''Carapus acus'' 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 ...

of bony fish in the family

Carapidae Pearlfish are marine fish in the ray-finned fish family Carapidae. Pearlfishes inhabit the tropical waters of the Atlantic, Indian, and Pacific Oceans at depths to , along oceanic shelves and slopes. They are slender, elongated fish with no sca ...

, the pearlfishes, and is native to the Mediterranean Sea and the Atlantic Ocean. It lives as 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 ...

in association with a

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 ...

, spending the day inside its host and emerging at night to feed.

Description

''Carapus acus'' is a laterally-compressed, elongated fish growing to a length of about . The head measures about one eighth of the total length and there are 84 to 92

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

e, which gives the fish great flexibility. The continuous

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...

and the

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

run the whole length of the body. There is no caudal fin and the tail ends with a point. The gut loops back and the

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 ...

is located just behind the head, in front of the pectoral fins. This fish is translucent, with a number of silvery or reddish-gold iridescent spots on the operculum and the thoracic region. The

peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesoth ...

lining the body cavity is an opaque silvery colour.

Distribution

''Carapus acus'' is native to 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 ...

and eastern

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 ...

; it is common in the western half and around the coasts of Spain, France, Italy, Sardinia and the Balearic Islands. It is less common in the

Adriatic The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkans, Balkan Peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Sea) ...

and

Aegean Sea The Aegean Sea ; tr, Ege Denizi (Greek language, Greek: Αιγαίο Πέλαγος: "Egéo Pélagos", Turkish language, Turkish: "Ege Denizi" or "Adalar Denizi") is an elongated embayment of the Mediterranean Sea between Europe and Asia. It ...

Ecology

''Carapus acus'' usually lives inside the

respiratory tree The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to th ...

or the

body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, and ...

of sea cucumbers. Neither the species nor the size of the host seem important in host choice, and the fish makes use of whatever sea cucumber it happens to encounter; the most common hosts are '' Parastichopus regalis'' and '' Holothuria tubulosa'', while others include, '' H. poli'', '' H. arguinensis'', '' H. sanctori'' and '' H. mammata''. The fish swims close to the seabed searching for a suitable host. When it finds one, it positions its head beside the sea cucumber's anus, from which a current of water is being expelled. Either it then bends its flexible tail around, inserts it in the sea cucumber's anus and works its way backwards inside, with the tail entering first, or less commonly, it thrusts its head into the orifice and enters with a sharp flick of its tail, turning round when inside the host. In either case, it positions itself with its head close to or protruding from the sea cucumber's anus. Because the fish's anus is located just behind its head, it is able to defecate into the open water without fouling its host. Pearlfishes are able to produce sounds by means of certain muscles connected to the

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled Organ (anatomy), organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their curren ...

, with each species producing a sound unique to its species. The function of these sounds is unclear, various hypotheses have been proposed including finding a mate, indicating the size of the fish while it is concealed in its host, and establishing a territory. Sometimes more than one fish can occupy the same host, and mating inside the host is possible. Breeding takes place between July and September. A cluster of eggs is produced which rises to the water surface. The eggs hatch into

fish fry A fish fry is a meal containing battered or breaded fried fish. It usually also includes french fries, coleslaw, macaroni salad, lemon slices, tartar sauce, hot sauce, malt vinegar and dessert. Some Native American versions are cooked by coati ...

after three days. When these larvae have grown to a length of about they undergo

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

into juveniles, becoming elongated and laterally flattened. At this time they move to the seabed to find a suitable host. Juvenile fish may nibble the reproductive organs or the gills of their host; this causes little damage because sea cucumbers have great regenerative powers. Older fish are nocturnal, emerging from their host and feeding on small crustaceans, worms, and fish fry, before returning to their hosts.

References

{{Taxonbar, from=Q2271222 Carapidae Fish of the Atlantic Ocean Fish of the Mediterranean Sea Fish described in 1768 Taxa named by Morten Thrane Brünnich