''Parastichopus regalis'', also known as the royal sea cucumber, is a

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 ...

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 ...

in the family

Stichopodidae The Stichopodidae are a family of sea cucumbers, part of the order Synallactida. Description Members of this family are mostly large or medium-sized holothuroids with a squarish cross-section, a flat ventral surface, and large, fleshy, cone-s ...

Description

''Parastichopus regalis'' grows to a length of about . The body is soft and somewhat flattened, and a clearly incised lateral fold with large papillae separates the dorsal surface from the ventral surface. The dorsal surface is mottled brown, and the ventral surface is rather paler, with a darker longitudinal streak in the centre. The bony

sclerite A sclerite (Greek , ', meaning "hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly ...

s that strengthen the integument include tables, terminal plates and perforated plates; the sclerites in the

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s that surround the mouth include elongated, branched, perforated and arched rods.

Distribution and habitat

''Parastichopus regalis'' is found in the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

, along the coasts of Africa in the eastern Atlantic Ocean, and in the tropical and subtropical western Atlantic Ocean, including 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 ...

. It is typically found on sand or other soft substrates, at depths down to about ; it is commonest in the depth range.

Ecology

In a

commensal 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 ...

arrangement, ''Parastichopus regalis'' is often host to the pearlfish ''

Carapus acus ''Carapus acus'' is a species of bony fish in the family Carapidae, the pearlfishes, and is native to the Mediterranean Sea and the Atlantic Ocean. It lives as a commensal in association with a sea cucumber, spending the day inside its host and e ...

'', which enters the sea cucumber's respiratory tree via its

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

. The fish lives inside the sea cucumber during the day, emerging into the open water at night to feed. Two fish may occupy a single host, and a pair have been known to breed inside their host. When stressed, it autotomises its internal organs, regenerating them again later.

Status

This sea cucumber is a common species. It is edible and is eaten in parts of its range but is not a targeted species, being caught as

bycatch Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juve ...

during deepwater trawling.

References

{{Taxonbar Stichopodidae Fauna of the Atlantic Ocean Fauna of the Mediterranean Sea Animals described in 1817 Taxa named by Georges Cuvier