) (Palmer 1990). If the rhythm does not persist under constant
conditions, the process is not considered to be endogenous but instead is triggered
by periodic external stimuli (Palmer 1973).
Although endogenous clocks often persist for several cycles in constant
conditions, they constantly need to be reset by external cues, commonly referred to
as zeitgebers (zeit = time, geber = giver). These cues provide the organism with
information about the state of the external cycle that is being tracked and are used
to adjust the internal clock so that the appropriate phase relationship between the
2
biological process and external cycle is maintained (Roenneberg et al. 2003).
However, when the external cycle is shifted for any reason, the zeitgeber is applied
at a time that is different than expected, resulting in a phase advance or delay in the
internal clock. This property is commonly used in laboratory experiments to
determine which external cues serve as zeitgebers for resetting the endogenous
rhythm.
Early work on internal clocks focused on diel rhythms with period lengths
of about 24 h. These types of rhythms are referred to as circadian (circa = about,
diem = day) and are common features of plants, animals, and protists (Menaker
1969). They are typically synchronized with the light-dark cycle and the transitions
between day and night commonly serve as zeitgebers (Menaker 1969). In marine
environments, circadian rhythms are often involved in the diurnal vertical
migratory behavior of planktonic organisms (Enright and Hamner 1967, Forward
1988) and the activity of isopods, amphipods and fish (Palmer 1974, Godin 1981,
Forward 1988, Macquart-Moulin and Kaim-Malka 1994).
In addition to the diel cycle, a variety of other cyclic phenomena can govern
the life of organisms. For instance, in coastal and estuarine environments, shortterm cyclic changes in sea level (i.e., tides) are the result of the gravitational force
of the moon and sun on large water masses and cause periodic changes in
environmental conditions (e.g., temperature, salinity, pressure, turbulence,
3
inundation). The relative position of the moon and sun with respect to the earth
affect the amplitude of the tides and the corresponding changes in physical
parameters. Twice each synodic month (about 29.5 days), the three celestial bodies
are aligned and the gravitational force of the sun and the moon on the earth’s water
masses are additive. The extremes in tidal amplitude occur at full and new moon
and are referred to as spring tides. In contrast, when the moon and the sun are
positioned at right angles to each other (1st and 3rd quarter moon), the forces are
non-additive and the amplitude of the tide is minimal. These periods are referred to
as neap tides. Lunar rhythms have a periodicity of 29.5 days whereas semi-lunar
rhythms are synchronized with the spring-neap tidal cycle with a period length of
14.75 days (Palmer 1974).
On a daily basis, water level varies periodically due to the attraction of the
water masses by the moon and the rotation of the earth on its axis. However, the
timing and periodicity of the tides can vary greatly from one location to the next.
Most areas experience semi-diurnal tides with a 12.4 h periodicity. Each day, there
are two high tides and two low tides. However, other areas are subjected to diurnal
tides, with only one high tide and one low tide occurring each day with a period of
24.8 h. Finally, some areas are under the influence of mixed tide, where there are
usually two high tides and two low tides a day of unequal magnitude (Palmer
1973).
4
Regardless of the tidal regime, many environmental conditions change
predictably according to the state of the tides and include water temperature,
salinity, and turbulence. Those factors are typically out of phase with changes in
water level, yet have the same period (Uncles et al. 1985). Rising tides and
flooding currents typically result in an increase in hydrostatic pressure and salinity
and a decrease in temperature. On the other hand, during falling tides and ebbing
currents, in coastal and estuarine areas organisms typically experience a decrease in
hydrostatic pressure and salinity and an increase in temperature. Water turbulence
also varies with the tides. Turbulence is typically minimal near the time of slack
water but increases and peaks near the time of maximum currents (both ebb and
flood). The cyclic changes in environmental parameters are therefore used to
resynchronize the endogenous rhythms with the tidal cycle.
ENTRAINMENT OF CIRCATIDAL ACTIVITY RHYTHMS
Endogenous clocks regularly need to be readjusted to maintain synchrony
with the tidal cycle. Therefore, in many littoral, estuarine, and coastal organisms,
entrainment of the internal clock by an appropriate zeitgebers is necessary (Palmer
1973). To demonstrate that an environmental cue serves as the zeitgeber for a
circatidal rhythm, laboratory experiments can be conducted where the animal is
exposed to an artificial cycle applied in antiphase to the natural cycle. In the case
where the exogenous cue effectively serves as a zeitgeber, a shift in the timing of
5
the rhythm is observed during the entrainment period. Following placement in
constant conditions, the rhythm is no longer in phase with the natural tides, but
matches the imposed cycle. This procedure has previously been used to
demonstrate that changes in salinity, temperature, and pressure serve as the
entrainment cue to synchronize the locomotory behavior of Carcinus maenas with
local tides (Taylor and Naylor 1977; Bolt and Naylor 1986; Reid and Naylor 1990;
Warmann and Naylor 1995). Similarly, on beaches, turbulence and mechanical
agitation associated with the tide entrain activity rhythms of interstitial organisms
such as isopods that actively swim during flood tide and burrow during ebb tide to
avoid being carried away by the falling tide (Enright 1965, Jones and Naylor 1970).
Cyclic changes in environmental factors also entrain the behavior of intertidal
species such as the isopod Eurydice (Jones and Naylor 1970) so that activity
periods are synchronized with cycles of inundation (Palmer 1973). Forward et al.
(1986) also successfully entrained the timing of larval release by female
Rhithropanopeus harrisii using salinity cycles. In blue crabs Callinectes sapidus,
Ziegler (2002) found that the circatidal rhythm in larval release exhibited by
ovigerous females can be entrained by simulated tidal changes in salinity.
RHYTHMIC ACTIVITY IN CALLINECTES SAPIDUS
Like many marine invertebrates, the blue crab Callinectes sapidus possess a
complex life cycle that includes both pelagic and benthic phases. Larvae (zoeae)
6
are released in euryhaline areas of estuaries and transported to shelf waters where
they undergo development, as higher salinity is necessary for survival (Epifanio
1995). Following metamorphosis to postlarvae (megalopae), blue crabs are
advected shoreward in surface waters and are transported up estuary by flood
currents (for review see Forward and Tankersley 2001). Upon reaching nursery
areas, postlarvae metamorphose to juveniles and migrate further upstream to lowsalinity areas. Adult blue crabs mate in oligohaline regions before females migrate
down-estuary toward coastal areas and eventually spawn near inlets to the ocean.
The migration of female blue crabs for spawning is typically divided into
two phases (Forward et al. 2003a). During Phase I, female crabs leave low salinity
areas and travel to the mouth of the estuary. Upon reaching high salinity areas,
migration ceases. Phase II immediately follows Phase I if females reach the mouth
of the estuary early enough in the reproductive season (Forward et al. 2003a).
Otherwise, crabs overwinter and Phase II is initiated the following spring
(Tankersley et al. 1998). In either scenario, Phase II of the migration begins with
the extrusion and attachment of fertilized eggs to the abdomen to form a large egg
mass or “sponge”. Ovigerous crabs then use tidal currents to migrate further
seaward toward offshore areas (Tankersley et al. 1998; Forward et al. 2003a, Carr
et al. 2004, Hench et al. 2004).
7
To accomplish this migration, females migrate vertically in and out of the
water column in synchrony with the changes in direction and magnitude of the tidal
currents (Tankersley et al. 1998). This process, known as selective tidal-stream
transport (STST), results in rapid movement up or down estuary or between
estuaries and coastal areas (reviewed by Forward and Tankersley 2001). Net
horizontal transport is accomplished by entering the water column during one phase
of the tide and remaining on or near the bottom during the alternate phase. Thus,
STST can be classified as either ebb-tide (ETT) or flood-tide (FTT) transport
depending upon the timing of migration relative to tidal phase and the direction of
travel. Ovigerous females utilize ETT to reach spawning areas. Once embryonic
development is complete, larvae are released during morning ebb tides (Zeigler
2002). Using this transport mechanism, female crabs ensure better survival and
development of the newly hatched zoeae since they are unable to tolerate low
salinity conditions within the estuary (Costlow and Bookhout 1965). Following
release, post-spawning females reverse direction and re-enter the estuary using FTT
(Tankersley et al. 1998).
During the spawning migration, females alternate between two behaviors
(1) migratory restlessness and (2) egg maintenance. Migratory restlessness consists
of periods of increased activity and vertical swimming and has been previously
shown to be under the control of an endogenous clock (Forward et al. 2003b).
8
Peaks in activity occur during the time of expected ebb tide, therefore promoting
seaward transport (Forward et al. 2003b). Alternatively, egg maintenance behavior,
consisting of preening the egg mass with the walking legs and vigorous abdominal
pumping to aerate the eggs, is also controlled by an internal clock and is
concentrated during periods of flood tide and (Forward et al. 2003b).
Consequently, migratory restlessness results in periods when crabs are in the water
column and transported seaward by ebb currents and pumping occurs when crabs
are expected to be on the benthos and avoiding being transported by flood currents.
The synchrony between the pumping activity and the migratory restlessness with
the tidal current are likely to be resynchronized with the local tidal conditions
through zeitgebers such as temperature, pressure, salinity, or turbulence.
OBJECTIVES AND HYPOTHESIS
The objective of this study was to determine whether tide-associated
changes in temperature serve as an entrainment cue for the circatidal activity
rhythms (migratory restlessness and abdominal pumping) exhibited by ovigerous
blue crabs during the spawning migration. Previous studies demonstrated that
other crustaceans are entrained by changes in temperature (Reid and Naylor 1990).
For example, Bolt et al. (1989) found that the circatidal locomotory rhythm of the
green crab Carcinus maenas is entrained by cycles in temperature. Similarly,
9
Holmström and Morgan (1983a) successfully entrained the ebb-tide swimming
rhythm of the amphipod Corophium volutator using temperature cycles.
Previous studies by Forward et al. (2003b) indicated that ovigerous blue
crabs from the Newport River Estuary, NC exhibit circatidal rhythms in activity
and egg maintenance behavior. Therefore, since the present experiments were
conduced using a population of crabs from the Indian River Lagoon, FL, a
preliminary study was conducted to test whether females from the area have similar
tidal rhythms in activity as those reported for the Newport River Estuary, NC
population. Peaks in migratory restlessness and vertical swimming behavior were
hypothesized to occur during the expected time of ebb tide at the collection site,
whereas egg maintenance behavior was anticipated to be concentrated during the
expected time of flood tide. Second, to determine if temperature serves as the
zeitgeber for the two behaviors exhibited by ovigerous C. sapidus, crabs were
exposed to artificial cycles in temperature with a period comparable to the tidal
cycle (12.42 h) at the collection site. If temperature serves as the zeitgeber, the
rhythms (i.e., migratory restlessness and pumping) were expected to shift and
become synchronized with the imposed cycle. If entrainment did not occur,
activity patterns of the crabs were expected to remain in phase with the natural
cycle or become arrhythmic.
10
MATERIAL AND METHODS
COLLECTION AND MAINTENANCE OF ANIMALS
Ovigerous blue crabs Callinectes sapidus Rathbun were collected using
commercial crab traps during the spawning season from June-November 2003 and
March-October 2004. Traps were deployed near Sebastian Inlet, FL, USA
(27° 50”N, 80° 28”W) and checked daily. This area is part of the Indian River
Lagoon system and experiences semidiurnal tides with an average amplitude of
0.6 m (Smith 1990). Following collection, crabs were classified according to the
developmental stage of the embryos using the scheme described by De Vries et al.
(1983). For all experiments, crabs were grouped into two categories, early- and
mid-stage, based on egg yolk content and embryo eye development. Early-stage
eggs are yellow/orange in color, contain embryos that lack eyespots, and are
> 6 days from hatching (Stages 1-4 of De Vries et al. 1983). Mid-stage eggs are
typically light-orange to rusty-brown in color, contain embryos with newly formed
eyespots, and are 3-6 days from hatching (Stages 4-7 of De Vries et al. 1983). Crab
size (i.e., carapace width, defined as the distance between the tips of the lateral
spines) was measured and recorded.
11
CIRCATIDAL RHYTHMS IN MIGRATORY RESTLESSNESS AND
ABDOMINAL PUMPING
Forward et al. (2003b) found that ovigerous blue crabs with early- and midstaged embryos collected near Beaufort Inlet, NC possess endogenous circatidal
rhythms in egg maintenance behavior (abdominal pumping) and migratory
restlessness (swimming activity) that are synchronized with flood and ebb currents,
respectively. To verify that crabs from the Indian River Lagoon, FL possess
similar circatidal rhythms, swimming activity and egg maintenance behavior of
ovigerous females with early- and mid-stage embryos was monitored under
constant conditions.
Each trial consisted of placing crabs individually in clear plastic aquaria
(30 cm x 18 cm x 10 cm) filled with sea water (35 psu). Water within the aquaria
was aerated continuously and changed daily. Temperature was maintained at 2425 ºC. Containers were illuminated continuously with low-level red light supplied
by two incandescent bulbs (25 W). Crab activity was monitored continuously for a
period of 96 h using a closed-circuit video camera (Panasonic) connected to a timelapse video recorder (Panasonic model AGRT600A).
