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Shell Games with Hermit Crabs

I admit when I saw a line of hermit crabs holding onto each other (roughly in order of size) stretching across the sandy path at Two Foot Bay, on the northeast side of Barbuda, I thought it was about sex. I was wrong; it was about real estate.

Hermit crabs don't have a hard shell and need protection against sun and predation for their soft abdomen. They solve their housing dilemma by occupying discarded gastropod (snail) shells. As hermit crabs grow they need to find larger shells and often they are not lucky enough to stumble onto the correct sized vacant shell. What we were watching was a community shell swap among Caribbean hermit crabs (Coenobita clypeatus). This phenomenon was documented by researchers from Tufts University (Rotjan and Lewis) in Belize. They call it a 'synchronous vacancy chain'. The classic chain starts when a large vacant shell appears and the crabs arrange themselves into a line of decreasing size, starting with the largest crab holding onto the empty shell. The crabs swap shells, each moving up in size as it is vacated by the slightly larger crab ahead of it. A single vacant shell kicks off an entire chain of shell vacancies that ultimately leads to many crabs getting new, and generally improved, housing. The exact details of each vacancy chain may vary, but the end product of upsizing the shell is accomplished. If a crab finds a shell that is a 'fit' there would be no chain and the simple swap – one crab to a larger shell – would be called 'asynchronous vacancy chain' because it takes place over an extended period of time.

Shell switching is fast; the crab holds the new shell with the aperture facing upward and releasing its abdominal grip on the old shell, it rapidly swings its abdomen over to occupy the new shell. It gets more confusing when a crab decides to move into a shell that is not vacant. We watched a crab successfully evict the tenant of a shell by rocking it until the occupant started to emerge in effort to right the shell and at that point the aggressor snatched him out. The displaced crab jammed himself into the smaller shell and scurried off.

Shell fit is important because a snug shell protects the crab from predation. If a crab is able to find the perfect shell it would ultimately be able to completely close itself in, its large claw totally blocking the opening on the shell.

As crabs grow they need to discard their old exoskeleton (their skin) and that process is called molting. Small crabs may molt several times a year, while larger crabs (they can be as large as a baseball) may only molt once a year or even less frequently. Molting allows crabs to re-grow lost appendages. Crabs usually bury themselves in the sand or soil to molt. This provides them with protection from predation and increased humidity to help shed the old skin. If they cannot bury themselves they dig a hole and orient the opening of the shell downward to the sand. Those watching hermit crabs molt have noted that they often eat their old exoskeleton. They hypothesize that the crabs do this to harvest the calcium lost in the molt.

Devi Sharp is a retired wildlife biologist and is exploring the birds of the Caribbean with her husband, Hunter, on their sailboat Arctic Tern. Chuck Shipley is a former professor of computer science and an avid amateur photographer. He and his wife Barbara live aboard their trawler Tusen Takk II in the Caribbean.

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