After reproducing, they return inland, and their young follow at a later time. Some are canned and others are sold fresh to restaurants. In the United States three main kinds of crabs are caught for their meat. Blue crabs are caught commercially in open water by trawls or are removed from the mud with dredges. Various kinds of stone, rock, and sand crabs are gathered along coasts for food, but they are not commercially important.
Some hermit crabs have symbiotic relationships with sponges.
A hermit crab living in a cast-off snail shell may find that a sponge larva has moved in with it. The larva grows into a sponge whose unpleasant odor and taste discourage the crab's predators. The crab carries the sponge when it moves from place to place, and the sponge is able to filter water from the new sites. The back of another kind of crab is always covered with a growth of sponge, which serves as camouflage. Hermit Crabs as Pets.