J .E. Angel J. Exp. Mar. Biol. Ecol. 243 2000 169 –184
175
Activity was monitored for 1 h intervals using a Sony Hi 8 mm video camcorder. The two hermit crabs in each pair were placed on either side assignment to sides was
random of a glass aquarium 400 32103250 mm divided down the middle by a
perforated, opaque plastic tank divider Penn-Plax, New York. A black-on-white paper grid of 25 mm squares was affixed to the outside of the tank’s bottom surface. There
were an equal number of squares marked out on each side of the divider. Seawater 24
8C, 30 ppt covered the crabs, and was replaced for each new pair of crabs observed. The hermit crabs acclimated to their new environment for 1 h.
Each pair of hermit crabs was then recorded for 1 h. The recording room was free of visual and auditory distractions, such as human movement. Both hermit crabs in each
pair received the same light intensity approximately 350 lux from an overhead fluorescent lamp, and because the tank divider was perforated, the paired crabs shared
the same seawater environment.
To analyze the data, the videotape was reviewed at high speed. Activity level was defined as the number of squares on the grid crossed by each hermit crab during the 1 h
of observation. To determine shell treatment effects on activity level, a t statistic was calculated.
3. Results
3.1. Growth rate Shell fit significantly affected crab growth rate df
512, t53.316, P50.0062. For 11 of 13 pairs, the crab in the tightly fitting shell grew at a slower rate. Mean growth
rate 6S.E.M. for 13 hermit crabs in tight shells shell adequacy index50.5 was 0.060.1
mg day. Mean growth rate 6S.E.M. for 13 hermit crabs in shells of preferred fit shell
adequacy index 51.0 was 0.460.1 mg day. The mean difference in growth rates
between shell treatments was significant. 3.2. Predation risk
During the observation periods, predators handled both hermit crabs in seven out of the 10 trials observed. In the eleventh trial, there was no observation period. In these
seven cases where both hermit crabs were handled, all the hermit crabs with tight shell fit were wounded, while none of the hermit crabs with preferred shell fit were harmed. In
the three cases in which only the hermit crab with tight shell fit was attacked during the observation period, all three were wounded.
I presumed that the predator encountered both hermit crabs during the additional 2–3 nights in which it continued to have access to the hermit crabs. During this time, nine of
the 11 hermit crabs with tight shell fit were eaten; the remaining two hermit crabs with tight shell fit were fatally wounded Table 1. In contrast, all but one of the hermit crabs
with preferred shell fit remained unharmed throughout the study. Mortality after long-term exposure 48
1 h to the predator was significantly affected by shell fit Table 1, P
,0.0001, Fisher exact test.
176 J
.E. Angel J. Exp. Mar. Biol. Ecol. 243 2000 169 –184 Table 1
a
Status of hermit crabs after 48 1 h exposure to predators
Shell adequacy index Hermit crab status
Dead Alive
0.5 tight fit 11
1.0 preferred fit 1
10
a
Each of 11 pairs of hermit crabs P . longicarpus, one with shell adequacy index 1.0 preferred fit, and the
other a shell adequacy index of 0.5 tight fit, was exposed to a distinct predator C . irroratus. The category
‘Dead’ includes hermit crabs that were eaten or fatally wounded. P ,0.0001, Fisher exact test.
3.3. Feeding rate There was no significant difference in feeding rate between the paired hermit crabs in
the two shell treatments of the first feeding rate experiment Fig. 2; df 513, t51.043,
P 50.3158. The mean feeding rate6S.E.M. of 14 hermit crabs in shells of preferred fit
shell adequacy index 51.0 was 2.760.2 mg h. The mean feeding rate6S.E.M. of 14
hermit crabs in tightly fitting shells shell adequacy index 50.5 was 2.360.2 mg h.
In the second feeding rate experiment, the mean feeding rate 6S.E.M. of the 20 hermit
crabs prevented from switching shells control treatment was the same in the first 2 weeks, 19.3
61.1 mg h, as in the second 2 weeks, 19.860.6 mg h Fig. 3A, df519, t
50.4632, P50.6485. The control hermit crabs maintained constant feeding rates during their month of captivity in the laboratory. The 17 hermit crabs that were allowed
to choose larger shells shell switchers increased their shell size by an average 6S.E.M.
Fig. 2. Feeding rate as function of shell fit for hermit crab P . longicarpus. Each bar above the line shows the
mean 6S.D. of three measurements for a hermit crab having a shell adequacy index of 0.5; each corresponding
bar below the line shows the mean 6S.D. for its partner, a hermit crab having a shell adequacy index of 1.0.
n 514 pairs.
J .E. Angel J. Exp. Mar. Biol. Ecol. 243 2000 169 –184
177
Fig. 3. Feeding rate as function of change in shell fit for hermit crab P . longicarpus. For both graphs, the
dashed line Y 5X indicates where points would fall if mean feeding rates were identical in the first 2 weeks
and second 2 weeks of measurement. S.D. shown for second 2 weeks only. A Control: hermit crabs confined to the small shells for 4 weeks, n
520. B Shell switchers: hermit crabs that switched to larger shells after 2 weeks, n
517.
178 J
.E. Angel J. Exp. Mar. Biol. Ecol. 243 2000 169 –184
of 19.3 60.9 over the original shell size. However, there was again no significant effect
of shell treatment on mean feeding rate; mean feeding rate 6S.E.M. pre-switch5
16.8 61.1 mg h, and post-switch515.161.4 mg h Fig. 3B, df516, t51.391, P5
0.1832. 3.4. Activity level
Activity levels for the hermit crabs ranged from 6 to 3598 squares crossed in 1 h. In 10 out of 19 pairs, the hermit crab in the smaller shell was more active; in seven cases,
the hermit crab in the shell of preferred fit was more active; in two cases, the hermit crabs were equally active Fig. 4A. The mean activity level
6S.E.M. for hermit crabs in tightly fitting shells was 474
6181 squares crossed h. The mean activity level6S.E.M. for hermit crabs occupying shells of preferred fit was 346
675 squares crossed h. This difference in mean activity level was not significant Fig. 4B; df
518, t50.7885, P
50.4407.
4. Discussion