Under these conditions crabs were expected to oscillate between (1)
“migratory restlessness,” characterized by active swimming against the sides of the
chamber, and (2) “egg maintenance”, characterized by preening of the egg mass
with the walking legs and rhythmic abdominal pumping. Time-lapse recordings
12
were used to quantify the frequency of these two activities at 30 min intervals.
Migratory restlessness was defined as the percentage of time spent actively
swimming, whereas egg maintenance was defined as the number of abdominal
pumps per 0.5 h period. The experiment was replicated 5 times with crabs with
early- and mid-stage embryos.
Prior to statistical analysis, all time series were filtered using a low pass
filter to remove frequencies greater than 0.33 Hz (periodicities below 3 h) and a
detrend function (Matlab 7) was used to fit a linear least-squareline to the time
series to remove any linear trend prior to further analysis. Each time series was
analyzed for periodicity using autocorrelation (Chatfield 1989). Recurring peaks in
the autocorrelograms that exceed 2 / N were considered to be statistically
significant at P < 0.05 (Dowse and Ringo 1989). Dominant periodicities in the
time series were further confirmed using maximum entropy spectral analysis
(MESA) following the procedure described by Dowse and Ringo (1989). The
synchrony between activity rhythms and the expected tidal cycle and between the
pumping activity and migratory restlessness was determined using cross-correlation
analysis (SPSS 11.5).
13
ENTRAINMENT OF ACTIVITY RHYTHMS USING SIMULATED TIDAL
CHANGES IN TEMPERATURE
To test the hypothesis that cycles in temperature serve as the entrainment
cues for the rhythms in migratory restlessness and abdominal pumping observed in
ovigerous blue crabs, females with early-stage embryos were placed in an
experimental system that exposed them to simulated tidal changes in temperature
that were in antiphase to natural cycles at the time of collection. Thus, if
temperature served as a zeitgeber for the rhythms, activity patterns were expected
to shift during the entrainment period to match the new cycle.
The apparatus for producing simulated tidal cycles in temperature was
similar to the one described by Reid et al. (1989) (Figure 1). Ovigerous crabs were
placed in the inner compartment (33 cm x 17.5 cm x 20 cm) of a water-jacketed
acrylic chamber (39.5 cm x 24 cm x 20 cm) filled with 11.5 L of filtered (< 5 µm)
sea water (35 psu). Temperature inside the chamber was controlled by circulating
water from a refrigerated water bath and heater system through a sealed water
jacket surrounding the inner chamber. To avoid the buildup of metabolites, aerated
sea water from a 190 L external reservoir was pumped continuously (30 ml min-1)
through the inner chamber containing the crab.
Sinusoidal oscillations in temperature with periodicities matching those
found in a semi-diurnal tidal regime (12.42 h) were produced using a Gateway
P5 120 desktop computer and associated A/D converter (Metrabite DAS-16) and
14
Figure 1. Diagrammatic representation of the apparatus used to subject crabs to
cycles in temperature. Water connections are indicated by solid lines and
electrical connections are indicated by dashed lines. SW: sea water
supply. The black arrows indicate the direction of the flow.
15
control software (Labtech Notebookpro 8.1, Laboratory Technologies
Corporation). The output from a calibrated temperature probe and meter placed
inside the experimental chamber was monitored by the computer every 0.2 s and
compared to a preprogrammed set-point. The difference between the two values
was used to adjust the temperature of the water flowing through the water jacket by
turning on and off a series of heaters placed inside the refrigerated water bath. The
set-point was adjusted every 10 min and oscillated between 20 °C and 25 °C with a
period of 12.4 h, thus simulating the changes that occur in areas with semi-diurnal
tides. The temperature within the experimental chamber was recorded every
2 minutes.
For each trial, a crab with early stage embryos was placed in the chamber
and was subjected to cycles of temperature for 96 h. This period has been shown to
be sufficient to entrain the locomotory behavior of Carcinus maenas (Bolt and
Naylor 1986). Both migratory restlessness and pumping activity were monitored
and recorded continuously using the closed-circuit video system described above.
Following the entrainment period, conditions inside the experimental chamber were
held constant and activity was monitored for an additional 96 h. The experiment
was replicated 5 times using different crabs. In addition, 4 trials where the
temperature in the animal chamber was held constant at either 20 ºC or 22.5 ºC for
192 h were conducted and served as controls for the effect of time on the
16
endogenous rhythms exhibited by ovigerous crabs (e.g., loss of rhythms due to
starvation). Differences in the behaviors between the crabs subjected to
temperature cycles and control crabs were used to assess the success of
entrainment.
Prior to statistical analysis, each activity record was divided into two time
series (1) entrainment (first 96 h) and (2) post-entrainment (last 96 h in constant
conditions). High frequency noise in the time series was removed using digital
filter that removed frequencies greater than 3 h. Additionally, data sets that
displayed linear trends in the behavior (e.g., increase in pumping associated with
egg development) were detrended. As previously described, rhythmicity was tested
using autocorrelation analysis and the length of significant periodicity was
confirmed using MESA. For the behaviors displaying a circatidal rhythm, the
synchrony of the time series with the imposed temperature cycle experienced
during the entrainment period was assessed using a cross-correlation analysis.
During this phase, a significant rhythmicity was expected with a periodicity
approximating 12.4 h. Additionally, when rhythms in both pumping activity and
migratory restlessness were present and had periods approximating the tidal cycle
(i.e., 12.4 h), a cross-correlation analysis was performed to determine the phase
relationship between the two behaviors (SPSS 11.5). A similar procedure was used
to assess the relationship between any circadian rhythms and the day:night cycle.
17
RESULTS
ENDOGENOUS RHYTHMS IN MIGRATORY BEHAVIOR AND
ABDOMINAL PUMPING
Time series of the migratory restlessness and the pumping behavior
exhibited by five crabs with early-stage embryos are depicted in Figures 2 -16. All
trials were terminated before larval release. The time series for crabs 2A and 4A
were detrended since baseline pumping activity increased gradually during the
observation period. Only two crabs displayed a circatidal rhythm in the pumping
activity, indicated by significant peaks in the autocorrelation plots and
corresponding peaks in the MESA spectra at 12.95 h and 13.87 h (Figs. 3 and 6,
respectively). When compared to the expected tidal cycle in the field, peaks in
pumping occurred 1 h before high tide, corresponding to the time of flood tide in
the field. The remaining three crabs displayed no significant rhythms in pumping
(Figs. 9, 12 and 15).
Similar results were observed in the time series of migratory restlessness.
Rhythmic activity was detected in the time series Crabs 1A and 4A (Figs. 2 and
13). However, significant peaks in the autocorrelation plot occurred at 24.19 h
(Figs. 4 and 13) indicating a circadian rhythm in activity. The migratory
restlessness of Crabs 1A and 4A was correlated with the diel cycle, with the peaks
in activity occurring in the middle of the night at 0:00 (maxCC = 0.553) and 02:30
18
(maxCC = 0.451), respectively. Crabs 2A, 3A and 5A were arrhythmic and
displayed relatively little activity during most of the observation period (Fig. 7, 10
and 16).
Pumping Activity (# Pumps / 0.5 h)
Tide
19
25
20
15
10
5
Migratory Restlessness (% Activity/0.5 h)
0
100
80
60
40
20
0
0
12
24
36
48
60
72
84
96
108
Time (hours)
Figure 2. Actographs of pumping activity (upper panel) and migratory restlessness
(lower panel) for Crab 1A under constant conditions. The expected
light:dark and tidal cycles in the field are indicated at the top of the
figures. The top graph depicts the number of abdominal pumps per half
hour. The bottom figure indicates the proportion of the sampling period
(0.5 h) the crab exhibited migratory restlessness (%). Time is expressed in
hours starting at midnight on July 10, 2003.
20
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1800
12.95 h.
1600
Spectral Density
1400
1200
1000
800
600
400
200
0
0
12
24
36
Period (h)
Figure 3. Correlogram (top) and MESA spectrum (bottom) for the time series of
pumping activity for Crab 1A (Fig. 2). Dashed horizontal lines in the
upper figure indicate the 95% confidence intervals. Period lengths
corresponding to significant peaks in the autocorrelation plots are
provided.
21
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
24.19 h
3
Spectral Density (x 10 )
50
40
30
20
10
0
0
12
24
36
Period (h)
migratory restlessness for Crab 1A (Fig. 2). Dashed horizontal lines in
the upper figure indicate the 95% confidence intervals. Period lengths
provided.
Tide
22
35
30
25
20
15
10
5
0
100
80
60
40
20
0
0
12
24
36
48
60
72
84
96
108
Time (hours)
(0.5 h) the crab exhibited migratory restlessness (%). Time is expressed in
hours starting at midnight on July 10, 2003.
23
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
250
13.87 h
Spectral Density
200
150
100
50
0
0
12
24
36
Period (h)
provided.
24
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1400
1200
Spectral Density
1000
800
600
400
200
0
0
12
24
36
Period (h)
the upper figure indicate the 95% confidence intervals.
Tide
25
25
20
15
10
5
0
100
80
60
40
20
0
0
12
24
36
48
60
72
84
96
108
Time (hours)
(0.5 h) the crab exhibited migratory restlessness (%). Time is expressed
in hours starting at midnight on July 18, 2003.
26
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
180
160
Spectral Density
140
13.87 h.
120
100
80
60
40
20
0
0
12
24
36
Period (h)
provided.
27
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
2.5
3
2.0
1.5
1.0
0.5
0.0
0
12
24
36
Period (h)
Tide
28
45
40
35
30
25
20
15
10
5
0
100
80
60
40
20
0
0
12
24
36
48
60
72
84
96
108
Time (hours)
29
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1200
Spectral Density
1000
800
600
400
200
0
0
12
24
36
Period (h)
upper figure indicate the 95% confidence intervals.
30
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
24.19 h.
3
6
4
2
0
0
12
24
36
Period (h)
the upper figure indicate the 95% confidence intervals. Period lengths
provided.
Tide
31
70
60
50
40
30
20
10
0
100
80
60
40
20
0
0
12
24
36
48
60
72
84
96
108
Time (hours)
32
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
2000
1800
1600
Spectral Density
1400
1200
1000
800
600
400
200
0
0
12
24
36
Period (h)
upper figure indicate the 95% confidence intervals.
33
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
800
Spectral Density
600
400
200
0
0
12
24
36
Period (h)
34
ENTRAINMENT OF ACTIVITY RHYTHMS USING SIMULATED TIDAL
CHANGES IN TEMPERATURE
Five crabs were subjected to 96 h of simulated tidal cycles in temperature
(entrainment period) followed by 96 h of constant conditions (post-entrainment
period). For each crab, the time series and the results of the autocorrelation and
MESA were grouped together (Figs. 17-41), sequentially depicting the time series
(Figs. 17, 22, 27, 32, and 37), statistics of the pumping behavior during the
entrainment (Figs. 18, 23, 28, 33, and 38) and post-entrainment period (Figs. 19,
24, 29, 34, and 39) and statistics of the migratory restlessness during the
entrainment (Figs. 20, 25, 30, 35, and 40) and post-entrainment period (Figs. 21,
26, 31, 36, and 41).
During the entrainment period, all crabs displayed a rhythm in egg
maintenance behavior, with periods ranging from 11.43 h to 13.87 h (Figs. 18, 23,
28, 33 and 38). Following the entrainment period, Crabs 2B, 4B and 5B (Fig. 24,
34 and 39) exhibited a circatidal rhythm, with periods ranging between 12.14 h and
12.95 h. Crab 1B displayed a circadian rhythm with a dominant period of 24.19 h
(Fig. 19). There was no significant rhythmicity in the pumping activity of Crab 3B
(Fig. 29). When compared to the simulated temperature cycle during the
entrainment period, peaks in pumping activity of Crabs 1B, 2B, 4B and 5B ranged
between 0.5 h and 1.5 h before maximal temperature (Table 1). However, during
the post-entrainment period, peaks in pumping activity of Crabs 2B and 4B
35
Temperature (ºC)
25
36
25
24
23
22
21
20
20
15
10
5
0
Migratory Restlessness (% Activity)
100
80
60
40
20
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (hours)
Figure 17. Actographs of pumping activity (# pumps/0.5 h) and migratory
restlessness (% time spent active) for Crab 1B subjected to simulated
tidal changes in temperature for the first 96 h and subsequently
maintained under constant conditions for an additional 96 h. The
expected light:dark cycle and the imposed temperature cycle are
indicated in the top panel. Time is expressed in hours starting at
midnight on August 20, 2003.
37
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1600
12.95 h.
1400
Spectral Density
1200
1000
800
600
400
200
0
0
12
24
36
Period (h)
temperature for 96 h (entrainment phase). Dashed horizontal lines in the
autocorrelation plots indicate the 95% confidence intervals. Dominant
periods in the MESA spectra corresponding to significant peaks in the
autocorrelation plots are provided.
38
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1200
24.19 h.
