Home sweet fossil home

Natural History, Dec, 2001 by Kristen L. Weir

Large or small, landlubbers or seafarers or both, hermit crabs have one feature in common: they generally spend their lives inside the empty shells of snails or other mollusks. When these crabs grow too large for one adopted home, they search for another. But if high-quality shells are in short supply, they must improvise. Some make do with a broken shell or even with a piece of bamboo or a hollow mangrove root. Now there is evidence that at least one type of hermit crab has found another solution.

Zoologist David K.A. Barnes, of University College Cork in Ireland, recently discovered that the large, semiterrestrial hermit crab Coenobita rugosus sometimes moves into fossilized marine snail shells that have fallen out of the eroding coastal limestone of southwestern Madagascar.

Barnes found that the crabs preferred the shells of local mollusks, but when these modern homes weren't available the hermits inspected fossil shells for suitability and sometimes moved into them (though nearly all the fossils Barnes examined had structural hindrances that made them unusable). He predicts that this curious behavior may eventually change, however. Local fishermen collect mollusks along the shore and, working at the high-tide mark, remove the living animals from their casings, leaving behind piles of empty shells. Most are too large for hermit crabs. But as fishermen increasingly exploit the local mollusk populations, they'll soon have to settle for smaller animals. In time, the discarded shells may be diminutive enough to provide homes for hermit crabs, freeing them from the need to survey the emerging fossils. ("Ancient Homes for Hard-up Hermit Crabs," Nature 412, 2001)