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

Pea crab

The New Zealand pea crab, Pinnotheres novaezelandiae, is a small, parasitic crab that lives most commonly inside New Zealand green-lipped mussels. Adult females are about the size and shape of a pea, while adult males are smaller and flatter. Adult New Zealand pea crabs are completely reliant on...

 which lives harmlessly within a large edible clam

Clam

The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

. This species is a symbiont of Tresus capax in its mature stage.

Description

Pea crabs, family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Pinnotheridae

Pinnotheridae

Pinnotheridae is a family of pea crabs. Pea crabs are tiny soft-bodied crabs that live commensally in the mantles of certain bivalve molluscs...

, are small crabs that live symbiotically

Symbiosis

Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 with clam

Clam

The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

s, tubeworms, sea cucumbers, 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

Rostrum (anatomy)

The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....

 and no teeth between the eyes.

The carapace can be up to 15 millimetre (0.590551181102362 in) wide and 7 mm (0.275590551181102 in) long. The carapace and walking legs are often covered in seta

Seta

Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....

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

Intertidal zone

The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...

 of Prince of Wales Island, Alaska

Prince of Wales Island, Alaska

Prince of Wales Island is one of the islands of the Alexander Archipelago in the Alaska Panhandle. It is the fourth-largest island in the United States and the 97th-largest island in the world....

 to Newport Beach, California

Newport Beach, California

Newport Beach, incorporated in 1906, is a city in Orange County, California, south of downtown Santa Ana. The population was 85,186 at the 2010 census.The city's median family income and property values consistently place high in national rankings...

, most common in Puget Sound

Puget Sound

Puget Sound is a sound in the U.S. state of Washington. It is a complex estuarine system of interconnected marine waterways and basins, with one major and one minor connection to the Strait of Juan de Fuca and the Pacific Ocean — Admiralty Inlet being the major connection and...

.

Ecology

P. faba are only known to mate in Tresus capax 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 capax while the males and juveniles roam around the mantle cavity. Juveniles can be found in most other clams, notably the butter clam, Saxidomus giganteus.

Some writers consider the relationship of P. faba with its hosts to be commensalistic

Commensalism

In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...

 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.