Spectral Density
1000
800
600
400
200
0
0
12
24
36
Period (h)
activity for Crab 1B (Fig. 17) subjected to constant conditions for 96 h
(post-entrainment phase). Dashed horizontal lines in the autocorrelation
plots indicate the 95% confidence intervals. Dominant periods in the
MESA spectra corresponding to significant peaks in the autocorrelation
plots are provided.
39
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
30
12.95 h.
3
25
20
15
10
5
0
0
12
24
36
Period (h)
restlessness for Crab 1B (Fig. 17) subjected to a simulated tidal cycle in
40
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
60
27.61 h.
3
50
40
30
20
10
0
0
12
24
36
Period (h)
restlessness for Crab 1B (Fig. 17) subjected to constant conditions for
96 h (post-entrainment phase). Dashed horizontal lines in the
Temperature (ºC)
41
25
24
23
22
21
20
40
35
30
25
20
15
10
5
0
100
80
60
40
20
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (hours)
midnight on September 27, 2003.
42
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
0.8
0.6
-3
13.87 h.
0.4
0.2
0.0
0
12
24
36
Period (h)
43
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1.2
12.95 h.
-3
1.0
0.8
0.6
0.4
0.2
0.0
0
12
24
36
Period (h)
plots are provided.
44
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
40
27.62 h.
3
30
20
10
0
0
12
24
36
Period (h)
45
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
3.5
12.14 h.
3
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
12
24
36
Period (h)
Temperature (ºC)
46
25
24
23
22
21
20
35
30
25
20
15
10
5
0
100
80
60
40
20
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (hours)
midnight on November 9, 2003.
47
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
160
27.62 h.
140
Spectral Density
120
100
80
60
40
20
0
0
12
24
36
Period (h)
48
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
600
Spectral Density
500
400
9.72 h
300
200
100
0
0
12
24
36
Period (h)
plots indicate the 95% confidence intervals.
49
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
800
Spectral Density
600
400
200
0
0
12
24
36
Period (h)
autocorrelation plots indicate the 95% confidence intervals.
50
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
3.0
3
2.5
2.0
1.5
1.0
0.5
0.0
0
12
24
36
Period (h)
Temperature (ºC)
51
26
24
22
20
40
35
30
25
20
15
10
5
0
100
80
60
40
20
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (hours)
midnight on August 16, 2004.
52
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
250
Spectral Density
200
150
12.14 h.
100
50
0
0
12
24
36
Period (h)
53
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
300
Spectral Density
250
200
11.43 h.
150
100
50
0
0
12
24
36
Period (h)
plots are provided.
54
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
20
27.62 h.
3
15
10
5
0
0
12
24
36
Period (h)
restlessness activity for Crab 4B (Fig. 32) subjected to a simulated tidal
cycle in temperature for 96 h (entrainment phase). Dashed horizontal
lines in the autocorrelation plots indicate the 95% confidence intervals.
Dominant periods in the MESA spectra corresponding to significant
peaks in the autocorrelation plots are provided.
55
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
35
13.87 h.
3
30
25
20
15
10
5
0
0
12
24
36
Period (h)
Temperature (ºC)
56
25
24
23
22
21
20
20
15
10
5
0
100
80
60
40
20
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (hours)
midnight on October 12, 2004.
57
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
12.95 h.
1000
Spectral Density
800
600
400
200
0
0
12
24
36
Period (h)
58
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
100
12.95 h.
Spectral Density
80
60
40
20
0
0
12
24
36
Period (h)
plots are provided.
59
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
10000
32.1 h.
3
8000
6000
4000
2000
0
0
12
24
36
Period (h)
60
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
3
400
300
200
100
0
0
12
24
36
Period (h)
61
occurred 5.5 h and 4.5 h after maximal temperature, respectively (Table 1). In
contrast, Crab 5B displayed maximal pumping 2.5 h before maximal temperature
(Table 1).
The phase relationship between the temperature cycle and pumping was
compared for Crab 2B and 4B. When exposed to temperature cycles, the acrophase
between the pumping activity of Crab 2B and the temperature cycle varied (peaks
in pumping occurred between 2 h before and after high temperature; Fig. 42).
Pumping activity of Crab 4B for the first two cycles of the experiment occurred 4 h
to 6 h before high temperature. In the subsequent cycles, maximal pumping
occurred in phase with the simulated changes in temperature (i.e., 0 h phase lag)
(Fig. 43). Following placement under constant conditions, peaks in pumping
gradually shifted such that peaks in activity occurred during the time of expected
minimum temperature. However, the timing of behaviors following placement in
constant conditions differed among crabs. The activity cycle for Crab 2B was
slightly shorter than the rhythm exhibited during the entrainment phase
(0.92 shorter; Fig. 23 and 24), whereas the rhythm for Crab 4 was slightly longer
(0.71 h longer; Fig. 42 and 43).
As with pumping activity, Crabs 1B and 5B also exhibited rhythms in
migratory restlessness during the entrainment period with dominant periods of
12.95 h and 12.14 h, respectively (Figs. 20 and 40). However, during the post-
62
Entrainment phase
Phase Lag (h after max temperature)
12
Constant conditions
10
8
6
4
2
0
-2
-4
0
20
40
60
80
100
120
140
160
180
200
Time (h)
Figure 42. Phase relationship between pumping activity of Crab 2B and actual (left
panel) and expected (right panel) changes in temperature. Phase lags
are expressed relative to the time of maximum temperature (y-axis).
Thus, positive and negative lags indicate that peaks in pumping activity
occurred that many hours after or before peaks in temperature.
220
63
Phase Lag (h after max temperature)
Constant conditions
Entrainment phase
4
2
0
-2
-4
-6
-8
0
20
40
60
80
100
120
140
160
180
200
220
Time (h)
Figure 43. Phase relationship between pumping activity of Crab 4B and actual (left
panel) and expected (right panel) changes in temperature. Phase lags
are expressed relative to the time of maximum temperature (y-axis).
Thus, positive and negative lags indicate that peaks in pumping activity
occurred that many hours after or before peaks in temperature.
64
entrainment period, Crab 1B displayed a circadian rhythm (27.62 h; Fig.21) in
migratory restlessness, whereas Crab 5B was arrhythmic (Fig. 41). Crabs 2B and
4B displayed rhythms with periods of 27.62 h period during the entrainment period
(Figs. 25 and 35). Yet, after placement in constant conditions, they exhibited
rhythms which were more consistent with a circatidal rhythm (12.14 h and 13.87 h
period; Fig 26 and 36). The time series of migratory restlessness for crab 3B was
arrhythmic during both the entrainment and post-entrainment periods (Figs. 30 and
31). When compared to the expected temperature cycle, peaks in the rhythms of
migratory restlessness for Crabs 1B and 5B occurred 6 h and 3 h after peaks in
temperature (Table 1). During the post-entrainment period, peaks in migratory
restlessness of Crabs 2B and 4B occurred 2 h and 3 h after the expected maximal
temperature, respectively (Table 1).
To control for the effects of time on the activity rhythms of crabs, four
ovigerous females were monitored for the same time period as experimental crabs
but were not exposed to cycles in temperature (22.5 ºC and 20 ºC) for 192 h. The
results were reported in a similar fashion as entrainment trials. For each crab, the
time series and the results of the autocorrelation and MESA were grouped together
(Figs. 44-63), sequentially depicting the time series (Figs. 44, 49, 54, and 59),
statistics of the pumping behavior during the first (Figs. 45, 50, 55 and 60) and last
96 h (Figs. 46, 51, 56, and 61) of the trial and statistics of the migratory restlessness
65
during the first (Figs. 47, 52, 57, and 62) and last 96 h (Figs. 48, 53, 58, and 63) of
the trial. During the first 96 h, none of the crabs displayed a rhythm in pumping
activity (Figs. 45, 50, 55, and 60) and only Crab 4C exhibited a significant rhythm
in migratory restlessness (27.61 h period; Fig. 62) whereas the other did not
exhibited a rhythm in migratory restlessness (Figs. 47, 52, and 57). Similarly, none
of the crabs displayed a significant rhythm in pumping during the second half
(96 h-192 h) of the trial (Figs. 46, 51, 56, and 61). Only Crab 1C exhibited a
rhythm in migratory restlessness during the second half of the experiment. The
dominant period of the rhythm was 24.17 h, suggesting that it might be circadian
(Fig. 48). The other crabs were arrhythmic during the last 96 h (Figs. 53, 58, and
63) of the trial.
66
40
35
30
25
20
15
10
5
0
100
80
60
40
20
0
0
12
24
36
48
60
72
84
96 108 120 132 144 156 168
Time (Hours)
conditions for 192 h. The expected light:dark cycle is indicated in the
top panel. Time is expressed in hours starting at midnight on June 16,
2004.
67
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
160
140
Spectral Density
120
100
80
60
40
20
0
0
12
24
36
Period (h)
activity for Crab 1C (Fig. 42) subjected to constant conditions for the
first 96 h of the experiment. Dashed horizontal lines in the
68
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
500
Spectral Density
400
300
200
100
0
0
12
24
36
Period (h)
last 96 h of the experiment. Dashed horizontal lines in the
69
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
4
3
3
2
1
0
0
12
24
36
Period (h)
restlessness for Crab 1C (Fig. 42) subjected to constant conditions for
the first 96 h of the experiment. Dashed horizontal lines in the
70
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
7
24.17 h.
3
6
5
4
3
2
1
0
0
12
24
36
Period (h)
the last 96 h of the experiment. Dashed horizontal lines in the
71
25
20
15
10
5
0
100
80
60
40
20
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (Hours)
top panel. Time is expressed in hours starting at midnight on June 25,
2004.
72
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
250
Spectral Density
200
150
100
50
0
0
12
24
36
Period (h)
73
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
50
Spectral Density
40
30
20
10
0
0
12
24
36
Period (h)
74
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
14
3
12
10
8
6
4
2
0
0
12
24
36
Period (h)
75
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
6
Spectral Density
5
4
3
2
1
0
0
12
24
36
Period (h)
76
30
25
20
15
10
No Data
5
0
100
80
60
40
20
No Data
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (Hours)
top panel. Time is expressed in hours starting at midnight on July 4,
2004.
77
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
60
Spectral Density
50
40
30
20
10
0
0
12
24
36
Period (h)
78
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
120
Spectral Density
100
80
60
40
20
0
0
12
24
36
Period (h)
79
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
200
Spectral Density
150
100
50
0
0
12
24
36
Period (h)
80
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1.2
5.26 h
3
1.0
0.8
0.6
0.4
0.2
0.0
0
12
24
36
Period (h)
81
45
40
35
30
25
20
15
10
5
0
100
80
60
40
20
0
0
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204
Time (Hours)
top panel. Time is expressed in hours starting at midnight on July 13,
2004.
82
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
500
Spectral Density
400
300
200
100
0
0
12
24
36
Period (h)
83
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
1200
Spectral Density
1000
800
600
400
200
0
0
12
24
36
Period (h)
84
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
100
3
80
60
40
20
0
0
12
24
36
Period (h)
85
1.0
Autocorrelation
0.5
0.0
-0.5
-1.0
-48
-24
0
24
48
Lag (h)
25
3
20
15
10
5
0
0
12
24
36
Period (h)
86
DISCUSSION
Previous studies indicated that ovigerous blue crabs Callinectes sapidus
possess a circatidal rhythm in egg maintenance behavior, migratory restlessness,
and swimming activity (Forward et al. 2003b). Peaks in migratory restlessness and
swimming activity occur during ebb tides in the field, thus transporting females
down-estuary toward spawning grounds. During flood tide, female crabs remain on
the bottom and peaks in pumping activity are observed (Tankersley et al. 1998,
Forward et al. 2003b). In the preliminary and control trials of the present study,
only three of the nine crabs tested displayed rhythms in pumping, migratory
restlessness, or both activities. Behaviors of the other female crabs were
arrhythmic, which conflicts with earlier results for crabs collected from Beaufort
Inlet, NC. Ovigerous crabs collected from this area prior to larval release exhibited
clear circatidal rhythms in both pumping and migratory restlessness (Forward et al.
2003b).
One possible explanation for the discrepancy between the results of the
present study and those described by Forward et al. (2003b) lies in the differences
in the characteristics of the estuaries where crabs were collected for the studies.
The properties of the Indian River Lagoon (IRL) differ markedly from the Newport
River Estuary, NC (Forward et al. 2003b). In the IRL, changes in environmental
87
conditions associated with the tides are confined to relatively small area (10 km2) in
the immediate vicinity of the inlets (Smith 1990). Thus, most of the IRL is
considered non-tidal or microtidal (Smith 1990). Conversely, most of the Newport
River Estuary experiences strong semi-diurnal tides of about 0.80 m amplitude
(Churchill et al. 1999). As a result, changes in the physical parameters are more
pronounced relative to the tides in the IRL and are less affected by stochastic
events, such as storms. In addition, tidal changes are not limited to the area
adjacent to the inlet (Luettich et al. 1999). Therefore, ovigerous crabs located in
areas short distances away from the inlet in the IRL may not experience the
appropriate entraining cue or one of an appropriate magnitude to entrain the
endogenous rhythm. On the other hand, females from the Newport River Estuary
located several km up-estuary will be subjected to detectable tidally related changes
in environmental conditions.
