A tidal rhythm in P. crassipes
M. Holdren, D. Sandberg 1
INTRODUCTION
The lined shore crab, Pachygrapsus crassipes Randall,
inhabits the rocky intertidal region of the western coast
of North America. Generally it is found from the zone of
lower tides up to the high splash zone in areas where there
are pools, crevices, and boulders. P. crassipes may be
found higher and lower if vertical crevices are present and
it is able to migrate up and down with incoming and receding
tides. They are also found in tide pools of varying depths.
Crevices offer protection from wave action and from predators
as well as providing a damp surface to prevent the crab from
desiccating. Close associations with tide pools aid the
crab in this respect (Hiatt,1948).
Previous studies on P. crassipes include the major
work on its biology and molting cycle (Hiatt,1948), osmotic
regulation (Gross,1955,1957,1958; Prosser et al.,1955), and
reproduction (Boolootian,1965; Bovbjerg,1960a). Virtually
no previous work has been done with regard to rhythmic
activity patterns. Studies conducted by Hiatt indicated
P. crassipes was primarily active during the hours of dark¬
ness. This observation was also reported by Bovbjerg (1960b)
and Knudsen (1964).
Endogenous rhythms have been discovered in other crabs.
An endogenous tidal rhythm has been discovered in a fiddler
crab by Palmer (1975). He found the rhythm could persist
in the laboratory for up to 5 weeks, but was most often
damped out rather quickly. The fiddler crab was most active
at low tide.
A tidal rhythm in P. crassipes
M.Holdren, D.Sandberg
In preliminary studies done during the month of April 1978
few crabs were seen at night, contradicting the observations
of Hiatt, Bovbjerg, and Knudsen. This contradiction and
the question of whether P. crassipes displays an activity
rhythm comprise the focus of this investigation.
MATERIALS AND METHODS
Field Studies
Field studies on P. crassipes were carried out at
Mussel Point, Pacific Grove, California between 26 April and
3 May 1978. One area observed was between the +2 and 8 ft.
tide level and measured 7x8 meters. It was massive granite
containing large horizontal and vertical rock surfaces,
crevices, and part of a large pool, into which seawater
would flow with incoming tides. A lower intertidal observation
area was from the -1 to +3 ft. tide level, and measured
5x5 meters. Observations were made by sitting on top of
a tall boulder and remaining relatively still. The crabs
did not seem to notice the observer during the day unless the
observer moved close to the crab or shadows passed near the
crab. During the day observations were made from above
with a pair of Bushnell Sportview 7x35 binoculars. At
night observations were made using a Barsom infrared viewer.
The area was continuously monitored over a 48 hour
period during the study. At 8 hr. intervals weather con¬
ditions, the temperature of rock surfaces and crevices were
recorded. Whenever a crab was sighted notes were made of
A tidal rhythm in P. crassipes
M.Holdren, D.Sandberg
(1) the time (2) the height in the intertidal where the crab
was seen (3) the type of surface it was on, i.e. rock, crevice,
or pool (4) if the surface was wet or dry, and (5) if the
crab's activity was solitary or social. If the crab was
moving, the direction of movement, either toward or away from
the action of the waves, was recorded.
Brief observations of the area were made during the nights
of 2 May and 3 May 1978, when lower low water occurred shortly
after midnight. These observations were made with the infra¬
red viewer.
Activity Wheel Experiments
Six crabs with carapaces measuring 1.5-2.5 cm. across
were captured and each placed into activity wheels (Figure 1).
The activity wheels, made of plexiglass, revolve freely about
a stationary rod. Complete revolutions of each wheel were
recorded as deflections using an Esterline Angus strip chart
recorder. Each wheel containing a crab was placed in a part¬
ition of a light tight box. Running seawater was kept at a
level to just cover the lower edge of the revolving wheel(Fig. 2).
Food was provided in the form of Macrocystis pyrifera, attached
in a bundle to the side of the cage. This procedure was used
in experiments with three light regimes: LD 12:12, DD, and
LL.
The light source was a flourescent tube which provided
a light intensity of 2.4 pE/m/sec at a distance of 1 ft.
In experiments where activity was very high, (LD), data from
the recorder was scored on a scale of O to 5, with O indicating
no activity during a one hour period and 5 indicating
A tidal rhythm in P. crassipes
M.Holdren, D.Sandberg
continuous activity throughout the hour. The activity of each
crab was scored for every hour of the study. The data were
normalized as hourly percentages of total activity which were
then summed and plotted as mean values for all experimental
animals.
In activity wheel experiments in DD and LL, activity
was plotted as summed hourly deflections for each individual.
