The pea crab, ''Pinnotheres pisum'', is a small

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

in the family

Pinnotheridae The Pinnotheridae are a family of tiny soft-bodied crabs that live commensally in the mantles of certain bivalve molluscs (and the occasional large gastropod mollusc species in genera such as '' Strombus'' and ''Haliotis''). '' Tunicotheres moser ...

that lives as a

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...

clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two she ...

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...

s, and other species of

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

Description

Pea crabs are small crustaceans about the size of a pea or dime, with a "smooth dorsal surface of the carapace, or upper exoskeleton". The

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

of males is hard and circular and has eyes and antennae extending from their fronts, and the

cheliped A chela ()also called a claw, nipper, or pinceris a pincer-like organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through New Latin '. The plural form is chelae. Legs bearing a chela are called chelipeds. ...

s are more robust in males than in females, which have more elongated chelipeds. The bodies of the female pea crabs are often

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...

and show the inner organs and gonads as yellow and red, with the males being a "more yellowish-grey with patches of brown".

Ecology

The relationship between the pea crab and its host is one of

parasitism Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

, rather than

commensalism Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...

, since the host may be harmed by the crab's feeding activities. The pea crab relies solely on its host for food, safety, and oxygen. Pea crabs have a variety of hosts, the most important of which are

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

. The pea crab lives in the

mantle cavity The mantle (also known by the Latin word pallium meaning mantle, robe or cloak, adjective pallial) is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of ...

of these hosts. Other hosts, in addition to

s, include

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

s and

sand dollar Sand dollars (also known as a sea cookie or snapper biscuit in New Zealand, or pansy shell in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are k ...

s. ''Pinnotheres'' can be found inside sand dollars, in the rectum of

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, in the tubes of parchment worms, in the burrows of mud shrimp, or in the gills of sea squirts. Little is known about the pea crab's feeding habits, but in the related

oyster crab The oyster crab (''Zaops ostreus'') is a small, whitish or translucent crab in the family Pinnotheridae. Ecology It is less than half an inch across, and lives inside the gills of an oyster or a clam. It uses the oyster for protection and live ...

(''Zaops ostreus''), larval stages feed on

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

brought in by the oyster, while adults feed by taking the food that is a part of the oyster's diet, as well as what is not. The feeding process can be harmful to the crab's host when it feeds on the mucous strings that help carry the food to the host's mouth.

Mating

A male pea crab will rub the edge of a shellfish containing a female pea crab for hours until the shellfish opens and allows the male pea crab to enter.Nationalgeographic.com 2015-05-08 Crab Tickles Shellfish for Hours to Find Love
/ref> A study by New Zealand researchers Oliver Trottier and Andrew Jeffs from

University of Auckland , mottoeng = By natural ability and hard work , established = 1883; years ago , endowment = NZD $293 million (31 December 2021) , budget = NZD $1.281 billion (31 December 2021) , chancellor = Cecilia Tarrant , vice_chancellor = Dawn F ...

shows this behaviour on a similar parasitic pea crab, ''Nepinnotheres novaezelandiae''.

Etymology

''Pinnotheres'' is

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

for "guard of Pinna" and ''pisum'' is

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

for a

pea The pea is most commonly the small spherical seed or the seed-pod of the flowering plant species ''Pisum sativum''. Each pod contains several peas, which can be green or yellow. Botanically, pea pods are fruit, since they contain seeds and d ...

, in reference to the shape of the crab.

References

{{Taxonbar, from=Q2234799 Pinnotheroidea Crabs of the Atlantic Ocean Parasitic crustaceans Crustaceans described in 1767 Taxa named by Carl Linnaeus