The oyster crab, Zaops ostreus, is a small, whitish or translucent crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

 in the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Pinnotheridae

Pinnotheridae

Pinnotheridae is a family of pea crabs. Pea crabs are tiny soft-bodied crabs that live commensally in the mantles of certain bivalve molluscs...

. It is less than half an inch across, and lives inside the gills of an oyster

Oyster

The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....

 or a clam

Clam

The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

. It uses the oyster for protection and lives on the food that the oyster gets for itself. It is found in oysters in the North Atlantic Ocean. The crabs are eaten as a delicacy.