The migratory restlessness of Crab 1A was more pronounced during the
expected night ebb than during the day ebb (Fig. 2, p.19). A circatidal rhythm may
have been present but was overridden by a circadian clock. In all the trials
combined, a total of six crabs displayed circadian rhythms in either pumping
activity (one crab) or migratory restlessness (five crabs). These results indicate that
diel differences in activity observed in migrating crabs (Carr et al. 2004) could be
the result of a circadian clock that partially suppresses activity during the day. In
88
one instance during the entrainment period, the crab displayed a rhythm with a
periodicity around 12.42 h in both migratory restlessness and pumping activity, but
when placed under constant condition circadian rhythms in both behaviors were
expressed (Crab 1B; Figs. 17-21, p.36-40).
The purpose of this study was then to determine if cycles in temperature
could entrain the endogenous rhythms exhibited by ovigerous C. sapidus. During
the entrainment period, most crabs displayed a rhythm in pumping activity with
periods ranging from 12.14 h to 13.87 h. Peaks in pumping occurred near the time
when temperatures were expected to be maximal which corresponds to times of ebb
tide in the field (Redfield 1980, Holmström and Morgan 1983a) (Table 1, p.35).
However, during the post-entrainment period, three of the five crabs displayed a
circatidal rhythm in pumping activity with periodicities ranging between 11.43 h
and 12.45 h. For two crabs, peaks in pumping occurred 4.5 h to 5.5 h after the time
of expected high temperature with period lengths of 12.96 h (Table 1, p.35). Since
the cycle in temperature was set so that maximum temperature occurred at the time
of high tide in the field, maximal pumping thus occurred 4.5 h to 5.5 h after natural
high tide. Because the tidal and current cycles in the field are not perfectly
synchronized with each other (SBE occurring approximately 2.5 h to 3 h after high
tide at Sebastian Inlet, FL; Smith 1990), peaks in egg maintenance behavior
89
observed during the post-entrainment period occurred during the time of expected
ebb tide.
During the entrainment period, all but one crab exhibited a circatidal
rhythm in pumping activity (Table 1, p.35) with peaks occurring 1.5 h to 0.5 h
before the time of high temperature (Table 1, p.35). During the summer, warm
water conditions are typically associated with low tides and therefore ebbing
current (Holmström and Morgan 1983a). However, Forward et al. (2003b)
observed that the pumping activity of ovigerous crabs placed in constant conditions
occurred during the time of flood currents. The pattern in pumping activity
observed during the entrainment period may be the result of a physiological
response to the change in temperature (i.e., Q10 effect), rather than an endogenous
clock. This masking effect, defined as an external stimulus suppressing the
endogenous biological process, is commonly observed in organisms possessing
internal clocks (Roenneberg et al. 2003).
The increase in pumping activity with higher temperature could be the
result of 1) decreased oxygen concentration, 2) increased in metabolic rate of the
female or 3) increase in oxygen consumptions by the embryo. With an increase in
temperature, the amount of oxygen in solution declines and therefore less oxygen
would have been available for both the female and the eggs. This alone could be
sufficient to elicit behavioral responses from the ovigerous crabs (i.e., increase in
90
ventilation and pumping rates) to meet the oxygen demands of the embryos. For
example, ovigerous Cancer pagurus detect low oxygen concentrations in the egg
mass and are able to make adjustment through abdominal pumping (Naylor et al.
1999). It is possible that ovigerous blue crabs possess the same capabilities.
Under those conditions, the increased oxygen demands of the embryo are met
through an increase in the abdominal pumping rate (Wheatly 1981). Since blue
crabs have a limited ability to physiologically cope with low oxygen conditions
(see Tankersley and Forward 2005 for review), an increase in activity, associated
with an attempt to leave the area under hypoxic conditions, and higher ventilation
rates are observed (Lowery and Tate 1986; Das and Sickle 1994). However, the
studies preformed by Lowery and Tate (1986) and Das and Sickle (1994) focused
on juvenile and adults. The behavioral response of ovigerous blue crabs to lower
oxygen concentrations are unknown and could involve an increase in migratory
restlessness and pumping rate to provide more ventilation and therefore oxygen to
the embryos as observed for ovigerous C. pagurus (Naylor et al. 1999).
Second, as with most poikilothermic organisms, the physiological processes
of the blue crab vary with temperature which affects activity levels and behavioral
patterns. Oxygen uptake by adult blue crabs doubles over the temperature range of
15 ºC to 25 ºC (Q10 = 2; Mauro and Mangum 1982). Similarly, over the same range
of temperature, ventilation and heart rates also increase with Q10 of 4 and 2,
91
respectively (Mauro and Mangum 1982). As a result, the pumping activity of
females under warm water conditions can be expected to be higher as well. In this
study, it was estimated that the pumping rate of ovigerous C. sapidus was
multiplied by about 2.5 for a 5 ºC increase in temperature (Q10 = 6.25 ± 4.25).
Third, the oxygen demand of developing embryos is expected to increase with
temperature, as it has been documented in other crab species (Wheatly 1981).
However, response to increased oxygen demand does not explain the results
obtained in the post-entrainment period, where the pumping behavior of ovigerous
blue crab was synchronized with the expected temperature cycle. Entrainment did
not occur since the behavior exhibited during the post entrainment period shifted
with respect to the expected cycle in temperature (Figs. 42 and 43, p.62 and 63).
Therefore, ovigerous blue crabs appear to respond to an exogenous change in
temperature but it does not serve as a cue for entraining their endogenous rhythms
in pumping and migratory restlessness.
The present results however contrast with previous work (Taylor and
Naylor 1977, Hasting 1981, Reid and Naylor 1990). In the aforementioned studies,
activity peaks displayed a change in phase relationship relative to the natural tidal
cycle during the entrainment period. As a result, the behavior matched the imposed
cycle. When subsequently placed in constant conditions, the organism exhibited a
circatidal rhythm that was in phase with the simulated cycle (Taylor and Naylor
92
1977, Hasting 1981, Reid and Naylor 1990). However the organisms used in those
studies were maintained in the laboratory prior to testing and their original
circatidal rhythm was lost (Taylor and Naylor 1977, Hasting 1981, Reid and Naylor
1990). When the animals are collected from the field and subjected to simulated
tidal changes, the typical pattern is for the rhythm to gradually shift to match the
imposed cycle (Holmström and Morgan 1983a, Bolt and Naylor 1986, Akiyama
2004). In C. maenas entrained to salinity cycles that are 6 h out of phase with the
natural cycle, phase shifting in locomotor activity occurs over several days at an
average rate of 1 h per day (Bolt and Naylor 1986). In the present experiment,
females that responded to high temperature through increase in pumping activity
did so either immediately after being placed in the experimental chamber and
subjected to the temperature cycle (Crab 2B) or within 3 cycles (Crab 4B).
However, once in constant conditions, the timing of the pumping activity gradually
shifted to occur 6 h out of phase with the peaks in temperature.
The lists of zeitgebers used by ovigerous blue crabs to synchronize their
behavior with the tidal cycle are not restricted to temperature. Previous studies
indicated that multiple environmental factors may serve as entraining cues
(Holmström and Morgan 1983a, Reid and Naylor 1990). Salinity has been shown
to synchronize the circatidal larval release behavior of ovigerous blue crabs
(Tankersley et al. 2005) and the locomotory activity of C. maenas (Taylor and
93
Naylor 1977). However, changes in pressure associated with the tide are more
predictable and reliable than other environmental variables, and ovigerous C.
sapidus may therefore rely on pressure to synchronize their spawning behavior with
local tides. Pressure has been found to serve as a zeitgeber for circatidal activity
rhythms in several marine species, including adult C. maenas (Reid and Naylor
1990), the cumacean Dimorphostylis asiatica (Akiyama 2004), and fish (Gibson
1984; Northcott 1991). Additionally, Hench et al. (2004) suggested that pressure
served as a potential exogenous cue triggering the spawning behavior of
C. sapidus. Hench et al. (2004) studied the swimming behavior of tethered females
in the field. In situ measurements indicated that the cycle in pressure was closely
correlated with the swimming behavior. They observed that periods of migratory
restlessness were most closely associated with periods of ebb when pressure was
decreasing (i.e., water level is falling). It was therefore suggested that pressure
served as the zeitgeber for the endogenous rhythm in the spawning behavior
(Hench et al. 2004). However, in this field study the crab was subjected to a
variety of natural cues, and further testing under controlled laboratory conditions is
needed to assess if pressure effectively serves as an entraining cue.
94
CHAPTER 2
IMPACT OF TIME OF INITIATION AND DURATION OF
VERTICAL MIGRATION ON THE EFFECTIVENESS OF
SELECTIVE TIDAL-STREAM TRANSPORT
INTRODUCTION
Vertical migration is commonly used by estuarine and marine organisms to
take advantage of depth-varying currents for transport into or out of estuaries (e.g.,
breeding grounds and nursery areas; Sulkin 1984, Epifanio and Garvine 2001,
Forward and Tankersley 2001, Gibson 2003). Organisms living in areas under tidal
influence accomplish those horizontal migrations by migrating vertically in and out
of the water column in synchrony with the changes in direction and magnitude of
the tidal currents (Forward and Tankersley 2001, Gibson 2003). This process,
known as selective tidal-stream transport (STST), results in rapid movement up or
down estuaries or between estuaries and coastal areas (reviewed by Forward and
Tankersley 2001, Gibson 2003). Depending on the timing of migration relative to
tidal phase and the direction of travel, STST is classified as either flood-tide (FTT)
or ebb-tide transport (ETT). During FTT, shoreward or up-estuary transport is
accomplished by ascending into the water column during flood tide and descending
prior to the beginning of ebb tide. FTT is commonly used by organisms with
oceanic larval stage to invade nursery habitats and spawning grounds within
estuaries (reviewed by Forward and Tankersley 2001). Conversely, organisms
95
undergoing ETT enter the water column during ebb tide and remain on or near the
bottom during flood tide. This mechanism is commonly exhibited by estuarine and
coastal animals migrating from estuaries to undergo development in shelf areas
(reviewed by Forward and Tankersley 2001).
Previous modeling studies indicate that STST is most efficient when the
migratory behavior is perfectly synchronized with the dominant tidal constituent
(e.g., M2, S2, K1; Hill 1991a, 1995, Smith and Stoner 1993). Shifts in the phasing
of the migratory behavior with respect to the current cycle is expected to reduce the
efficiency of transport since during the same vertical migration event transport will
occur in both an up- and down-stream directions. However, the period of the active
phase does not need to match the tidal period in order for net displacement to occur
(Hill 1995, Smith and Stoner 1993). For example, Hill (1995) demonstrated that in
a semi-diurnal habitat diel vertical migration can result in several days of positive
net transport. In areas where the dominant tidal constituent is not semi-diurnal
(e.g., diurnal tides environment), the number of nights over which transport occurs
in the same direction will vary between several days to up to 6 months depending
upon the difference in periodicity of the two cycles (Hill 1995).
The rhythmic behavior underlying STST can be either under endogenous or
exogenous control. In organisms in which a circatidal rhythm mediates STST, the
phasing of the rhythm with the tidal cycle is set by external cues or zeitgebers
96
(Palmer 1973). Potential zeitgebers include environmental cues which typically
change with the tide, such as salinity, temperature, pressure, and turbulence
(reviewed by Forward and Tankersley 2001, Reid and Naylor 1990). Similarly, in
organisms where STST behavior is under exogenous control, both the initiation and
termination of the migratory phase are cued by environmental cues associated with
the tides (reviewed by Forward and Tankersley 2001).
However, although changes in water level and tidal currents have similar
periods, they are rarely perfectly synchronized (Redfield 1980). Thus, the cycle in
tidal currents usually lags the cycle in water level (Redfield 1980). This
asynchrony is attributed to the fact that the tides have both standing and progressive
wave characteristics. Near coastal areas, tides propagate primarily as progressive
waves and slack before ebb (SBE) and slack before flood (SBF) occur at mid-tide
(Redfield 1980). In contrast, the reflection of the tidal wave in estuaries causes it to
interfere with itself thereby generating a standing wave (Redfield 1980). In this
case, currents are synchronized with the tides and SBE and SBF occur at the time
of high and low tide, respectively. Consequently, at any one location the relative
phase relationship between the tides and currents can vary between 0 h and 3 h
depending on the relative contribution of the standing and progressive wave
components (Redfield 1980). Since fluctuations in environmental factors are
generally closely associated with either water level or tidal currents, the zeitgeber
97
used to reset the endogenous clock or the environmental cues initiating or
terminating the active phase of STST behaviors may not be in perfect synchrony
with the current cycle and therefore the resulting behavior will display a similar
phase-lag. For example, tidal changes in pressure are typically synchronized with
changes in the water level, yet other variables, including temperature, salinity and
turbulence are more closely synchronized with tidal currents (Uncles et al. 1985).
Regardless of the underlying cue mediating STST, any asynchrony between the
migration and the current cycles is hypothesized to reduce the efficiency of STST.
