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Activity Patterns in Pagurus samuelis and
Pagurus granosimanus
I. Dispersion
Eldon E. Ball Jr.
Hopkins Marine Station of Stanford University
Pacific Grove, California
June 3, 1965
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Introduction
Many Crustaceans show rhythmic activity patterns which
continue even under conditions of constant illumination.
(Brown, 1961) According to Pardi and Papi (1961) hermit
crabs of the genus Pagurus are positively phototropic at
low and medium light intensities, but negatively phototropic
at high intensities. It was observed that two local species
of hermit crabs, Pagurus samuelis and Pagurus granosimanus,
were out moving around on the bottom of a pond in much
larger numbers during the night than during the day. In.
order to determine whether the Pagurus were responding to
a physiological rhythm or directly to light the numbers of
animals out in the open on the bottom of a pond were compared
over 24 hour periods of constant light, constant darkness,
and normal day-night conditions.
Materials and Methods
The Pagurus used in the following experiments were all
collected at the Great Tidepool on Point Pinos near Pacific
Grove. A 15 X 9 foot pond was divided in half by plastic
screen ånchored by rocks to give two areas 7.5 x 9 feet.
The water in the pond was usually maintained at a depth of
7 inches, although several times it became deeper due to
plugged drains. A section of sheet rock with an area of
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slightly over 4 square feet was supported on rocks on each
side of the screen in order to provide the crabs with shade.
The shells of the two species of Pagurus were painted
different colors (P. samuelis yellow and P. granosimanus
orange), but the screen between the two species was necessary.
since without it there would have been considerable inter-
specifio shell exchange. In experiments where blinded crabs
were used their shells were painted red and they were placed
with the normal crabs of their own species. There may have
been some shell exchange between blind and normal crabs, but
this did not appear to be a major source of error over the 24
hours which the experiments ran. The crabs were recounted
and checked as to species and condition of eyes before each
experiment.
In some cases dispersion had to be determined somewhat
arbitrarily because the crabs would pile up in a corner rather
than seeking shelter. When the crabs packed up in tight masses
and ceased their activity they were not counted as dispersed.
It was impossible to maintain a constant temperature in the
pond, so the temperature and illumination were both taken over
the entire 24 hour period. In all counts made at night a ruby
bulb was used for observing the crabs.
Constant illumination was obtained by suspending a bank
of lights (with a total wattage of 900) 2.5 feet above the
pond's surface at a distance of 3.67 feet from each end of the
pond. Gooseneck lamps containing 100 watt bulbs were placed
at each of the corners and in the center on both sides of the
pond. Unfortunately, the illumination achieved was far from
uniform, since it varied from 14-230 ft.-candles at different
points in the pond. Also, as can be seen from figures 4 and 5,
the maximum illumination obtained at night was less than 37
of that obtained from the sun.
It was impractical to produce constant darkness in the
pond due to its size. Instead of this, 25 crabs of each species,
which had been blinded by cutting off their eyestalks, were
added to the pond. It was later decided that these crabs did
not provide a satisfactory dark sample, so an experiment was
set up in a darkroom with constant darkness, a constant tem-
perature of 14'C, and the same population density as the out-
door pond.
Results
Both species of Pagurus seemed to show a very definite
pattern of dispersion with large numbers of crabs active at
night and a much smaller number active during the day when
they were kept in the outdoor pond. (figures 1-5) In every
case P. samuelis became active before P. granosimanus during
the late afternoon.
The constantly illuminated sample shows a pattern of
dispersion which does not differ greatly from the other two
samples kept under normal day-night conditions." The darkroom
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sample (figure 6) fails to show the drop in dispersion during
the day which was found in all other cases.
Discussion
It appears that the activity pattern shown by the two
species of Pagurus is a direct response to illumination.
The lack of any rhythmic behavior cycle in the crabs kept in
the darkroom would tend to support this hypothesis. Assuming
that the activity pattern is a direct response to light, the
failure to break this pattern with constant illumination might
be attributed to insufficient light intensity. Such an explan-
ation seems quite plausible when it is noted that the maximum
intensity which could be produced was less than 3% of that of
sunlight. It is possible that temperature may be a factor in
the pattern of dispersion exhibited by the crabs, but the in-
verse correlation between number of crabs active and illumin-
ation is much better than that between number of crabs active
and temperature.
