hermit crab home improvement

Among the favorite characters in the touch-tank in the museum at College of the Atlantic are the Acadian hermit crabs. Hermit crabs typically live in empty snail shells, which offer protection for their soft, slightly curved abdomens. There are some exceptions to this pattern: a few hermit crabs have straight abdomens and live in worm tubes; others, like the giant coconut crabs, don't use extra coverings at all during adulthood, relying solely on their chitinous exoskeleton for protection. The large Acadian hermit crabs we see here usually inhabit the old shells of moon snails (Lunatia heros) or ten-ridged whelks (Neptunea decemcostata). An odd thing about these shells is that they are often missing chunks along the margin of the shell opening. Odder still, the hermit crabs hack away these chunks themselves, once they've taken up residence in the shell. Diver Ed has captured this behavior on film, and he has routinely seen them doing it during his tens of thousands of hours underwater. Maybe they're getting the size of their shell JUST right. If they left it any bigger, a larger hermit crab might want it and could easily wrestle it away from the smaller resident. Hermit crabs are certainly known to compete for shells. But if customizing the shell is a way to limit competition from larger hermit crabs, why do we see this phenomenon only among the largest individuals? Do bigger hermit crabs have disproportionately larger and stronger claws, capable of pinching off pieces of shell margin, or do the largest snails have disproportionately thinner shells, making modification possible only for the hermit crabs that inhabit the largest shells? I suspect some size-related pattern, but of course I would suspect that. I'm rather obsessed with size-related patterns; my favorite mathematical expression is the allometric equation.