Food Preference in the Sand Fiddler Crab Uca pugilator

1
Food Preference in the Sand Fiddler Crab Uca
pugilator
Katie Whitworth and LaJeania Gibson Animal
Behavior, Department of Biology, Coastal Carolina
University
Introduction The fiddler crab, Uca
pugilator, is a semi-terrestrial crab that lives
in the intertidal zone of salt marshes along the
Gulf of Mexico and the lower East Coast of the
United States (Reinsel and Rittschof1995). These
deposit feeders are known to feed on detritus,
bacteria, algae, ciliates, and nematodes
(Robertson et al. 1980). The chemical
composition of the food crabs naturally consume
is an important factor in their food preference.
The two compounds used in this experiment,
carbohydrates and proteins, are essential
macromolecules to any living organism (WebMd).
Carbohydrates are found as a readily available
food source in the forms of sugars and starches
or can be stored in the body for later use as
calories. Proteins form the enzymes used to
carry out metabolic processes and can also be
used as energy when fat and carbohydrate reserves
run low. The objective of this experiment was to
see if Uca pugilator has a particular preference
for one nutrient over the other.
Results
Discussion Four carbohydrate and two
protein data points were dismissed because of
their negative values. One possibility for those
numbers might be water absorption from the sand
causing the bread and chicken to have a final
weight higher than the initial. Another
explanation could be the addition of sand to the
food by the crab itself. A possible explanation
for the crabs not preferring one food type over
the other could be due to the length of time they
were held before the tests were run. The crabs
were kept in the bucket for over 2 weeks with no
food, so after having reworked the soil they may
have been so hungry they would have eaten
anything. State-dependent behavior is known to
happen in other invertebrates (Bilde et al 2002).
We were surprised that most of the crabs lost
mass as we assumed that if they consumed food,
they would gain weight. The decrease in mass
could be due to the fiddler crabs high
evapotranspirative and metabolic rates (Power and
Cole 1976), perhaps the food was enough to keep
up energy levels but not necessarily induce
weight gain. The replications themselves may have
also played a role in misconstrued data, some
trials were run overnight while others were run
throughout daytime. Were all the crabs run at
the same time the data may have been more
consistent.
Figure 1. The average weight of nutrient eaten.
Materials and Methods The 20 fiddler crabs
used in this experiment were collected in
Murrells Inlet, South Carolina and held in a
bucket of moist mud. The trials were run four
crabs at a time using two ten gallon aquarium
tanks that were divided in two using plexiglass
and duct tape. Two Petri dishes were placed in
diagonal corners of the holding area on a
substrate of wet sand. One petri dish held 1.0g
(/- 0.1g.) of plain white bread (the
carbohydrate) the other contained 1.0g (/- 0.1g)
of breaded chicken nugget (the protein). Each
crab was weighed, its carapace width taken and
sex determined before being placed in the tank.
After 6 hours the crab was removed and reweighed.
The carbohydrate and protein were also reweighed
once the trial was over.
Literature Cited Bilde,T, Maklakov, A, Tayor, P.
and Lubin, Y. 2002. State-dependent decisions in
nest site selection by a wed-building spider.
Anim. Behav. 64 447-452. Powers, L. and Cole, J.
1976. Temperature variation in fiddler crab
microhabitats. J. exp. Mar. Biol. Ecol. 21
141-157. Reinsel K.A. and D. Rittschof.1995.
Environmental regulation of foraging in the sand
fiddler crab Uca pugilator. J. exp. Mar. Biol.
Ecol. 187 269-287. Robertson J.R., K. Bancroft,
G. Vermeer and K. Plaisier. 1980 . Experimental
studies on the foraging behavior of the sand
fiddler crab Uca pugilator. J. exp. Mar. Biol.
Ecol. 44 67-83.
Figure 2. Weight Change in the Fiddler Crabs
The average mass of carbohydrate eaten was 0.3g
and average mass of the protein was 0.2g. A
t-test determined that there is no significant
difference in which macromolecule the sand
fiddler crabs fed off of (t0.982, d.f.16,
P.170). Some data points were excluded from the
test because their mass actually increased during
the trial (see Discussion). Two crabs were
observed to eat the carbohydrate less than a
minute after it had been placed in the tank
whereas none ate the protein as quickly. Out of
the 20 crabs tested, 2 were found to gain weight,
12 lost weight and 6 showed no change in their
mass.
24 protein per serving (5 pieces) for a 2000
calorie diet.