Activity Patterns in Pagurus samuelis and
Pagurus granosimanus
II. Locomotor Activity
Eldon E. Ball Jr.
Hopkins Marine Station of Stanford University
Pacific Grove, California
June 1, 1965
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Introduction
It was observed that two local species of hermit crabs,
Pagurus samuelis and Pagurus granosimanus, were active in
much greater numbers at night than during the day. In order
to determine whether the crabs move at different speeds at
different times of day the following experiments were carried
out.
Materials and Methods
Concentric circles were laid out on a piece of sheet
rock at distances of 20, 30, 40, and 50. cm. and it was
placed at the bottom of a pond in approximately 10 inches
of sea water. Ten Pagurus granosimanus and ten Pagurus
samuelis of approximately equal size were then selected.
A small weighted cage (plastic strawberry carton) was
used to retain the Pagurus in the center of the circles until
the start of the experiment. One minute after the investi-
gator had left the pool the cage was raised using a string
attached to a pole. The time required for five of the ten
crabs to pass out of the 50 cm. circle was then recorded.
In a small number of cases one of the crabs clung to the cage
and was carried out of the circles. When this happened the
crab which was carried away was ignored, and the time required
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for five crabs to leave the 50 cm. circle was taken as usual.
The trials were run on even hours (e.g. 1400, 1600, etc.)
at various times over a period of 6 days with trials every
two hours over a 24 hour period on the last day. Two runs
were generally taken with each species at each trial. The
species were alternated on these runs. The same crabs were
used throughout the 6 days of the trials. Night observations
were made by using a ruby bulb, although in some cases the
pond was illuminated by light from nearby windows.
Results
The combined data from all runs (fig. 3) seems to show
that the crabs move more slowly at night than during the day,
but the otherwise fairly regular trend is interrupted at
2200 when the time required for the crabs to get out of
the 50 cm. circle falls off rather sharply. The data for
the two species considered individually (figures 1 and 2)
are much less regular than the combined data, although' in
both cases the same trend of slow movement at night and
faster movement by day is evident.
Discussion
Figures 1, 2, and 3 all show essentially the same
trend, with the crabs moving slower at night than during
the day. The reason for the interruption of this trend
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at 2200 is at this time unknown. Temperature probably does
not play a major role in determining these activity patterns,
since the greatest activity occurs at night (2200) after the
water has cooled considerably. Also, the rate of movement
begins to speed up in the morning while the temperature is
still falling. Intensity of illumination may well prove to
be a major factor in determining how fast the crabs move.
In order to draw any conclusions about the possible effects
of light or physiologisal rhythms on the rate of locomotion
in Pagurus a much larger number of trials would be required,
preferably under conditions where temperature and illumination
could be controlled.
Summary
Data are presented on the rate of locomotion of Pagurus
granosimanus and Pagurus samuelis at different times of the
day and night. There appears to be a general tendency for
the crabs to slow down at night, but there is an unusual in-
crease in the speed with which the crabs move at 2200. More
experiments under controlled conditions are needed.
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Legends for Figures
Figure 1. Time (in seconds) required for 5 of 10 Pagurus
granosimanus to pass out of a 50 cm. circle at different
Hours of day (width of column indicates number of trials
Figure 2. Time (in seconds) required for 5 of 10 Pagurus
samuelis to pass out of a 50 cm. cirèle at different hours
of day (width of column indicates number of trials)
Figure 3. The data from figures 1 and 2 are here combined
to give the average time (in seconds) required for 5 of 10
Pagurus to pass out of a 50 cm. circle at different hours
oi day (width of column indicates number of trials)
820
0600
1200
180
2400
Time
2oon
0600
1200
240
8
Time
800
0600
1800
240
Time