Position in the intertidal is possibly correlated to the
difference in the times at which activity begins to pick up
during the late afternoon in the two speoies. The P. samuelis
are normally found higher in the intertidal, (Belknap and
Markham, unpublished) and may thus have become adapted to
greater light intensities.
There are at least two possible explanations for the
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similarity between the activity patterns of the blind and
the normal crabs. One would be that the crabs possess some
type of dermal light sense. Another possibility, which seems
more likely, is that the normal behavior of the blinded crabs
during the bright part of the day is changed by the behavior
of the other crabs in the pond with them. These possibilities
should be tested.
Summary
Pagurus samuelis and Pagurus granosimanus show a rhythmic
activity pattern in which most crabs are active at night while
the number active during the day is greatly reduced. This
pattern of dispersion appears to be an inverse response to
intensity of illumination. Pagurus samuelis consistently
becomes active earlier in the afternoon than does Pagurus
granosimanus.
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Legends for Figures
Figure 1. Relation between percent of crabs active (left
axis), illumination (right axis, smaller numbers), and
temperature (right axis, larger numbers) over a 24 hour
period under normal day-night conditions (100 normal crabs
of each species)
—
heavy solid line-normal Pagurus samuelis (P.s.n.
heavy dots and dashes-normal Pagurus granosimanus (P.g.n.
light dashes-temperature (T)00-----
heavy dots-illumination in foot candles (I).........
rectangle shows variation in illumination due to clouds
Figure 2. Relation between percent of Pagurus samuelis
active (left axis), illumination (right axis, smaller nun-
bers), and temperature (right axis, larger numbers) over
a 24 hour period under normal day-night conditions (100
normal crabs plus 25 blind crabs)
Legend as in figure 1 with the addition of:
light solid line-blind Pagurus samuelis (P.s.b.)
Figure 3. Relation between percent of Pagurus granosimanus
active (left axis), illumination (right axis, smaller num-
bers), and temperature (right axis, larger numbers) over a
24 hour period under normal day-night conditions (100 normal
crabs plus 25 blind crabs)
Legend as in figure 1 with the addition of:
light dots and dashes-blind Pagurus granosimanus (P.g.e.
Figure 4. Relation between percent of Pagurus samuelis
active (left axis), illumination (right axis, smaller num-
bers), and temperature (right axis, larger numbers) over
a 24 hour period of constant illumination (100 normal crabs
plus 25 blind crabs)
Legend as in figure 1 with the addition of:
light solid line-blind Pagurus samuelis (P.s. 6.)
—.—
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Figure 5. Relation between percent of Pagurus granosimanus
active (left axis), illumination (right axis, smaller num-
bers), and temperature (right axis, larger numbers) over a
24 hour period of constant illumination (100 normal crabs
plus 25 blind crabs)
Legend as in figure 1 with the addition of:
light dots and dashes-blind Pagurus granosimanus (P.g.b.)-
Figure 6. Percent of crabs active over 24 hours of constant
darkness and constant temperature in a darkroom (32 normal
crabs of each species in two ponds 9.7 X 1.8 feet,
Legend as in figure 1.
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Footnotes
1. The reason for the unusually high humber of P. gran-
ogipanus and the unusually low number of P. samuelis active
at the start of this eiperiment is unknown. The sharp drop
in the number of crabs active at 2300 may have resulted
from the investigator disturbing the pond while taking light
readings.
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Literature Cited
Belknap,R. and J. Markham. 1965. On the Intertidal and Sub-
tidal Distribution of Pagurus (Fabricius). unpublished
class manuscript, Hopkins Marine Station
Brown, Frank A. Jr. 1961. Physiological Rhythms, pp. 401-430
in: (Waterman,T.H.,ed.) The Physiology of Crustacea.
Vol. II-Sense Organs, Integration and Behavior. New
York and London, Academic Press: viii+ 681 pp.
Pardi,L. and F.Papi. 1961. Kinetic and Tactic Responses,
p.371 in: (Waterman,T.H.,ed.) The Physiology of Crus-
tacea, Vol. II-Sense Organs, Integration and Behavior.
New York and London, Academic Press: viii + 681 pp.
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