Vertical migratory behaviors associated with STST are often influenced by
light, which suppresses circatidal swimming activity and the responses to tiderelated environmental cues (Forward and Tankersley 2001, Gibson 2003, Carr et al.
2004, Forward and Cohen 2004). Thus, in many species, including the American
eel Anguilla rostrata (Parker and McLeave 1997), ovigerous females (Tankersley
et al. 1998, Carr et al. 2004) and megalopae (Forward et al. 2003a) of the blue crab
Callinectes sapidus, and numerous brachyuran zoeae (Paula 1989, Forward and
Tankersley 2001), STST only occurs during the dark phase. When the appropriate
tidal phase for transport occurs in darkness, maximum daily transport occurs.
However because the period length of the tidal and diel cycles differ (i.e. 12.4 h vs.
24 h), tides occur nearly one hour later each day. Therefore, as the active phase
98
shifts with respect to the day-night cycle, less of the tidal phase occurs in darkness
and transport is expected to be reduced.
The objective of this study was to use a coupled tidal current-vertical
migration model to assess the effects of the timing and duration of vertical
migratory activity on the efficiency of STST. Three models were developed to
investigate the effects of: 1) the synchrony and duration of vertical migration; 2)
the timing of the initiation of the active (migration) phase relative to the tidal cycle
and; 3) diel differences in activity (e.g., suppression of vertical migration during the
day) on net displacement. We used the well studied STST behaviors exhibited by
the blue crab Callinectes sapidus as a basis for the model algorithms. Blue crabs
have been shown to exhibit both FTT and ETT during their life cycle. Postlarvae
(megalopae) use FTT behavior to enter estuaries following development in shelf
waters (De Vries et al. 1994, Olmi 1994). The timing of their active phase is
controlled by exogenous cues. Ascent in the water column is triggered by an
increase in salinity near the beginning of flood tide (Tankersley et al. 1995). Once
in the water column, sustained swimming is cued by the presence of turbulence,
and descent to the bottom is triggered by the decline in turbulence associated with
SBE (Welch et al. 1999, Tankersley et al. 2002). Two separate external cues are
therefore responsible for each triggering the ascent and the descent in the water
column. However, the relationship of each cue with the tidal cycle is different.
99
The timing of the ascent phase (i.e., increase in salinity) with the tidal currents may
fluctuate greatly depending of external factors (e.g., freshwater runoff,
precipitation) whereas the timing of the descent phase will be less variable and will
occur near SBE. As a result, the time spent swimming in the water column will not
be expected to be constant but will change according to the environmental
conditions. In contrast, ovigerous blue crabs utilize ETT to migrate seaward to
spawn (Tankersley et al. 1998). This behavior is mediated by an endogenous clock
which controls the initiation and termination of the active phase (Forward et al.
2003a). Additionally, FTT behaviors are often suppressed by the presence of light
during the day (Tankersley et al. 1995). Thus, if cues responsible for initiating the
ascent response occur during the light phase, megalopae fail to respond and FTT is
not initiated (Tankersley et al. 1995). Similarly, a descent response and premature
termination of FTT is triggered by the onset of light (Tankersley et al. 2002). As
with megalopae, the presence of light likely inhibits the expression of the rhythm
such that ETT occurs primarily during the dark phase (Tankersley et al. 1998, Carr
et al. 2004, Hench et al. 2004). However, the suppression of the rhythm buy light
could not be confirmed by under laboratory conditions (Forward and Cohen 2004).
100
MATERIAL AND METHODS
The interaction between periodic vertical migration and tidal currents was
modeled using procedures similar to those described by Hill (1995). The model
was separated into two components (a) vertical swimming activity and (b) tidal
current velocity. Vertical migration in and out of flow was represented by a simple
step-function with M(t) = 0 indicating periods during STST when the organism was
either on or near the bottom and not subjected to tidal currents and M(t) = 1
indicating periods when the organism was in the water column and transported
passively by tidal flow (Fig. 64). Previous models of vertical migration in tidal
flows have concluded that simple square-wave functions are sufficient to describe
the migratory behavior associated with STST (Hill 1991a, b, 1995). The period of
the vertical migration cycle (Tm) was fixed at 12.42 h, which is consistent with the
migratory behavior exhibited by organisms living in areas with semi-diurnal tides
(see Forward and Tankersley, 2001 for review; Fig. 64). The duration of the active
phase, or “in flow” portion of each migratory cycle (i.e., the period time spent in
the water column), was Tw.
Tidal current velocity was treated as a sinusoidal and spatially uniform
wave with a period equal to the lunar semidiurnal tidal constituent (i.e., M2;
101
Ebb
U(t)
Flood
T
Tm
M(t)
Tw
D(t)
Ti
0
12
24
36
48
Time (h)
Figure 64. Diagram representing tidal currents [U(t)], vertical migration pattern
[M(t)] and diel pattern [D(t)] used to calculate net transport resulting
from STST behaviors. M(t) represented the vertical activity pattern of a
simulated organism undergoing flood-tide transport. Thus, M(t) = 0
when the organism was out of the flow and M(t) = 1 when it was in the
flow. Tw represented the duration of the active phase when the simulated
organism was present in the water column and subjected to tidal flow. A
similar square wave was used to simulate diel activity D(t) where
D(t) = 0 represented periods when activity is suppressed (daytime) and
therefore the organism was out of the flow and D(t) = 1 represented
periods when the activity was not suppressed (e.g., nighttime). The
light:dark cycle was set at 14:10 for all simulations.
102
12.42 h). Current speed U(t) at any given time t was expressed using the following
equation:
U (t ) = U a cos(
2π
t)
T
(1)
where Ua was the amplitude of the tidal currents and Τ the tidal period (Fig. 64;
12.42 h). Thus, for all simulations T and Tm were equal. For simplification, current
amplitude was set at Ua = 1.0 m s-1 (based on Hill 1991a, b). To differentiate
between the direction of flow, and therefore transport, flood and ebb currents were
treated as positive and negative values, respectively.
Horizontal displacement (Xi) of a vertically migrating organism at any given
time (t) was calculated by summing the product of the two wave forms [M(t) and
U(t)] at each time interval (∆t)
i
X i = ∑ M (t ) U (t ) ∆t
t =0
Thus, the organism was assumed to be transported at the same speed as the currents
(i.e., passive transport) when in the water column and to be stationary (i.e., no
displacement) during the inactive phase. All simulations were based on FTT
behaviors since comparable simulations of ETT would yield identical results but
with displacement occurring in the opposite direction.
(2)
103
EFFECT OF SYNCHRONY BETWEEN VERTICAL MIGRATION AND TIDAL
CURRENTS ON TRANSPORT
The first series of simulations examined the effects of a phase-lag between
tidal current velocity [U(t)] and vertical migration [M(t)] on the efficiency of
transport (referred to as Migration-Flow Synchrony Model). This algorithm was
design to model STST behaviors which are mediated by an endogenous rhythm in
migration, such as those exhibited by ovigerous blue crabs during the spawning
migration (Forward et al. 2003b). Synchrony was altered by varying the phase
angle or acrophase (φ) between M(t) and U(t) from -90° and 0° (Fig. 65a). Thus,
the initiation of the active phase occurred between 3.11 h and 0 h before SBE. The
duration of the active phase (Tw) was constant and equal to half of the vertical
migration cycle (i.e., ½ Tm or 6.21 h; Fig. 65a). Net transport was monitored for 10
tidal cycles (i.e., 124 h) at acrophases of 0º, 30°, 60º and 90º. The relative effect of
each phase lag on transport was determined by comparing net displacement after 10
tidal cycles to control values in which the current and vertical migration cycles
were perfectly synchronized (i.e., φ = 0º).
In the previous model, the duration of the active phase of the migration was
assumed to last half a tidal cycle (6.21 h). However, the duration of migration will
vary based on difference in behavior among species. Consequently, a second
model algorithm (Migration Duration-Flow Synchrony Model) was developed
104
U(t)
a) Shifts in acrophase
M(t)
φ
0
12
24
Time (h)
36
48
U(t)
b) Shifts in acrophase and duration of swimming
φ
M(t)
Tw
0
12
24
36
48
24
36
48
Time (h)
U(t)
c) Shifts in the time of initiation
M(t)
∆ Ti
0
12
Time (h)
Figure 65. Diagram depicting the parameter [i.e., acrophase (φ), duration (Tw) and
time of initiation (Ti)] describing the timing and duration of migration
and their variations for the different model algorithms. The upper panel
of each graph shows the current cycle [U(t)] and the lower panel
indicates the active [M(t) = 1] and inactive [M(t) = 0] phases of
migration. The full line (bottom plot) represents the vertical migratory
behavior of Tw = 6.21 h and φ = 0º. The dotted line indicates how
timing and duration of the vertical migratory behavior was varied. The
equation for [U(t)] was multiplied by M(t) to obtain instantaneous speed
of an organism undergoing STST. This result was then integrated to
calculate net transport. a) Migration-Flow Synchrony Model where the
acrophase φ varied. b) Migration Duration-Flow Synchrony Model
where both the acrophase φ and swimming duration Tw varied.
c) Initiation of Migration Model with variations in the time of initiation
of the active phase (Ti).
105
where both the synchrony (acrophase: φ) and the duration of the migration (Tw)
varied simultaneously (Fig. 65b). For this model, transport after 10 cycles (i.e.,
124 h) was compared for simulations in which the acrophase varied between -90°
and 0° and the activity phase ranged between 0 h and 6.21 h. Transport was
expected to be maximal for φ = 0° and Tw = 6.21 h. Net displacement under these
conditions was used to evaluate the relative impact of changes in the synchrony,
duration, and timing of the swimming behavior on overall net transport.
TRANSPORT
The second set of simulations examined the effect of the timing of the
ascent phase relative to tidal currents on transport (Initiation of Migration Model).
The model was designed to mimic STST behaviors in which the initiation and the
termination of migration were mediated by exogenous cues associated with flood
currents and slack water, respectively (Fig. 65c). For all trials, current velocity
[U(t)] and the vertical migration behavior [M(t)] had the same period and were
synchronized [i.e., acrophase (φ = 0)]. The end of the active phase was fixed so
that it coincided with SBE. Consequently, the duration of the active phase (Tw) was
altered by delaying the time of the ascent phase relative to SBF. Initiation times
(Ti) relative to SBF varied between 0 h and 3.11 h. Thus, when Ti = 0 h, ascent
coincided with SBF and lasted for the entire duration of flood tide. When Ti
106
was > 0 h, entry into the water column was postponed until later during flood
phase. Thus, the duration of activity ranged between 6.21 h (Tm/2) and 3.11 h
(Tm/4). Net transport was compared for 10 tidal cycles (i.e., 124 h) at four different
initiation times: Ti = 0 h, 1.0 h, 2.1 h and 3.1 h corresponding to 100%, 83%, 67%
and 50% of the flood tide period. For each initiation time (Ti), the relative decrease
in net transport was compared to values obtained at Ti = 0 h.
EFFECT OF DIEL CYCLE ON TRANSPORT
The effects of diel differences in activity on the efficiency of STST was
modeled by adding a second step-function D(t) to the Migration-Flow Synchrony
Model and Initiation of Migration Model described above (Eq. 2; Fig 65 a and c),
i
X i = ∑ M (t ) U (t ) D (t ) ∆t
t =0
such that D(t) = 0 indicated periods of suppressed activity (i.e., diurnal inactivity),
and D(t) = 1 indicated periods of activity (i.e., nocturnal activity) (Fig. 1). The
period length D(t) = 24 h and the relative duration of the inactive:active phases of
the cycle (i.e., light: dark cycle) were set to 14:10 h to reflect conditions during the
summer when STST behaviors are common in coastal and estuarine organisms
(Forward et al. 2003a). At time t = 0, the phasing of the tidal and the diel cycle was
such that SBF and sunset occurred at the same time.
(3)
107
In the Diel Migration-Flow Synchrony Model transport occurred whenever
the activity phase (M(t) = 1) coincided with the dark phase of the L:D cycle
(D(t) = 1). Thus, entry into and out of the water column was determined by both
animal activity [M(t)] and the day:night cycle [D(t)]. Net daily transport over 60
tidal cycles (about 30 days) was computed at four different acrophases (φ ): 0°, 30°,
60°, 90°. In the Initiation of Migration Model, entry into the water column and
transport only occurred when the initiation of the activity phase (Ti) occurred
during the dark phase (D(t) = 1). Thus, if Ti occurred before sunset, the organism
remained out of the flow for the entire activity cycle. Similarly, the onset of light
at sunrise resulted in the termination of the active phase. The time of initiation of
activity therefore varied relative to SBF but required absence of light, whereas the
termination of the activity phase was assumed to be triggered by either cues
associated with SBE or the onset of light. Net daily transport over 60 tidal cycles
(about 31.5 days) was compared for four different values of Ti: 0 h, 1.0 h, 2.1 h and
3.1 h. This longer duration (60 cycles) was chosen over the one used previously
(10 cycles) in order to assess the periodicity in daily transport. The synchrony
between the flow cycle and the diel cycle varied due to differences in periodicities
(12.4 h vs. 24 h) which cause the flow cycle to occur ~1 h later each day (Barnwell
1976).
