The pea crabs generally are commensals of larger invertebrates, including polychaete and echiuroid worms, pelecypods, large chitons, keyhole limpets, sea urchins, sea cucumbers, and ascidians. The species generally associate with only one general type of host: those that live with polychaetes usually do not associate with mollusks and vice versa, for example. The associations do not seem to be species-specific.

Pinnotherids have a rounded to rectangular carapace, often wider than long. Usually, the carapace is soft. Most pinnotherids are colored whitish to yellowish, but some, such as Opisthopus transversus Rathbun, 1893; may have a spotted carapace and banded legs. The front, eye and orbits may be reduced. The antennules fold transversely.

Sexual dimorphism is pronounced in certain pinnotherids, with males being smaller and having a less inflated body than the females. In others, the chelae of the male are sturdier than those of the female. Pereopods 2–5 may differ in size and shape from anterior to posterior. The dactyls of pereopods 2–5 may be modified for crawling, grasping, or gripping the surface of a host.

Members of the family Pinnotheridae lately have undergone taxonomic revision. The generic placement of tropical species has been revised; males and females of sexually dimorphic groups have been matched so that some names applied only to one sex have been put into synonymy. For references prior to 1970, the account by Schmitt et al. (1973) remains valuable. The key given here follows the work of Davidson (1968) and Zmarzly (1992). Zmarzly's study of species of Pinnixa should be consulted for additional anatomical details and illustrations. A key by Campos-Gonzalez (2007) to the intertidal pinnotherids of central California to Oregon includes new drawings and valuable information. That key includes several species, such as Parapinnixa affinis Holmes, 1900; that have not been reported north of Point Conception.

Pinnotheres holmesi Rathbun, 1918 and Pinnotheres nudus Holmes, 1895 (part) seem to be synonyms of the Atlantic oyster crab Zaops geddesi (Miers, 1880). This crab may have been introduced with the oyster Crassostrea virginica in the early twentieth century but has not been reported in the area since then. Pinnotheres nudus as originally described, however, may be a synonym of Opisthopus transversus.