''Tunicotheres'' is a

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

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 ...

s 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 moseri ...

, and ''Tunicotheres moseri'' is the only

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

in the genus. This crab lives commensally in the atrial chamber of a small

ascidian Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" ...

(sea squirt). It is found in the tropical western

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

, the

Caribbean Sea The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...

and the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...

Taxonomy

Mary J. Rathbun Mary Jane Rathbun (June 11, 1860 – April 4, 1943) was an American zoologist who specialized in crustaceans. She worked at the Smithsonian Institution from 1884 until her death. She described more than a thousand new species and subspecies and ...

's original description of this species, she placed it in the then broadly defined '' Pinnotheres''. It remained there for most of the 20th century until Ernesto Campos, while revising the

, noted that most species with a 2-segmented palp on the third

maxilliped An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including anten ...

were anomalous in ''Pinnotheres''. This led to the

segregation Segregation may refer to: Separation of people * Geographical segregation, rates of two or more populations which are not homogenous throughout a defined space * School segregation * Housing segregation * Racial segregation, separation of humans ...

of multiple genera, of which ''Tunicotheres'', defined in 1996, was one.

Description

Members of this family are tiny, soft-bodied crabs commonly known as pea crabs. Males of this species have

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

widths of up to and females of up to .

Ecology

This crab has been found as an

endosymbiont An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within" ...

of several species of

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

tunicate including '' Styela plicata'', '' Phallusia nigra'', ''

Molgula occidentalis ''Molgula occidentalis'' is a species of marine invertebrate of the family Molgulidae. The scientific name of the species was validated and published for the first time in 1883 by Traustedt. ''URMO: UNESCO-IOC Register of Marine Organisms''. Land ...

'' and '' Polycarpa spongiabilis''. Usually, a single crab occupies the atrial chamber of a single tunicate. Questing males will seek to enter tunicates that are occupied by unberried females (those not brooding eggs), but will avoid those containing males, or females carrying eggs and developing larvae. ''Tunicotheres moseri'' has a reduced developmental cycle, with the eggs and all larval stages being retained in the brood pouch of the female. The developing young finally leave the abdominal pouch and the host tunicate as first

instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ass ...

juveniles. The crabs are found in solitary residence in their tunicate host far more often than would be expected statistically. This seems to be due to

territoriality In ethology, territory is the sociographical area that an animal consistently defends against conspecific competition (or, occasionally, against animals of other species) using agonistic behaviors or (less commonly) real physical aggression. ...

, but does not occur because of aggression between a resident and an invading crab; the resident crab stations itself close to the atrial siphon and in most cases the intruder is deterred from attempting to enter. Additionally, the crabs seem to use an agonistic avoidance strategy and likely detect, by chemical or tactile cues, which tunicates are already occupied, moving on promptly from occupied hosts to find unoccupied ones.

References

{{Taxonbar, from=Q11848823 Pinnotheroidea Monotypic crustacean genera Crustaceans of the Atlantic Ocean Fauna of the Caribbean Crustaceans described in 1918