Optimal Foraging in Wild Hermit Crabs

1
Optimal Foraging in Wild Hermit Crabs Meredith
Wright and Jessica Barley Animal Behavior,
Department of Biology, Coastal Carolina University
Introduction Hermit Crabs in the wild live
on the beach and are commonly known as
scavengers. However, recent studies have shown
that hermit crabs actively choose their food.
This is because every animal needs to have a
balanced diet, which they could not get if they
were only scavengers. We used hermit crabs from
islands off of Southern Florida for our research.
The focus of this study was to determine what
foods hermit crabs preferred and whether or not
they would still choose based on accessibility of
the food. We use the optimal foraging theory to
try to prove that crabs will prefer a food with
higher nutritional value. By applying this
theory, we can test whether a crab would spend
more energy to obtain a food with higher
nutritional value. Hypothesis Wild hermit crabs
will have a food preference. Wild hermit crabs
will choose food that is less preferable, but
more accessible. Methods This is a
photograph of the 10-gallon habitat that was set
up to house the 20 hermit crabs used in this
experiment.

A ten gallon tank was set up as a habitat for the
hermit crabs and the crabs were numbered 1-20.
There were four food groups, with three food
types in each. For each trial, an individual crab
was placed in a white plastic dish tub,
equidistant from the three food types (Fig. 1).
As soon as a crab started eating a food type,
that was recorded as preferred. This method was
repeated for all 20 crabs for each food group. If
a crab died, it was recorded and it was left out
of the remaining trials.
Food 1 Crab
Food 2
Food 3
Fig. 1 Top View (Preference Trials)
Chart 1. The crabs overwhelmingly chose papaya
over apple and orange. Fifteen times as many
crabs chose papaya as did orange. (n19,
x218.09, df2, Pgt5.991)
Chart 2. There was no significant difference
between the meat groups. Not even twice as many
crabs chose ham as did roast beef. (n17,
x2.8249, df2, Plt.5.991)
Chart 5. This is the optimal foraging study,
chosen because there was such a significant
difference for the fruit group. Twice as many
hermit crabs chose papaya as did orange, however,
the results were not significant. (n16, x22.25,
df1, Plt3.841)
Chart 3. We tested crabs preferences for
vegetables. At least 8 times the number of crabs
chose broccoli and green beans over carrots.
(n18, x26.33, df2, Pgt5.991)
Chart 4. This group tested food naturally found
in the crabs environment. Three times as many
crabs chose leaves as did bark. (n18, x24.33,
Plt5.991)
Conclusions Hermit crabs do have a preference.
They prefer papaya, green beans, ham and leaves.
However, only the papaya and green beans were
significant. The preference is not based on
color, as seen by the results for papaya,
carrots, and oranges. The results for optimal
foraging showed that the crabs will still choose
papaya over oranges, even though it is less
accessible. However, by statistical analysis,
with the chi-square method, the results are not
significant because there is too small a
population or too little degrees of freedom.
For the final trial, a preferred food type was
placed up on a Petri dish and the least preferred
food item was placed easily accessible to the
crab (Fig. 2). Whichever food item was preferred
was recorded.
Food 1
References Thacker, R.W. 1998. Avoidance of
recently eaten foods by land hermit crabs,
Coenobita compressus. Animal Behaviour. 55
485-496. Wight, K. et al. 1990. Food aversion
learning by the hermit crab Pagarus granosimanus.
Biological Bulletin. 178 205- 209.
Crab
Food 2
Fig. 2 Side View (Optimal Foraging)