These values were normalized as hourly percentages of total
activity to get a grouped plot for the DD study. Differences
between activity at rising and falling tides and differences
in activity at high and low tides were calculated using a
Chi-square test.
Simulated Natural Environment
In addition to field observations and recordings of
individuals in activity wheels, observations were also made
on groups of crabs in captivity. A large fiberglass tub (Fig. 3),
approximately 1 me, located out of doors was used for these
studies. Intertidal rocks were arranged in the tub to
simulate the natural environment. A few mm of sea water
flowed across the bottom and algae was present for food. A
Panasonic dim light TV camera was positioned four to five
ft above the center of the tub to view the entire surface.
Activity was recorded on video cassettes for five minutes
every hour, and viewed on a Panasonic closed circuit TV monitor.
Each observation was scored for the number of crabs that were
visible. A crab was defined as "active" if it could be seen
and positively identified as a crab, either by its shape or
by movement. In other words, it had to be out in the open.
A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
or, if partly obscured, it had to move about in order to
be considered active. Nearly all crabs counted did move
around during the sample periods.
Natural Light Cycle
For the first set of observations using the set-up
described above, fifteen crabs, measuring about 3 cm across
the carapace, were collected and placed in the tub the day
before observations began. The population density in the
tub did not exceed densities that can be found in the field.
The light conditions under which the observations were
made were sunlight during the day and a 25 watt incandescent
bulb with a red plastic filter at night. The light intensity
was low; the minimum required by the sensitive dim light
camera to resolve the crabs and would not register on a
meter sensitive to 1 ft candle. But in order to control
for possible effects of the red light on the crabs, an infra¬
red viewer was used for part of one night to count the
crabs. This device allows the user to see objects illuminated
with infrared radiation and requires no visible light. The
numbers of crabs seen this way did not differ noticeably
from those obtained with visible red light. This is in
agreement with Hiatt's claim that artificial illumination
does not disturb their foraging habits.
Dim Light
It was also of interest to see how a group of crabs
would respond to a situation where the light intensity
did not change from day to night. Such a situation would
A tidal rhythm in P. crassipe
M.Holdren,D.Sandberg
be free of one type of external information about time of day.
The same materials and methods as before were used for
the second set of observations, except that the tub was
covered with two layers of 3 mil black plastic. This covering
was entirely opaque to visible sunlight. However, it was
found that a very faint glow of light did get through the
fiberglass sides of the tub during the day. Therefore, a
completely constant light condition was not achieved. The
red lamp illuminated the tub the entire time.
RESULTS
Field Studies
Activity during the 48 hr study peaked around lower low
water between the hours of 0800 and 0900. A second peak was
observed on the second day shortly after sunset, as noted
by Hiatt (1948). During periods of high activity (Fig. 4) the
majority of crabs were seen either feeding on or moving across
dry, exposed rocks. As the tide rose the animals moved away
from the action of the waves, into crevices. Although very
few social interactions were noticed, those observed usually
lasted only a few seconds. Crabs seen in crevices were usually
wedged in motionless. Often a bubbling sound could be heard
coming from the crabs in crevices. On several occasions
individual crabs were seen covering themselves with froth.
No reference to either behavior could be found.
Crabs were observed on lower rocks at lower low water
during the nights of 2 May and 3 May 1978. The number seen
A tidal rhythm in P. crassipes
M.Holdren,D. Sandberg
at night during lower low water was comparable to the number
observed at lower low water during the day, although no
quantitative information was recorded.
LD 12:12 - Activity Wheel
A plot of mean percent activity versus time shows that
more activity occurs during the daylight hours (Fig. 5).
A Chi-square test comparing activity in light and dark shows
that there is significantly more activity in light than in
dark (.Olpg.025).
Peaks of activity occur during hours of darkness. The
means of these activity peaks, excluding activity in light,
were calculated by averaging the percent activity over three
or four hours to either side of the hour of greatest activity.
A least squares linear regression on the relationship between
the means and corresponding days, shows a slope of .87.
Multiplied by 60 minutes, this slope represents a daily
phase advance of 51.9 min. The correlation coefficient for
this linear regression is.92 (Sokal and Rohlf, 1969).
DD - Activity Wheel
Two of the 3 crabs recorded in DD show significantly more
activity during rising tides when analyzed individually.
There was no definitive pattern in the activity of the three
crabs with respect to low or high tides, i.e. one crab was
more active at high tide, another was more active at low tide,
and there was no significant difference between activity at
high and low tides for another crab.
When the data for the crabs were grouped, a substantial
peak of activity occurs at hours between low and high tide
A tidal rhythm in P. crassipes
M.Holdren, D.Sandberg
between the hours of 1400 and 2400 (Fig. 6). The slope
of the regression line for these peaks is .94, representing
a phase advance of 56.6 minutes per day. The correlation
coefficient is.86.