108
The effect of seasonal variations of the day:night cycle on net daily
transport was simulated for both Diel Migration-Flow Synchrony and Diel
Initiation of Migration Models with φ and Ti set as constant at 0º and 0 h. The
length of the night varied between 10 h and 14 h representing summer to winter
conditions, respectively. The simulations were performed over 100 tidal cycles
(~50 days). Again, this longer duration was chosen over the one used previously
(10 cycles) in order to assess the periodicity in daily transport. For each duration of
the day:night cycle tested, the periodicity in daily transport was obtained. The
pattern in daily transport and the amount of daily net displacement over this period
was compared for the different day:night cycles (10:14, 11:13, 12:12, 13:11 and
14:10).
109
RESULTS
EFFECT OF SYNCHRONY BETWEEN VERTICAL MIGRATION AND TIDAL
CURRENTS ON TRANSPORT
As expected, maximum net displacement occurred when tidal currents and
the activity cycle were phase locked (φ = 0°) and declined as the phase lag between
the two cycles increased (Fig. 66). Regardless of the acrophase, net displacement
over time occurred as a series of saltatory steps (Fig. 66). Yet, because of the
sinusoidal nature of the flow cycle, transport declined by less than 13.5% for phase
lags < 1 h (i.e., φ < 30°) compared to conditions in which the two cycles were phase
locked (Fig. 66). However, net displacement decreased by more than 50% when
the phase lag was increased to 2 h (i.e., φ = 60°; Fig. 66). The relative effect of the
phase shifts between the two cycles on transport was independent of the duration of
the simulation (i.e., number of cycles).
In the Migration Duration-Flow Synchrony Model both the duration of the
activity phase and the synchrony between the two cycles were varied. Small
deviations from the control values (φ = 0º, Tw = 6.21 h) had relatively little impact
on net transport (Fig. 67). However, for phase angles above 30º and durations less
than 5.18 h (16.66% reduction) net displacement decreased by more than 20%
compared to control values (Fig. 67). Net transport covaried with both the
110
150
φ = 0h
φ = 1h
φ = 2h
φ = 3h
Transport (km)
125
100
75
50
25
0
0
24
48
72
96
120
Time (h)
Figure 66. Effect of acrophase between vertical migratory behavior and current
flow on net transport during STST (Migration-Flow Synchrony Model)
over ten tidal cycles. The phase relationship (φ) between the cycle in
tidal currents and the vertical migratory behavior varied between 3.11 h
and 0 h before SBF (φ = -90°, -60°, -30° and 0°). The period length of
the tidal currents and the vertical migration behaviors were both set at
12.4 h. The time the organism spent in the flow was 6.21 h.
111
6.21
5.17
130 km
110 km
Duration (h)
4.14
90 km
70 km
3.10
50 km
2.07
30 km
10 km
1.03
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Acrophase (degrees)
Figure 67. Effect of duration of the active phase and acrophase between vertical
migratory behavior and current flow on net transport during STST
(Migration Duration-Flow Synchrony Model) over ten tidal cycles.
The solid lines represent combinations of acrophase and swimming
duration resulting in identical net transport. The acrophase (φ) varied
between -90° and 0° and the activity phase (Tw) ranged between 0 h and
6.21 h.
112
acrophase and the swimming duration. Increasing acrophase and decreasing
duration resulted in less transport (Fig. 67). As a result, there were combinations of
acrophase and activity phase duration that produced identical net displacement as
indicated by the isolines of transport on Figure 67. Longer active phases were
necessary to produce the same net transport at greater acrophases.
TRANSPORT
A delay in the time of initiation of activity (Ti) resulted in a decline in net
transport that corresponded to the reduction in the length of the active phase. As
with the Migration-Flow Synchrony Model, displacement occurred in a series of
saltatory steps (Fig. 68). Transport remained unidirectional since the active phase
was terminated at SBE and animals were out of the flow when the currents were
ebbing. When the initiation of swimming occurred 1.0 h, 2.1 h and 3.1 h (16.66%,
33.33% and 50% reduction in Tw) after SBF, net transport was reduced by 6.5%,
25% and 50% of maximal transport obtained for Ti = 0 h (initiation occurred at
SBF), respectively (Fig. 68). Thus, small changes in the swimming duration had
relatively little impact on net transport. However, the impact on transport increased
by more than 25% compared to control values when the initiation of migration was
postponed by more than 2 h. The decline in net transport with an increase in the
delay in the initiation of the active phase was therefore not linear. For the same
113
150
Ti = 0 h
Ti = 1.0 h
125
Ti = 2.1 h
Transport (km)
Ti = 3.1 h
100
75
50
25
0
0
24
48
72
96
120
Time (h)
Figure 68. Effect of the delay in the time of initiation of swimming on net transport
during STST (Initiation of Migration Model) over ten tidal cycles. The
delay in the initiation time (Ti) with respect to slack before flood ranged
between 0 h and 3.11 h. The termination of swimming always occurred
at slack before ebb. The time the organism spent in the flow therefore
varied according to Ti. The period length of the tidal currents and the
vertical migration behaviors were both set at 12.4 h. The tidal current
amplitude was 1.0 m s-1.
114
increase in the delay of initiation, the reduction in transport was less for Ti < 2.1 h
but increased rapidly for Ti > 2.1 h.
EFFECT OF DIEL CYCLE ON TRANSPORT
The addition of a diel cycle to the Migration-Flow Synchrony and the Initiation of
Migration Models resulted in daily variations in net transport. For both models, net
daily transport remained high and was constant during periods when one of the two
daily migratory phases (Tw) occurred entirely during the dark phase (Fig. 69). As
the acrophase between the tidal and the day:night cycles increased, net daily
transport varied between 14.2 km day-1 and 6.1 km day-1, when the active phase
coincided with the tidal current (φ = 0º). As a result, there was a cyclic pattern in
net daily transport with a periodicity of 14.79 days which was equal to a semi-lunar
cycle. In the Combined Diel and Migration-Flow Synchrony Model, when the end
of one of the daily migratory periods took place near the beginning of the dark
phase, two short periods of transport occurred at both dusk and dawn. In terms of
daily displacement, the total transport of the two migration events was never as
great as displacement resulting from a single migratory phase that occurred entirely
during the dark phase. Those days when two short periods of transport occurred at
dusk and dawn represented time in the transport cycle when daily displacement was
reduced (Fig. 69). When most of both active phases took place during the light
phase, transport was reduced the most (Fig. 69). Because of difference in the
115
φ = 0°
φ = 30°
φ = 60°
φ = 90°
Daily Transport (km)
15
10
5
0
-5
0
5
10
15
20
25
30
Time (days)
Figure 69: Daily horizontal displacement resulting from selective tidal vertical
migrations that are influenced by the combined effects of tidal and diel
cycles (Combined Diel and Migration-Flow Synchrony Model). The
duration of the active phase was fixed at 6.21 h per cycle but vertical
swimming was suppressed during the day. The phase relationship (i.e.,
acrophase) between tidal currents and the vertical migratory behavior
varied between 3.11 h and 0 h before SBF (φ = -90°, -60°, -30° and 0°).
At t = 0, the acrophase between the tidal currents and the day: night
cycle was 0º. Tidal current amplitude was 1.0 m s-1and the day: light
cycle set was at 14:10.
116
period length of the diel and the tidal cycles, this cycle repeated itself every
14.79 days resulting in an apparent semi-lunar rhythm in transport that was
independent of the spring-neap cycle since the amplitude of the tidal currents was
constant. With an increase in phase-lag between migratory behavior and tidal
currents, net displacement declined and, for larger acrophases, even reversed
direction for some portions of the 14.79 day cycle (for φ < 60º). At φ = 90º, daily
transport took place in one direction for half of the semi-lunar cycle and in the
other for the other half (Fig. 69). As the phase lag between the tidal cycle and the
swimming behavior increased, daily net transport decreased following the same
pattern observed in the Initiation of Migration Model. However, transport
remained constant during days when the entire active phase took place at night
(Fig. 69). When the duration of the day:night cycle was varied, the same pattern
and periodicity (i.e., 14.79 day cycle) in net displacement was observed and net
transport increased with increasing duration of the night (Fig. 70). When the dark
phase was greater than 12.42 h, at least one of the active phases took place in
entirely darkness (Fig. 70). The relationship between the acrophase and the
reduction in transport was similar to what was observed for the Migration-Flow
Synchrony Model. Small shifts (φ < 30º) in synchrony between the tidal cycle and
the swimming behavior had relatively little effect on net displacement, but shifts in
the acrophase by more than 60º reduced net transport by more than 50%.
117
Daily transport (km)
20
15
10
5
14:10
13:11
12:12
11:13
10:14
0
0
10
20
30
40
50
Time (days)
Figure 70. Net daily displacement resulting from cycles in vertical migration that
included both tidal and diel components (Combined Diel and MigrationFlow Synchrony Model ; Fig. 69). Swimming duration was set to 6.21 h
per cycle but vertical swimming was suppressed during the light phase.
All simulations started at SBF and sunset. Lines represent transport for
light:dark cycles between 10:14 and 14:10.
118
A similar cyclic pattern in displacement with a 14.79 days periodicity was
observed in the results of the Combined Diel and Initiation of Migration Model.
Daily transport was constant and maximal when the entire active phase occurred
during the dark phase (Fig. 71). The duration of maximum transport increased
when the delay in the time of initiation of the active phase increased. The number
of days over which transport was maximal and constant increased with an increase
in the delay in the time of initiation relative to SBF (Fig.71). When cumulative net
transport was calculated over a single semi-lunar cycle (14.79 days), net
displacement was maximal when the initiation of swimming was delayed by 1 h
(Ti = 1.0 h). Under the same conditions, net transport was slightly greater (1.7 %)
than what was observed for the control (Ti = 0 h). However, when the initiation
occurred 2.1 h and 3.1 h after SBF net transport over one semi-lunar cycle
decreased by 5.2% and 36.3%, respectively, relative to Ti = 0 (Fig. 72). For a
day:night cycle of 14:10, no net transport was observed for a period of
three consecutive days during each semi-lunar cycle, regardless of the time of
initiation (Fig. 71). During these periods, the ascent phase took place just before
the end of the light phase and the entire subsequent active phase was suppressed by
light. The shift in the initiation of the active phase was caused by differences in
period lengths of tidal and the diel cycles. Therefore, the time of initiation occurred
an hour later every day. Additionally, the time in the semi-lunar cycle over which
119
Ti = 0 h
Ti = 1.0 h
15
Ti = 2.1 h
Ti = 3.1 h
10
5
0
0
5
10
15
20
25
30
Time (days)
Figure 71. Daily horizontal displacement resulting from tidal vertical migration that
is influenced by the combined effects of tidal and diel cycles (Combined
Diel and Initiation of Migration Model). The delay in the initiation time
(Ti) with respect to slack before flood ranged between 0 h and 3.11 h.
The swimming phase was not initiated if Ti took place during the day.
The termination of swimming always occurred at slack before ebb and
sunset. Periodicities of the tidal currents and the vertical migration
behaviors were both set at 12.4 h. At t = 0, the acrophase between tidal
currents and day: night cycle was 0º. The tidal current amplitude was
1.0 m s-1 and the day: light cycle set was at 14:10.
120
140
Cumulative Transport (km)
120
100
80
60
40
20
0
0.0
1.0
2.1
3.1
Time of Initiation (h after SBF)
Figure 72. Cumulative net transport after 14.79 days for simulated organisms in the
Combined Diel and Initiation of Migration Model. Delays in the
initiation of the vertical migration period of 0 h, 1.0 h, 2.1 h, and 3.1 h.
Migration did not occur if the time of ascent took place during the light
phase and migration was terminated at the onset of light or at SBE.
121
no transport occurred was not the same depending on the time of initiation of the
active phase and took place one day earlier in the 14.79 days cycle for each hour
delay in transport (Fig. 71). As for the Combined Diel and Migration-Flow
Synchrony Model, variations in the day:night cycle had no effect on the pattern or
period length of the cycle in daily transport (14.79 days; Fig. 73). However,
cumulative net transport over one transport cycle increased with the length of the
night since more time is available for transport (Fig. 73).
122
20
14:10
13:11
12:12
11:13
10:14
15
10
5
0
0
10
20
30
40
50
Time (days)
Figure 73. Net daily displacement resulting from cycles in vertical migration that
included both tidal and diel components (Combined Diel and Initiation
of Migration Model; Fig. 71). The delay in the initiation time (Ti) was
set at Ti = 0 h but the entire migratory period was suppressed if time of
initiation occurred during the light phase. Migration was terminated at
the onset of light and at slack water. All simulations started at SBF and
sunset. Lines represent transport for light:dark cycles between 10:14
and 14:10.
