oyster crab
noun
1.
a pea crab,
Pinnotheres ostreum,
the female of which lives as a commensal within the mantle cavity of oysters.
Origin
1750-60
British Dictionary definitions for
oyster crab
oyster crab
noun
1.
any of several small soft-bodied crabs of the genus
Pinnotheres,
esp
P. ostreum,
that live as commensals in the mantles of oysters