''Pinnixa faba'', known as the pea crab, mantle pea crab or large pea crab, is a

pea crab The pea crab, ''Pinnotheres pisum'', is a small crab in the family Pinnotheridae that lives as a parasite in oysters, clams, mussels, and other species of bivalves. Description Pea crabs are small crustaceans about the size of a pea or dime, wit ...

which lives harmlessly within a large edible clam. This species is a symbiont of ''

Tresus capax ''Tresus capax'' is a species of saltwater clam, marine bivalve mollusk, common name the fat gaper, in the family Mactridae. It also shares the common name horse clam with '' Tresus nuttallii'' a species which is similar in morphology and lifes ...

'' and ''

Tresus nuttallii ''Tresus nuttallii'', common name the Pacific gaper, is a species of saltwater clam, a marine bivalve mollusk in the family Mactridae. It also shares the common name horse clam with ''Tresus capax'', a species which is similar in morphology an ...

'' in its mature stage.

Description

Pea crabs,

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Pinnotheridae, are small crabs that live symbiotically with clams,

tube worms A tubeworm is any worm-like sessility (zoology), sessile invertebrate that anchors its tail to an underwater surface and secretes around its body a mineral tube, into which it can withdraw its entire body. Tubeworms are found among the following t ...

sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...

s, and other fauna. Usually they feed on the results of their host's filtering, or in the case of sea cucumbers they live in the cloaca feeding off of the results of digestion and reproduction. They have no rostrum and no teeth between the eyes. The carapace can be up to wide and long. The carapace and walking legs are often covered in

seta In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. Th ...

e which can collect the material being filtered by the host. The species is distinguished by the tips of the dactyls, which are noticeably curved, and by the rounded nature of the outer eye orbits. ''P. faba'' is indistinguishable from '' Pinnixa littoralis'' without magnification. ''P. littoralis'' has a more angular shape to the outer eye orbit while ''P. faba'' does not leave a significant gap when it closes its chelapeds.

Distribution

''P. faba'' is found among its hosts in the intertidal regions of

Prince of Wales Island, Alaska Prince of Wales Island (Tlingit: ''Taan'') is one of the islands of the Alexander Archipelago in the Alaska Panhandle. It is the fourth-largest island in the United States (after Hawaii, Kodiak Island, and Puerto Rico) and the 97th-largest i ...

Newport Beach, California Newport Beach is a coastal city in South Orange County, California. Newport Beach is known for swimming and sandy beaches. Newport Harbor once supported maritime industries however today, it is used mostly for recreation. Balboa Island, Newport ...

, most common in

Puget Sound Puget Sound ( ) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea. It is located along the northwestern coast of the U.S. state of Washington. It is a complex estuarine system of interconnected ma ...

Ecology

''P. faba'' are only known to mate in ''

'' and ''

'' where somehow juveniles are prevented from maturing until one member of the breeding couple dies or the juvenile finds another host. In a breeding couple the female will remain in the visceral fold feeding on the material filtered by ''Tresus'' clams while the males and juveniles roam around the mantle cavity. Juveniles can be found in most other clams, notably the butter clam, ''

Saxidomus giganteus ''Saxidomus gigantea'' is a large, edible saltwater clam, a marine bivalve mollusk in the family Veneridae, the venus clams. It can be found along the western coast of North America, ranging from the Aleutian Islands to San Francisco Bay. Commo ...

''. Some writers consider the relationship of ''P. faba'' with its hosts to be commensal while others consider it parasitic, though it is clear that ''P. faba'' causes minimal damage to the host. ''P. faba'' feeds on the filtered organic matter collected by its host.

References

{{Taxonbar, from=Q3933571 Pinnotheroidea Crustaceans of the eastern Pacific Ocean Crustaceans described in 1851