An anonymous reader writes "When it comes to abandoned snail shells that hermit crabs expropriate as mobile homes, size matters, for room to grow, room for eggs, and protection from predators. UC Berkeley evolutionary biologist Mark Laidre found that terrestrial hermit crabs on the Pacific shore of Costa Rica congregate in aggressive swap meets where one crab is forced from a relatively large shell, whereupon the rest trade up (one loser and multiple winners, pretty good odds). The loser gets the smallest shell, which means likely doom. Laidre and his colleagues note that most hermit crabs live in the ocean, where there are usually enough abandoned shells to go around so most can live, well, hermit-like lives without much interaction with fellow crabs. Not so on land, at least in Costa Rica."