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Friday, November 2, 2012

The hermit crab property swap

Hermit crabs live within discarded snail shells. As these homes cannot expand with their growing bodies, they must periodically move to larger homes. This isn’t a problem for marine hermit crabs, who can usually find an abundant selection of empty snail shells on the sea floor. However, shells are much scarcer on land, making it much more problematic for terrestrial hermit crabs to find new homes when the time comes to move up. If you add in the fact that the land crabs require modified snail shells, you get the makings for a communal property swap.

The terrestrial hermit crab Coenobita compressus lives inside a discarded snail shell and forages for plants and carrion along the Pacific coast from Mexico to Peru.
Credit: Mark Laidre, UC Berkeley

Terrestrial hermit crabs modify their homes by enlarging the entrance and the interior space. This is critical because, unlike marine hermit crabs, the land-dwelling crabs don’t fit very well into unmodified shells. When Mark Laidre from the University of California, Berkeley yanked crabs out of their shells and let them move into unmodified snail shells, he found that they were extremely vulnerable to predation. None of his unfortunate crabs survived for even twenty-four hours. If given a choice, land hermit crabs much prefer to move into homes that have already been modified by a prior tenant.

Left: A marine snail shell newly vacated by its gastropod owner.
Right: A shell that has been remodeled by a hermit crab.
Credit: Mark Laidre, UC Berkeley

This means that the easiest way to acquire a new home is to forcibly take it from its current occupant. Of course, the evicted crab now has to bully a third crab out of its shell, and so on. If one crab notices another crab eyeing its shell, it had better make sure there are other victims close by. Thus, the ordinarily solitary animals will aggregate in large numbers for a mass snail shell exchange. You can see an example below.

A free-for-all takes place whenever three or more hermit crabs congregate, with all crabs intent on displacing someone else to get a larger shell.
Credit: Mark Laidre, UC Berkeley 

Laidre, M. (2012). Niche construction drives social dependence in hermit crabs Current Biology, 22 (20) DOI: 10.1016/j.cub.2012.08.056