The Pea Crab is an unusual little parasite which you may just meet face-to-face. Pea Crabs live inside clams, mussels, and oysters, and people sitting down to a seafood meal occasionally find them. Those in the know consider them delicacies.
Pea Crabs fit their name. The largest females measure under an inch across. Full-grown males are much smaller. These tiny crabs live in the part of clams and similar creatures called the mantle, which, among other things, sifts food and oxygen from sea water. Positioned atop the mantle's gills, Pea Crabs snag bits of food, get oxygen, and enjoy the protection of their host's hard shell. Once full-grown, female Pea Crabs stay put, one per clam, oyster, or mussel. The crabs don't even develop hard shells of their own because they don't need to. The little male Pea Crabs wander, going from clam to clam. Around Puget Sound the big gaper, or horse, clams host 3 species of these little parasites. Young crabs may live in smaller bivalve species, moving to bigger ones like the gapers as they grow. Some kinds of Pea Crabs live in worm burrows or inside other types of marine creatures.
Especially for Pea Crabs living inside bivalves, "parasite" is the right label. The crab affects its host and not for the better. Besides diverting some of the clam's food, the crab damages the host's feeding apparatus as it grabs edible bits. This affects the bivalve's growth in various ways.
There are still many questions about how Pea Crabs live. They're just one more reminder of the great amount we don't know about an environment our activities affect more each day.