Jan A. Pechenik
Professor
Invertebrate Zoology and Marine Biology

Marine Hermit Crabs in Action

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 (Littorina littorea) showing the typical drill hole made by a predatory moon snail. 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.

Here's the video: http://www.youtube.com/watch?v=2kpSo8JVgSo

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.

Papers cited

Angel, J.E., 2000. Effects of shell fit on the biology of the hermit crab Pagurus longicarpus (Say). J. Exp. Mar. Biol. Ecol. 243: 169–184.

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.

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