Pea crabs (Pinnotheres ostreum) or oyster crabs (Zaops ostreus) are small soft-bodied crabs that live in bivalves such as oysters and mussels. They are kleptoparasites, which means they steal food from their host to survive. Once they enter an oyster, they live inside the oyster's gills and feed on the food that filters in. Since both crabs behave similarly, we will refer to both generally as "pea crabs."

Pea crabs find their oyster hosts very early on when both are still larvae. The crabs spawn a month after the oysters, which allows them to find oyster spat settled out of the water column. Pea crabs are free-swimming in the early stages to seek out their oyster hosts, but males remain free-swimming for life to find mates from oyster to oyster. Once female larvae find their hosts, they remain there until maturity and lay their eggs inside. That's why you may find more than one pea crab in an oyster.