LL - Activity Wheel
Statistical analysis of activity wheel recordings of
crabs in LL showed conflicting results. Again, data on
only three individuals was obtained due to equipment failure.
Though one crab showed more activity at rising tides than at
other times, another showed perfectly random activity with
regard to tides when subjected to a Chi-square test. The
third one turned the wheel almost incessantly and at what
appeared to be the same pace for many hours.
Simulated Natural Environment - LD
When the absolute numbers of crabs counted as active
each hour are graphed versus time, a repeating pattern
appears (Fig. 7). Very noticeable peaks of activity coin¬
cide with the hours around lower low water which occurs just
before noon on these days. Fewer crabs are active at other
times, particularly at night.
Simulated Natural Environment - Dim LL
Similarly, the graphed results of observations on crabs
in continuous dim light show peaks of activity at lower low
water during the day (Fig. 8). Note also the major burst
of activity that occurs on the second night at hours corre¬
sponding to low tide. This double peak occurs at the semi¬
lunar shift of lower low tides. There were not enough cycles
for analysis of phase shift.
A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
DISCUSSION
Early in our studies on activity patterns in Pachy-
grapsus crassipes field observations showed a peak of activity
at lower low water both during the day and at night. This
is in conflict with previous reports which indicate activity
during the hours of darkness (Hiatt, 1948; Bovbjerg, 1960b;
Knudsen, 1964). Since these studies indicated a tidal
rhythm of activity in field conditions we began studies in
the laboratory away from tidal influence and under a variety
of light regimes to investigate activity patterns and their
possible endogenous nature.
A 24 hour light-dark cycle is known to keep a tidal
locomotion rhythm at a strict tidal frequency, though under
constant conditions there is an average free running period of
24 hr 55 min (Enright, 1972), which is a phase advance of
the tidal period of 24 hr 51 min.
The presence of a tidal rhythm in P. crassipes is
supported by analysis in LD 12:12 where peaks occur with
rising tide during the hours of darkness. Analysis of
the means of the activity peaks shows a phase advance
of .9 min per lunar day. P. crassipes appears to maintain
a strict tidal rhythm in LD 12:12.
Analysis of activity peaks during rising tides in
DD during the subjective night shows a free running period
of 24 hr 56.5 min. This may indicate a circalunadian period
with a phase advance under constant dark conditions.
However, the sample size and period of the experiments is
A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
insufficient to establish this with certainty.
Further evidence to support the hypothesis of a tidal
rhythm comes from the study in the outdoor tub in constant
dim light. On the second day of the study low low water
and high low water were about equal in magnitude and shifted
positions from day to night. The crabs showed peaks of
activity at both of these low tides, apparently following
the tidal shifts. This suggests that the crabs' activity
rhythm anticipated the semilunar shift of low tides. This
result certainly deserves further investigation.
A small peak of activity was observed in the field
shortly after sunset on the second day of the 48 hour study.
No data were available for the first day during this time.
Activity after sunset was also reported by Hiatt (1948).
Therefore, both tidal factors and the natural light-dark
cycle may affect activity. A circadian activity rhythm in
the crabs was not conclusively shown by these data, but the
possibility of one can not be ruled out.
Crabs kept in activity wheels on a LD 12:12 regime
displayed greater activity when the light was on. The
environment was artificial in that the crabs did not have
crevices for hiding places. The reason for activity in the
light is open to speculation. The running could be an
escape response.
When exposed to constant light, the crab's behavior was
erratic. Since no two crabs exhibited the same behavior,
no conclusions can be drawn.
10
A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
Though field data are indispendible for an understanding
of Pachygrapsus crassipes behavioral patterns, these crabs
are so alert and cryptic that rigorous descriptions of their
movements in the field are difficult to obtain. The two
most successful methods in these studies were recording
from activity wheels and viewing small groups of crabs in
semi-natural aquaria.
These results suggest that P. crassipes has an endogenous
activity rhythm phased to the tides. Further studies are
necessary to determine the relationship between circatidal
and circadian variations in activity in the observed rhythm
in Pachygrapsus crassipes behavior.
SUMMARY
1. Studies in the field and in the laboratory removed from
tidal fluctations indicate a tidal rhythm of activity
in P. crassipes. The crabs are most active at lower low
water.
The phase advance in LD 12:12 was found to be .9 min
per lunar day. In DD the phase advance was found to
be 5.5 min per lunar day.
When the times of lower low water shifted from day to
night, crabs showed peaks of activity at both low tides.
This suggests that the crabs' activity rhythm follows
lower low water through the semi-lunar shifts of low tides.
A circadian rhythm was not found in this investigation.
