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THE NATURALIST
by Dave Evans

The Shell Game
July 23, 2001

On tidepool walks at Fort Abercrombie State Historical Park, I try to point out the series of life zones that correspond to amounts of time shore areas are covered by open water. In pools closest to the shoreline, we see, along with the periwinkles and acorn barnacles, some shells that move a bit differently and more quickly than the rest.

The shells turn out to be occupied by hermit crabs, and the fact they're encountered early on as we make our way out from the shoreline makes them instant attention-grabbers, especially among kids who are with us. In some of the pools, nearly every shell seems to have been expropriated by a hermit crab.

Like many other marine animals, hermits spend part of their lives as free-swimming juvenile larvae, in this stage looking more like miniature shrimp than they do hermit crabs. They can disperse far from the tidepool their parents inhabit, but eventually they'll settle down and metamorphose into miniatures of the adults. They're bizarre-looking creatures with a rigid and protective exoskeleton in front, but a soft, elongate, and asymmetrical abdomen. The front two claws are different sizes. The larger one can be held close to the body, blocking the entrance to the shell. The next four legs are the main walking legs, and hind pairs of legs at the end of the body are reduced to hooks used to secure the hermit within the shell. Upon going through this metamorphosis, the tiny hermit needs to locate a suitable shell immediately in order to protect its vulnerable abdomen.

Hermits seem obsessed with and rather possessive of their shells. Like other crustaceans (crabs, lobsters, and shrimp) they can only grow in size by shedding the exoskeleton. This is a complex growth process in itself, and hermits have the additional complication of needing to find a new shell to accommodate the newly-enlarged body after every one of these molts.

Shells become a hot commodity, and a shell-based economy prevails in the tidepools, with most hermit behavior focused on shells that are available for occupancy. Hermits spend much time fondling shells, tapping them with their antennae, measuring the openings and even trying on a few for size. In times of shell shortage, unoccupied shells will be hotly fought over. Sometimes a shell's previous owner will be forcibly ejected as the shell is checked out by an interloper. If the aggressor likes what it's found, the earlier occupant is left shell-less, presumably muttering to itself while searching for a new shell of its own. The actual change to a different shell can be rapid, as the hermit darts its abdomen down into the new accommodation.

Besides vying for shells, hermit crabs are interested in mating. The female releases her eggs and carries them on her abdomen, the male fertilizing them after they have left her body. In this way the eggs are protected by the mother's shell until the free-swimming larvae hatch and swim away. Marine biologists are trying to discover how the fertilization process occurs in nature--not as easy as it sounds because the reproductive organs of the crabs stay modestly covered by the shells during the process.

Some of our hermit species are associated with living sponges rather than snail shells. These have the advantage of increasing in size as the hermit grows. In this case, the young hermits select snail shells that become overgrown with sponge, the snail shells gradually dissolving. We don't know a great deal about this relationship, and whether it is based upon the shell originally chosen, whether the hermit carries small fragments of sponge on its body, or whether it might chemically signal sponge larvae to settle on its shell.

Then there's the question of handedness--some shells spiral to the left and others to the right, and the hermit's abdominal asymmetry seems to be determined by the spiralling of the first shell it occupies. Another hermit crab research problem is trying to determine at what point in the process the "decision" is made that a shell is suitable as a home, and the degree to which the spiraling of the shell must conform to the asymmetry of the crab's own abdomen. What information are the crabs gathering and integrating as they handle the shells, turning them over and over? Studies of this kind on these and other marine invertebrates are giving us insight on how the human nervous system processes information and, ultimately, how we make decisions ourselves.

Hermit crabs are one of the most common animals in Kodiak's tidepools, but they serve as fine examples of how many questions we have yet to resolve about even the most abundant organisms around us.


http://www.kadiak.org/naturalist/shell_game.html Updated 2001 August 20

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