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

crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...

in the family

Pinnotheridae Pinnotheridae is 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 moseri'' i ...

that lives as a

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

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 no ...

clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

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 () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

Etymology

''Pinnotheres'' is

Greek Greek may refer to: 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 of all kno ...

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

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

for a

pea Pea (''pisum'' in Latin) is a pulse or fodder crop, but the word often refers to the seed or sometimes the pod of this flowering plant species. Peas are eaten as a vegetable. Carl Linnaeus gave the species the scientific name ''Pisum sativum' ...

, in reference to the shape of the crab.

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 () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

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-shaped organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through Neo-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 light scattering by particles, scattering of light. On a macroscopic scale ...

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 close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...

, 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 fr ...

, 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

molluscs Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

. 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 or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...

s and

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

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

rectum The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...

sea cucumber Sea cucumbers are echinoderms from the class (biology), class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number be ...

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. Description and habitat Oyster crabs are small in size, typically measuring around 1 to 2 centimeters in width. They have a flattened bo ...

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

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

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. Sea otter with shells 2

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 the

University of Auckland The University of Auckland (; Māori: ''Waipapa Taumata Rau'') is a public research university based in Auckland, New Zealand. The institution was established in 1883 as a constituent college of the University of New Zealand. Initially loc ...

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

References

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

Etymology

Description

Ecology

Mating

See also

References