123
DISCUSSION
Migrations are an important part of the life cycle of numerous organisms
but are costly (e.g., energy, predation; Sinclair 1980, Forward and Tankersley 2001,
Gibson 2003). In areas where tidal currents are present, organisms often use tidal
currents to minimize the time spent swimming while maximizing transport and
therefore reduce the potential costs of migration (Forward and Tankersley 2001,
Gibson 2003). In this study, both the duration of the active phase and the
synchrony between the flow cycle and initiation of entry into the water column
were found to affect the efficiency of STST (i.e., total net transport). As expected,
maximum net transport occurred when the flow cycle and the rhythm in vertical
migration where phase locked and when swimming lasted for the duration of the
flood tide (i.e., φ = 0º and Tw = 6.21 h). These results are consistent with those of
Hill (1991a, 1995). However, since the underlying mechanism controlling
swimming activity may not always be in phase with the flow cycle, perfect
synchrony (i.e., φ = 0º) may not occur. Small deviations in the phase lag and the
initiation of the active phase had little effect on net displacement. However,
slightly longer delays (Ti = 2.1 h - 3.2 h) resulted in reductions in transport
exceeding 50 %. When the cues used to trigger the ascent in the water column are
closely associated with the tidal currents, such as temperature, salinity and
turbulence, transport may be significantly more efficient than when cues more
124
closely linked to water level, such as pressure, are used (Uncles et al. 1985). Such
cues may occur up to 3 h before the beginning of the current cycle, therefore
reducing the time available for transport (Uncles at al. 1985).
The Migration-Flow Synchrony Model mimicked the STST behaviors
exhibited by organisms that possess an endogenous clock. For any given
simulation, the timing of the initiation and termination of the active phase with
respect to the time of the tide occurred at the same time in each successive flow
cycle. External cues serving as zeitgebers reset the endogenous clock which in turn
maintains the synchrony between vertical migratory activity and current flow cycle.
This rhythm will be maintained, even if the cycle in the environmental cue varies or
conditions remain constant (Palmer 1973). On the other hand, if changes in the
physical conditions are significant due to non-tidal events, such as heavy rainfall or
strong winds (Redfield 1980, Peterson et al. 1986), then the timing, duration, and
amplitude of flood and ebb current may be altered. As a result, the active phase
may not remain synchronized with peaks in current. When the tidal cycle is altered
and the rhythm is not influence by external cues, short term changes in the timing
of the behavior are not possible and transport is likely reduced. Organisms in
which STST behaviors are mediated by exogenous cues (represented by the
Initiation of Migration Model) do not face the same problems since the
environmental parameters associated with the tides trigger the ascent and
125
termination of the active phase. Vertical migration is synchronized with the flow
cycle even if the timing of the latter changes. However, variations in
environmental conditions (e.g., temperature or salinity) could delay the triggering
of the ascent or descent phases of vertical migration. This could potentially delay
the timing of initiation of swimming duration, thus reducing the length of the active
phase and the efficiency of STST.
In the current study, the length of the active phase was either manipulated
directly (Migration Duration-Flow Synchrony Model) or varied as a consequence
of a delay in the timing of the ascent phase (Initiation of Migration and Combined
Diel and Initiation of Migration Models). The importance of the length of the
active phase in the energetic budget may be highly variable depending on the
characteristics of the organism (e.g., size, swimming ability, buoyancy). Although
the costs of upward and downward migration are fixed, the cost of sustained
swimming varies among organisms. For most planktonic species, the costs of
remaining in the water column are low since turbulence associated with the currents
is sufficient to maintain them in the upper portion of the water column (Sinclair
1980, Dill 1986, Gibson 2003). However, risk of predation when present in the
water column is often high, especially during the day (Sinclair 1980, Dill 1986).
As a result, even though energetic costs associated with migration are low, vertical
migratory behavior of zooplankton is often suppressed by the presence of light so
126
that transport occurs mostly at night when predation is reduced (Dill 1986, Christy
and Morgan 1998). Planktonic organisms for which it is costly to remain in the
water column even during the night (e.g., sustained swimming is necessary) need to
minimize the time spent in the water column without dramatically reducing net
displacement. In this case, short activity periods at small acrophases would result
in more transport and especially be less costly energetically (Fig. 67, p.111).
Finally, for nektonic organisms that are larger in size and less buoyant, such as
adult fish and crustaceans, sustained swimming is energetically costly but predation
pressure is reduced. Migration should therefore take place both during the day and
night but length of the active phase should be short and concentrated around the
time of maximum currents (i.e., during peak currents; Parker and McLeave 1997,
Carr et al. 2004). Such activity patterns have been observed in American eels
(Parker and McLeave 1997), young plaice (Gibson 2003), and ovigerous blue crabs
(Forward et al. 2003a, Hench et al. 2004).
The results obtained from the algorithms concur with transport observed in
migration studies of animals undergoing STST such as blue crabs C. sapidus and
American eels Anguilla rostrata. In the current simulations, maximal current speed
was 1.0 m s-1, which is close to the conditions found near many tidal inlets such as
Beaufort Inlet, NC where the spawning migration of ovigerous blue crab was
studied (Carr et al. 2004). Results from tracking studies of 8 ovigerous blue crabs
127
near the Beaufort Inlet, NC indicated that they migrated on average 4.8 km over
21.4 h (Carr et al. 2004). The results of the present simulations showed that crabs
undergoing STST can migrate more than 5 times this distance when the swimming
behavior is in perfect synchrony with the currents and still 3 times this distance for
an acrophase of 2 h and a swimming duration of 6.21 h assuming maximal current
speed was 1.0 m s-1 (Migration Duration-Flow Synchrony Model). However,
ovigerous blue crab do not stay in the water column during the entire time of ebb
but have been observed (both in the laboratory and tethered in the field) to migrate
by brief vertical swimming episodes (< 3 min) during the time of maximum ebb
tide (Forward et al. 2003b , Hench et al. 2004). Untethered crabs were active 1040% of ebb tide (night and day) but were more active when currents were high (4575% of the time) than when currents were low (< 15% of the time; Carr et al.
2004). Therefore, crabs near the Beaufort Inlet only need to be swimming for a
short period during the time of maximum ebb tide to be transported the required
distance for a 0º acrophase. Similarly, studies of homing and estuarine migration
behaviors of American eels, Anguilla rostrata, indicated that yellow and silver eels
used STST to migrate an average of 13.3 km over a period of 61.2 h (about 5 tidal
cycles; Parker and McCleave 1997). Eels were observed to migrate mainly during
mid-tide but swimming often ceased before the end of ebb. As a result, the
Migration Duration-Flow Synchrony Model with a swimming duration ranging
128
between 2.4 h and 4.65 h, a small acrophase (< 30º) and with swimming inhibited
during the day best represented the behavior of ovigerous blue crabs and migrating
eels.
When a diel cycle in activity was included in the models, net daily transport
varied with a period of about 14.79 days (Figs. 69 and 71, pp.115 and 119). This
pattern in daily transport was the result of differences in the period lengths of tidal
current (12.42 h) and diel cycles (24 h) that results in the tides occurring
approximately one hour later each day (Barnwell 1976). Periods of maximum
transport occurred when one of the active phases took place entirely during the
night. During days when most of the flood currents occurred during the light
phase, transport was partially or completely inhibited. As a result, periods of STST
migration should be concentrated during times in the lunar month when the active
phase occurs mostly at night. Christy and Morgan (1998) suggested that the
abundance of crustacean post-larvae depends upon the timing of the flood tide with
the day night cycle and not the amplitude of the tide. In this study, the observed
pattern was independent of the spring-neap cycle since only the M2 (12.42 h period)
component of the tide was included in the model and the amplitude of the current
flow was constant. However, depending on the location, the efficiency of transport
may vary since the time in the lunar cycle when the entire active phase occurs at
night will be different (Christy and Morgan 1998). Transport will be even greater
129
when nocturnal active phases occur during spring tides when currents are maximal.
Since the time in the spring-neap cycle when the entire flood occurs at night varies
spatially, the timing of initiation resulting in optimal transport will vary with
location. As a result, net transport of organisms whose migration is synchronized
with the time when the entire active phase takes place at night will be more
efficient than for those whose migration occurs during the time of spring tides
(Christy and Morgan 1998). Depending on the location, different larval population
may display a different vertical swimming behavior or respond differently to
exogenous cues to compensate for the difference (Queiroga et al. 1994).
In the Combined Diel and Initiation of Migration Model, the timing of the
cycles in daily transport depended upon the timing of the initiation of migration
relative to the tidal cycle as well as the diel cycle. This could have a significant
impact on organisms that also exhibit a semi-lunar rhythm in migration (e.g., larval
release of Decapods; reviewed by Forward 1987). Since tidal currents are typically
stronger during spring tides, net transport is expected to be maximal during this
period. However, the phasing of the day:night cycle with the current flow varies
throughout the lunar month with a 14.77 day period (Barnwell 1976). Therefore
the time of initiation can be delayed more and the duration of the active phase
shortened during periods of spring tide. Since overall net transport only varies
130
slightly (< 7 %) when the initiation is delayed by less than 2 h (Fig. 71, p119),
changes in cumulative transport due to an increase in Ti would be minor.
Results of the two simulations in which a diel component was included
suggest that peaks in larval recruitment should be observed during periods in the
lunar cycle when the entire active phase of species undergoing STST occurs during
the night. On the other hand, during days when most of the migratory periods take
place during the light phase, transport is reduced and recruitment is expected to be
low. Tankersley et al. (2002) observed that blue crab megalopae were most
abundant when SBE occurred in the middle of the night which coincided with the
time of neap tide in the study estuary but most importantly when most of flood tide
occurs in darkness. During this period of the lunar month, the rate of change in
salinity required to trigger vertical migration occurred after sunset and the active
phase took place entirely during the night. Up-estuary transport of megalopae was
therefore assumed to be maximized. In other brachyuran post-larvae, the time in
the lunar month when peaks in settlement and recruitment observed varies among
locations but always coincides with the time when the entire flood tide takes place
at night (Boylan and Wenner 1993, Metcalf et al. 1995, Christy and Morgan 1998,
Tankersley et al. 2002, Forward et al. 2004). The timing of the active phase in the
day:night cycle probably affect the efficiency of daily transport and therefore the
timing of recruitment and settlement than the spring-neap cycle.
131
In addition to the effect of the synchrony of the day:night and the tidal
cycles on transport, seasonal changes in the photoperiod were also found to have an
impact on STST. As expected, net transport increased with an increase in the
length of the dark phase (Figs. 70 and 73, pp. 117 and 122) since more time was
available for transport each night. Thus net displacement for winter migrators (e.g.,
menhaden larvae; Epifanio and Garvine 2001) should be greater since the number
of days over which the appropriate tide occurs in darkness is longer. Nevertheless,
most organisms undergo migrations at a specific time in the year and the timing
cannot simply be changed so that it occurs when the duration of the dark phase is
maximal. However, for organisms that do migrate during the winter the increased
length of darkness implies that for the same duration of migration (e.g. larval
period) more transport will occur, thus increasing dispersal. Similarly, less
migration days are required to result in similar transport than in summer, and could
result in better chances of survival as the larval period can be reduced.
132
LITTERATURE CITED
Akiyama T. 2004. Entrainment of the circatidal swimming activity in the cumacean
Dimorphostylis asiatica (Crustacea) to 12.5-hour hydrostatic pressure
cycles. Zoological Sciences 21:29-38.
Barnwell FH. 1976. Variation in the form of the tide and some problems it poses
for biological timing systems. In: De Coursey PJ, editor. Biological rhythms
in the marine environment. Columbia: University of South Carolina Press. p
161-187.
Bolt SRL, Naylor E. 1986. Entrainability by salinity cycles of rhythmic locomotor
activity in normal and eyestalk ablated Carcinus maenas (L). Marine
Behavior and Physiology 12:257-267.
Bolt SRL, Reid DG, Naylor E. 1989. Effects of combined temperature and salinity
on the entrainment of endogenous rhythms in the shore crab Carcinus
maenas. Marine Behavior and Physiology 14:245-254.
Boylan JM, Wenner EL. 1993. Settlement of brachyuran megalopae in a South
Carolina, USA, estuary. Marine Ecology Progress Series 97:237-246.
Carr SD, Tankersley RA, Hench JL, Forward RBJ, Luettich RAJ. 2004. Movement
patterns and trajectories of ovigerous blue crabs Callinectes sapidus during
the spawning migration. Estuarine, Coastal and Shelf Science 60:567-579.
Chatfield C. 1989. The analysis of time series: an introduction. New York. 241 p.
Christy JH, Morgan SG. 1998. Estuarine immigration by crab postlarvae:
mechanisms, reliability and adaptive significance. Marine Ecology Progress
Series 174:51-65.
Churchill JH, Blanton JO, Hench JL, Luettich RAJ, Werner FE. 1999. Flood tide
circulation near Beaufort Inlet, North Carolina: Implications for larval
recruitment. Estuaries 22(4):1057-1070.
Costlow JD, Bookhout CG. 1965. A method for developing brachyuran eggs in
vitro.212-215.
133
Cronin TW, Forward RBJ. 1988. The visual pigments of crabs. Journal of
Comparative Physiology 162:463-478.
Das T, Stickle WB. 1994. Detection and avoidance of hypoxic water by juvenile
Callinectes sapidus and C. similis. Marine Biology 120:593-600.
