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The Hermit Crab
by Takiyah Gray
This page is dedicated to sharing information about the hermit crab, an Arthropod in the crustacean family, and the sea creature that complicates the crabs' lives, the moon snail.
What
is a hermit crab?
A
hermit crab is a small crustacean, almost looking like a tiny lobster. It
has two pairs of walking legs, and a pair of claws, and a long torso. The
hermit crab's abdomens are tender, and, when out of its shell, exposed to
predators. The abdomen is curved in the shape of the snail and bends to hook
around the shell's main column. This seemingly queer position makes it hard
for the hermit crab to be ripped out of its shell. Aside, the small crabs
are ocean garbage cans, eating whatever available to them, including each
other. Many species of the hermit crab, such as the flat-clawed hermit crab
and the hairy hermit crab are found on rocky ocean bottoms. Many grow to 1.5
inches.
Why
do hermit crabs live in shells?
Half
of the hermit crab's body is completely uncovered, but the other, upper half
is just like that of another crab. The shell of a snail provides the hermit
crab with protection against predators and high salinity levels in the water.
The crabs can retract into the shell, ventilate the flow of water, and store
a female crab's eggs.
What
is the relationship between the hermit crab and the moonsnail?
The
moonsnail is known to drill a little hole onto the shells of other snails,
shells that are potential homes for hermit crabs. A hole in the shell means
that the shell is weaker than the average shell; creatures inside the damaged
shell would not be as protected from their enemies as those housed in an intact
shell.
Dr. Jan Pechenik and Sara Lewis did a study about the hermit crab's willingness to occupy a drilled shell. Over 900 gastropod shells were secured from a site in Nahant, Massachusetts. The hermit crabs in periwinkle shells were placed in plastic cups of seawater at 20 degrees Celsius. The seawater was changed daily and shrimp pellets were provided for the crabs' nourishment. In the first experiment, the crabs were offered two shells, one intact, and one with a drilled hole. The crabs, after inspection, usually choose the intact shell. A similar experiment was conducted in the dark, and the hermit crabs still strongly avoided the shells that were naturally and artificially drilled. Another experiment was run, with the hole artificially drilled on the opposite side of the shell that the hole is usually found on when a moonsnail drills into it. The same results were reached. In yet another experiment, Pechenik and Lewis used "18 hermit crabs that had been evicted from their shell [and] were given a choice between an intact periwinkle shell appropriate for a crab _ of that crab's wet weight and a naturally drilled shell of ideal size" (Pechenik, Lewis). In another experiment, 16 crabs were given the choice of an ideal sized shell with a drilled hole and an intact shell for a crab a quarter of that specific crab's weight. In the experiment, with the intact shell of half the size, most crabs picked the small shell, even though they didn't totally fit inside the shell. In the choice between the intact shell for a crab 25% the size and a drilled shell the perfect size, most of the crabs chose the larger, drilled shell, the intact shell being too small.
Much thanks to Dr. Pechenik for sharing his study and allowing me to use his info for our site.
Bibliography
Elwood,
Robert W., 1995, "Motivational change during resource assessment by hermit
crabs"
Lewis and Pechenik, February, 2000, "Avoidance of drilled gastropod shells by the hermit crab Pagurus longicarpus at Nahant, Massachusetts"
Palmer, Dave, "Hermit Crabs search for a shell to call home"
Links
www.elsevier.nl/locate/jembe