Pinnixa faba

How to Distinguish from Similar Species: Pinnixia littoralis, another common species, has an acute outer margin to its eye orbits and both sexes have a gap between the propodus and dactyl of the claw.

Primarily found within the fat gaper clam Tresus capax. Juveniles may be in other clam species.

Geographical Range: Prince of Wales Island, Alaska to Newport Beach, CA. This species is much more common than is P. littoralis in Puget Sound.

Pinnotherid crabs are true crabs which do not have a rostrum nor any teeth between the eyes or on the lateral margins of the carapace. The carapace is a squared oval or nearly circular, and usually wider than long. They are symbionts with worms, ascidians, bivalves, etc. Pinnixa faba has a carapace width at least 1.5 times the length, curved tips to the dactyls of its walking legs, the outer orbits of the eye sockets are rounded, and the female has no gap between the propodus and dactyl of her claw. The merus of the third leg in males is more than twice as long as wide. Males are hard-bodied and females are softer. The male has a tooth near the base of the propodus of the cheliped. Color variable, from white to orange. Male often has dark markings. Male carapace width to 1.75 cm, females to 2.5 cm.

Biology/Natural History: A number of crabs may be found inside a clam but usually only one pair is mature. The presence of the adults seems to hinder the maturation of the juveniles. The small male crabs move around but the larger females remain sheltered by a fringe of tissue, the visceral skirt, attached to the clam's visceral mass. They scrape plankton from the clam's feeding mucus from this fringe. They cause slight damage to the host. Alternate hosts include the clams Tresus nuttallii, Macoma nasuta, Mya arenaria, Saxidomus gigantea, Clinocardium nuttallii, Siliqua patula, and Protothaca staminea; the mantle folds of the sea hare Aplysia vaccaria, the atrial cavity of the tunicate Styela gibbsii, and in the cloaca of the sea cucumbers Caudina arenata, C. arenicola, and Cucumaria piperata. In Puget Sound adults seem to only be found in Tresus capax. In Washington, some females with eggs can be found year-round, and most raise two broods per year. A female usually carries 7000-8000 eggs. Breeding is interrupted when the females molt, from late August to October. Larvae are planktonic for 47 days, then seek a clam host. These crabs have a terminal molt. Females are large enough to lay eggs when they reach a carapace width of 12.8 to 14 mm but continue to grow to 19.7 mm.

Depth Range: Middle and low Intertidal and subtidal to 30 m