DeVries MC, Epifianio CE, Dittel AI. 1983. Lunar rhythms in the egg hatching of
the subtidal crustacean: Callinectes arcuatus Ordway (Decapoda:
Brachyura). Estuarine, Coastal and Shelf Science 17:717-724.
DeVries MC, Tankersley RA, Forward RBJ, Kirby-Smith WW, Luettich RAJ.
1994. Abundance of estuarine crab larvae is associated with tidal hydrologic
variables. Marine Biology 118:403-413.
Dill L. 1986. Animal decision making and its ecological consequences: the future
of aquatic ecology and behavior. Canadian Journal of Zoology 65:803-811.
Dowse HB, Ringo JM. 1989. The search for hidden periodicities in biological time
series revisited. Journal of Theoretical Biology 139:487-515.
Enright JT. 1965. Entrainment of a tidal rhythm. Science 147:864-867.
Enright JT, Hamner WM. 1967. Vertical diurnal migration and endogenous
rhythmicity Science 157:937-941.
Epifanio CE. 1995. Transport of blue crab (Callinectes sapidus) larvae in the
waters off mid-Atlantic states. Bulletin of marine science 57(3):713-725.
Epifanio CE, Garvine RW. 2001. Larval transport on the Atlantic Continental Shelf
of North America: a review. Estuarine, Coastal and Shelf Science 52:51-77.
Forward RB, Douglass JK, Kenney BE. 1986. Entrainment of the larval release
rhythm of the crab Rhithropanopeus harrisii (Brachyura: Xanthidea) by
cycles in salinity change. Marine Biology 90:537-544.
Forward RBJ. 1987. Larval release rhythms of decapod crustaceans: an overview.
Bulletin of marine science 41(2):165-176.
134
Forward RBJ. 1988. Diel vertical migration: zooplankton photobiology and
behaviour. Oceanography and Marine Biology: an Annual Review 26:361393.
Forward RBJ, Swanson J, Tankersley RA, Welch JM. 1997. Endogenous
swimming rhythms of the blue crab, Callinectes sapidus, megalopae: effects
of offshore and estuarine cues. Marine Biology 127:621-628.
Forward RBJ, Tankersley RA. 2001. Selective tidal-stream transport of marine
animals. Oceanography and Marine Biology: an Annual Review 39:305353.
Forward RBJ, Tankersley RA, Welch JM. 2003a. Selective tidal-stream transport
of the blue crab Callinectes sapidus: an overview. Bulletin of marine
science 72(2):347-365.
Forward RBJ, Tankersley RA, Pochelon PN. 2003b. Circatidal activity rhythms in
ovigerous blue crab, Callinectes sapidus: implications for ebb-tide transport
during the spawning migration. Marine Biology 142:67-76.
Forward RBJ, Cohen JH. 2004. Factors affecting the circatidal rhythm in vertical
swimming of ovigerous blue crabs, Callinectes sapidus, involved in the
spawning migration. Journal of Experimental Marine Biology and Ecology
299(2):255-266.
Forward RBJ, Cohen JH, Irvine RD, Lax JL, Mitchell R, Schick AM, Smith MM,
Thompson JM, Venezia JI. 2004. Settlement of blue crab Callinectes
sapidus megalopae in a North Carolina estuary. Marine Ecology Progress
Series 269:237-247.
Gibson RN. 1984. Hydrostatic pressure and the rhythmic behavior of intertidal
marine fishes. Transactions of the American Fisheries Society 113:479-483.
Gibson RN. 2003. Go with the flow: tidal migration in marine animals.
Hydrobiologia 503:153-161.
Godin J-CJ. 1981. Circadian rhythm of swimming activity in juvenile pink salmon
(Oncorhynchus gorbuscha). Marine Biology 64:341-349.
135
Hastings MH. 1981. The entraining effect of turbulence on the circa-tidal activity
rhythm and its semi-lunar modulation in Eurydice pulchra. Journal of
Marine Biology Association U.K. 61:151-160.
Hench JL, Forward RBJ, Carr SD, Rittschof D, Luettich RAJ. 2004. An empirical
test of a selective tidal-stream transport model: observations of female blue
crab (Callinectes sapidus) spawning migration. Limnology and
Oceanography. 49:1857-1870.
Hill AE. 1991a. A mechanism for horizontal zooplankton transport by vertical
migration in tidal currents. Marine Biology 111:485-492.
Hill AE. 1991b. Vertical migration in tidal currents. Marine Ecology Progress
Series 75:39-54.
Hill AE. 1995. The kinematical principles governing horizontal transport induced
by vertical migration in tidal flows. Journal of Marine Biology Association
U.K. 75:3-13.
Holmström WF, Morgan E. 1983a. Laboratory entrainment of the rhythmic
swimming activity of Corophium volutator (Pallas) to cycles of temperature
and periodic inundation. Journal of Marine Biology Association U.K.
63:861-870.
Holmström WF, Morgan E. 1983b. The effects of low temperature pulses in
rephasing the endogenous activity rhythms of Corophium volutator (Pallas).
Journal of Marine Biology Association U.K. 63:851-860.
Holmström WF, Morgan E. 1983c. Variation in the naturally occurring rhythm of
estuarine amphipod, Corophium volutator (Pallas). Journal of Marine
Biology Association U.K. 63:833-850.
Jones DA, Naylor E. 1970. The swimming rhythm of the sand beach isopod
Eurydice pulchra. Journal of Experimental Marine Biology and Ecology
4(188-199).
Lowery TA, Tate LG. 1986. Effect of hypoxia on hemolymph lactate and behavior
of the blue crab Callinectes sapidus Rathbun in the laboratory and field.
Comparative Biochemistry and Physiology 85A(4):689-692.
136
Luettich RAJ, Hench JL, Fulcher CW, Werner FE, Blanton OB, Churchill JH.
1999. Barotropic and wind-driven larval transport in the vicinity of a barrier
island inlet. Fisheries Oceanography 8(S2):190-209.
Macquart-Moulin C, Kaim-Malka R. 1994. Rythme circadien endogène
d'émergence et d'activité natatoire chez l'isopode profond Cirolana borealis
Lilljeborg. Marine Behavior and Physiology 24:151-164.
Mauro NA, Mangum CP. 1982. The role of the blood in the temperature
dependence of oxidative metabolism in Decapod Crustaceans. I.
Intraspecific responses to seasonal differences in temperature. The journal
of experimental zoology 219:179-188.
Menaker M. 1969. Biological clocks. Bioscience 19(8):2-10.
Metcalf KS, van Montfrans J, Lipcius RN, Orth RJ. 1995. Settlement indices for
blue crab megalopae in the York River, Virginia: temporal relationships and
statistical efficiency. Bulletin of marine science 57(3):781-792.
Naylor JK., Taylor EW, Bennett DB. 1999. Oxygen uptake of developing eggs of
Cancer pagurus (Crustacea: Decapoda: Cancridae) and consequent
behaviour of ovigerous females. Journal of the Marine Biological
Association of the United Kingdom 79:305-315.
Northcott SJ. 1991. A comparison of circatidal rhythmicity and entrainment by
hydrostatic pressure cycles in the rock goby, Gobius paganellus L. and the
shanny, Lipophrys pholis (L.). Journal of Fish Biology 39:25-33.
Olmi EJI. 1994. Vertical migration of blue crab Callinectes sapidus megalopae:
implications for transport in estuaries. Marine Ecology Progress Series
113:39-54.
Palmer JD. 1973. Tidal rhythms: the clock control of the rhythmic physiology of
marine organisms. Biological review 48:377-418.
Palmer JD. 1974. Biological clocks in marine organisms. New-York: WileyInterscience.
Palmer JD. 1990. The rhythmic lives of crabs. Bioscience 40(5):352-358.
137
Palmer JD. 2002. The living clock: The orchestrator of biological rhythms. New
York: Oxford University Press. 155 p.
Parker SJ, McCleave JD. 1997. Selective tidal stream transport by American eels
during homing movements and estuarine migration. Journal of Marine
Biology Association U.K.. 77:871-889.
Paula J. 1989. Rhythm of larval release of decapod crustaceans in the Mira Estuary,
Portugal. Marine Biology 100:309-312.
Peterson DH, Cayan DR, Festa JF. 1985. Interannual variability in biogeochemistry
of partially mixed estuaries: dissolved silicate cycles in Northern San
Francisco Bay. In: Wolfe DA, editor. Estuarine variability. New York:
Academic Press Inc. p 123-138.
Queiroga H, Costlow JD, Moreira MH. 1994. Larval abundance patterns of
Carcinus maenas (Decapoda, Brachyura) in Canal de Mira (Ria de Aveiro,
Portugal). Marine Ecology Progress Series 111:63-72.
Redfield AC. 1980. Introduction to tides: the tides of the waters of New England
and New York. Woods Hole, MS: Marine Science International. 108 p.
Reid DG, Bolt SRL, Davies DA, Naylor E. 1989. A combined tidal simulator and
actograph for marine animals. Journal of Experimental Marine Biology and
Ecology 125:137-143.
Reid DG, Naylor E. 1990. Entrainment of bimodal circatidal rhythms in the shore
crab Carcinus maenas. Journal of Biological Rhythms 5(4):333-347.
Reid DG, Naylor E. 1993. Different free-running periods in split components of the
circatidal rhythm in the shore crab Carcinus maenas. Marine Ecology
Progress Series 102:295-302.
Roenneberg T, Daan S, Merrow M. 2003. The art of entrainment. Journal of
Biological Rhythms 18(3):183-193.
Sinclair M. 1980. Marine Populations: an essay on population regulation and
speciation. Seattle. 252 p.
138
Smith NP. 1990. An introduction to the tides of Florida's Indian River Lagoon: II
Currents. Atmospheric and Oceanographic Sciences 53(3):216-225.
Smith NP, Stoner AW. 1993. Computer simulation of larval transport through tidal
channels: role of vertical migration. Estuarine, Coastal and Shelf Science.
Sulkin SD. 1984. Behavioral basis of depth regulation in the larval of brachyuran
crabs. Marine Ecology Progress Series 15:181-205.
Tankersley RA, Bullock TM, Forward RBJ, Rittschof D. 2002. Larval release
behaviors in the blue crab Callinectes sapidus: role of chemical cues.
Journal of Experimental Marine Biology and Ecology 273(1):1-14.
Tankersley RA, Forward RBJ. 1994. Endogenous swimming rhythms in estuarine
crab megalopae: implications for flood-tide transport. Marine Biology
118:415-423.
Tankersley, R.A. and R.B. Forward, Jr. 2005. Environmental physiology. In: The
Blue Crab, Callinectes sapidus. V. Kennedy and L.E. Cronin (eds).
Maryland Sea Grant, College Park, MD
Tankersley RA, McKelevey LM, Forward RBJ. 1995. Responses of estuarine crab
megalopae to pressure, salinity and light: implications for flood-tide
transport. Marine Biology 122:391-400.
Tankersley RA, Welch JM, Forward RBJ. 2002. Settlement times of blue crab
(Callinectes sapidus) megalopae during flood-tide transport. Marine
Biology 141:863-875.
Tankersley RA, Wieber MG, Sigala MA, Kachurak KA. 1998. Migratory behavior
of ovigerous blue crabs Callinectes sapidus: evidence for selective tidalstream transport. Biological Bulletin 195:168-173.
Tankersley RA, Ziegler TA, Butler K. 2003. Spawning behavior of the blue crab
Callinectes sapidus: Pattern of migration and timing of larval release.
Taylor AC, Naylor E. 1977. Entrainment of the locomotor rhythm of Carcinus by
cycles of salinity change. Journal of Marine Biology Association U.K.
57:273-277.
139
Uncles RJ, Jordan MB, Taylor AH. 1985. Temporal variability of elevation,
currents and salinity in a well-mixed estuary. In: Wolfe DA, editor.
Estuarine variability. New York: Academic Press Inc. p 103-122.
Warman CG, Naylor E. 1995. Evidence for multiple, cue-specific circatidal clocks
in the shore crab Carcinus maenas. Journal of Experimental Marine
Biology and Ecology 189:93-101.
Welch JM, Forward RBJ. 2001. Flood tide transport of blue crab, Callinectes
sapidus, postlarvae: behavioral responses to salinity and turbulence. Marine
Biology 139:911-918.
Welch JM, Forward RBJ, Howd PA. 1999. Behavioral responses of blue crab
Callinectes sapidus postlarvae to turbulence: implications for selective tidal
stream transport. Marine Ecology Progress Series 179:135-143.
Wheatly MG. 1981. The provision of oxygen to developing eggs by female shore
crabs (Carcinus maenas). Journal of Marine Biology Association U.K.
61:117-128.
Williams BG. 1969. The rhythmic activity of Hemigrapsus edwardsi. Journal of
Experimental Marine Biology and Ecology 3:215-223.
Ziegler TA. 2002. Larval release behavior of the blue crab, Callinectes sapidus:
entrainment cues and timing in different tidal regimes [Master Thesis].
Melbourne, FL: Florida Institute of Technology. 80 p.

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