Marine hermit crabs are pretty feisty, as you'll see in the
video link below. They have the unfortunate problem of requiring
other animals (snails) to make shells for them to live in.
Without the snail shell, the hermit crab is very vulnerable to
predators. You'll see a naked hermit crab at the beginning of
the video. Whenever I see a naked hermit crab like this, it
makes me want to melt some butter in a pan and add a little
garlic. Other potential predators probably feel much the same
way. Hermit crabs need to be in shells to keep from being eaten.
The snails that make the shells don't just give them away, of
course. In fact each snail is permanently attached to its shell
with a specialized columellar muscle that is cemented to the
inside of its shell. The hermit crabs have to wait for the
snails to die; then they can fight over the shells once the
shells become empty.
My lab showed some years ago (Angel, 2000) that hermit crabs of
this particular species (Pagurus longicarpus) are very
particular about the size of the shell they prefer to live in.
If you weigh a live, shell-less (naked!) hermit crab of this
species, you can predict quite accurately the size of the shell
it prefers to live in. In fact, in our study, differences in the
weights of naked hermit crabs predicted about 86% of the
variation in the sizes of the shells they preferred to live in.
If you're wondering why in this video the hermit crabs are
fighting so fiercely for each other's shells, it's because I
deliberately put most of the hermit crabs into shells that were
too small for them. So each crab was highly motivated to find a
larger shell to move into. "Your shell might be better than
mine; let me check it out!"
We also showed some years ago (Pechenik and Lewis, 2000) that
hermit crabs don't like living in shells that have holes in
them. In particular, they don't like living in shells that have
holes that were drilled into them by carnivorous snails called
moon snails. Moon snails hold onto the shells of other snails
and slowly drill beautifully round holes through the shells over
a period of many hours or even several days, using a combination
of acid secretion and rasping with a specialized tongue called a
radula. Once they have drilled through the shell they insert
their mouth—which is at the end of a long trunk, called a
proboscis—into the shell and eat the snail while it's still
alive inside the shell. What a way to go.
A periwinkle shell
drill hole made by
a predatory moon
Photo by J. Pechenik
Anyway, the Pechenik and Lewis paper (2000) showed that hermit
crabs would rather be in almost any shell other than one with
one of these drill holes. It turns out (Pechenik et al., 2001)
that hermit crabs in drilled shells are more vulnerable than
usual to predators, to salinity stress, and to being evicted
from their shells by other hermit crabs. If you look carefully
in the video, you'll see one hermit crab in a drilled shell that
looks very much like the shell pictured above. The shell is also
too small for it. That is one very desperate, unhappy, and
perhaps terrified hermit crab.
View the video:
After the filming, I allowed all the hermit crabs to choose new
shells, and then returned them the next day to the same beach
from which I had collected them.
Angel, J.E., 2000. Effects of shell fit on the biology of the
hermit crab Pagurus longicarpus (Say). J. Exp. Mar. Biol. Ecol.
Pechenik, J.A., Lewis, S. 2000. Avoidance of drilled gastropod
shells by the hermit crab Pagurus longicarpus at Nahant,
Massachusetts. J. Exp. Mar. Biol. Ecol. 253: 17-32.
Pechenik, J.A., J. Hsieh, S. Owara, S. Untersee, D. Marshall,
and W. Li. 2001. Factors selecting for avoidance of drilled
shells by the hermit crab Pagurus longicarpus. J. Exp. Mar.
Biol. Ecol. 262: 75-89.