Pinnotheroidea

Pinnotheroidea is a superfamily of small, soft-bodied crabs, commonly known as pea crabs or mussel crabs. These crabs are obligate symbionts, meaning they live their entire lives within the bodies of other marine invertebrates, most frequently bivalve mollusks like oysters, mussels, and scallops. They also inhabit other hosts, including echinoderms, ascidians, and polychaete worms.

The defining characteristic of pinnotheroidean crabs is their intimate association with their hosts. They typically reside within the mantle cavity of bivalves, feeding on food particles filtered by their host or on the host's gametes. In return for shelter and food, the pea crab is thought to offer some benefit to the host, though this is not always clear. Some studies suggest they may help keep the host clean by consuming parasites or detritus.

Pinnotheroidean crabs exhibit significant sexual dimorphism. Females are generally larger and more rounded, with a soft, uncalcified carapace that allows them to live within their host. Males are typically smaller, more mobile, and possess a harder carapace. Males must locate a receptive female within a host, mate, and then often continue their nomadic existence.

The superfamily Pinnotheroidea is divided into several families, including Pinnotheridae, which is the largest and most diverse. Species within this superfamily are found in marine environments worldwide, from tropical to temperate waters, and at various depths. Due to their small size and cryptic lifestyle within their hosts, they are often overlooked but play an important ecological role in their host-associated communities. Their impact on host populations can vary, with some interactions being commensalistic and others potentially parasitic, depending on the species and specific circumstances.