However, activity after sunset and the effect of light
on the activity patterns of crabs in running wheels
A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg 12
suggest that light plays an important role in modifying
the crabs'
behavior.
A tidal rhythm in
Joldren, D.Sandberg 13
ACKNOWLEDGEMENTS
We would like to thank our advisor, Chuck Baxter, for
encouragement at every stage of our study and for preparing
us well for our oral presentation. We are indebted to
Dr.Robin Burnett for assistance in many aspects of this
study, and Lawrence W. Harding, Jr. for careful reading of
this paper, many helpful suggestions, and coffee. We
appreciate our fellow students who were our ever-present
supportive companions as one 24.8 hour day led to the next.
A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
LITERATURE CITED
Boolootian, R.A. 1965. Aspects of reproductive biology in the
striped shore crab Pachygrapsus crassipes. Bull.
So. Calif. Acad. Sci. 64:43-44.
Bovbjerg, R.V. 1960a. Courtship behavior of the lined shore
crab, Pachygrapsus crassipes Randall. Pacif. Sci.
14:421-422.
Bovbjerg, R.V. 1960b. Behavioral ecology of the crab,Pachy¬
grapsus crassipes. Ecology 41:668-672.
Enright, J.T. 1972. A virtuoso isopod: Circa-lunar rhythms
and their tidal fine structure. J. Comp. Physiol.
77:141-162.
Gross, W.J. 1955. Aspects of osmotic regulation in crabs
showing the terrestrial habitat. Amer. Natur.
89:205-222.
Gross, W.J. 1957. A behavioral mechanism for osmotic
regulation in a semi-terrestrial crab. Biol.Bull.
113:268-274.
Gross, W.J. 1958. Potassium and sodium regulation in an
intertidal crab. Biol. Bull. 114:343-347.
Hiatt, R.W. 1948. The biology of the lined shore crab,
Pachygrapsus crassipes Randall. Pacif. Sci.
2:135-213.
Knudsen, J.W. 1964. Observations of the reproductive cycles
and common Brachyura and crablike Anomura of
Puget Sound, Washington. Pacif. Sci. 18:3-33.
Palmer, John D. 1973. Tidal Rhythms: The clock control of
rhythmic physiology of marine organisms. Biol.
Rev. 48:377-418.
14
A tidal rhythm in P. crassipes
M.Holdren, D.Sandberg
Palmer, John D. 1975. Biological clocks of the tidal zone.
Sci. Amer. 232(2):70-79.
Prosser, C.L., J.W. Green, and T.J. Chow, 1955. Ionic and
osmotic concentrations in blood and urine of
Pachygrapsus crassipes acclimated to different
salinities. Biol. Bull. 109:99-107.
Sokal, Robert R. and F. James Rohlf. 1969. Regression,
pp.424-425. Biometry, W.H. Freeman and
Co., San Francisco.776 pages.
A tidal rhythm in P. crassipes
M. Holdren, D. Sandberg
Figure 1. Activity wheel with revolving
cage, magnet, and switch.
—
A tidal rhythm in P. crassipes
M.Holdren,D.Sandbei
Figure 2. Activity wheels set up in light¬
tight box and supplied with running
seawater.
18

A tidal rhythm in P. crassipes
M.Holdren, D. Sandberg
Figure 3. Schematic diagram of the outdoor
tub as seen on the TV monitor
used for observations. The relative
sizes of the rocks and a crab in
relation to a 1 m tub are depicted.
5
S
A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
Figure 4. Total numbers of crabs observed at
half-hour intervals in field studies.
The position of the tide and the
day/night cycle are indicated.
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A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
The average percent of total activity
Figure 5.
of 4 crabs for 7 consecutive days from
6 to 12 May, 1978. These studies were
conducted in LD 12:12 Subjective
tidal conditions and the imposed light¬
dark cycle are indicated. The black
line represents a least squares linear
regression fit to the means of the
night activity peaks.
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04 08
12 46 20 0
TIME
M.Holdren,D.Sandberg
A tidal rhythm in P. crassipes
Average percent of total activity for
Figure 6.
3 crabs in activity wheels in DD for
6 consecutive days from 18 to 23 May,
1978. Subjective tidal conditions and
the light-dark cycle are indicated.
The black line represents a least
squares linear regression fit to the
means of the activity peaks occurring
between 1600and 2000.
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A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
Figure 7. Number of crabs observed active (n=15)
in the outdoor tub under natural light¬
dark conditions.
NUMBER OF CRABS


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A tidal rhythm in P. crassipes
M.Holdren,D.Sandberg
Figure 8.
Number of crabs observed active (n=15)
in the outdoor tub under constant dim
light.
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