Pea Crab Pinnotheres pisum
Typically about the size of a pea, the tiny pea crab is usually found inside the shells of the common mussel. Protected from predators in the mantle cavity of its host, it feeds on any plankton that becomes trapped on the mussel’s gills as water passes over them. Whether the presence of this guest is harmful to the mussel is unclear. Female pea crabs are substantially larger than males and have an almost translucent carapace through which their pink reproductive organs are visible. Males have harder, yellowish brown carapaces that protect them during the breeding season, which runs from April to October. During this time, males leave the safety of their host’s shell and swim around looking for females with which to mate. In regions where shellfish are harvested commercially, the pea crab is considered a pest.