The pinhead pearlfish, ''Encheliophis boraborensis'', is a species of slender,

ray-finned Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species. The ray-finned fishes are so called because their fins are webs of skin supported by bony or hor ...

fish in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

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 ...

found in the tropical Indo-Pacific Ocean; it normally lives inside 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 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 ...

such as the pineapple sea cucumber (''

Thelenota ananas ''Thelenota ananas'', also known as pineapple sea cucumber, oloturia ananas, tripang, prickly skin cucumber, pointed teat sea cucumber, armoured sea cucumber, giant sea cucumber, sand fish or prickly redfish, is a species of sea cucumber found in ...

'') or the leopard sea cucumber (''

Bohadschia argus ''Bohadschia argus'', the leopard sea cucumber, is a species of marine invertebrate in the family Holothuriidae. It is the type species of the genus ''Bohadschia''; Jaeger, 1833. Description ''Bohadschia argus'' is sausage-shaped with a smooth, ...

'').

Description

The pinhead pearlfish is a slender, tapering, eel-like fish that can grow to a length of . It has very small eyes and near the jaws are star-shaped

melanophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, ...

s which are more numerous in older specimens. This fish has no scales or pelvic fins, but does have short pectoral fins and an elongated dorsal fin with 31 to 42 soft rays. The equally long anal fin has 45 to 57 soft rays, and ''C. boraborensis'' is the only species in the family to have melanophores on its anal fins. It is a medium brown-coloured fish with a solid body and much enlarged

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 ...

that occupies most of its body cavity.

Distribution

The pinhead pearlfish is found in the Indo-Pacific Ocean at depths to . Its range extends from

Mauritius Mauritius ( ; french: Maurice, link=no ; mfe, label=Mauritian Creole, Moris ), officially the Republic of Mauritius, is an island nation in the Indian Ocean about off the southeast coast of the African continent, east of Madagascar. It incl ...

and the

Society Islands The Society Islands (french: Îles de la Société, officially ''Archipel de la Société;'' ty, Tōtaiete mā) are an archipelago located in the South Pacific Ocean. Politically, they are part of French Polynesia, an overseas country of the F ...

, northwards to

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the nort ...

and the

Yaeyama Islands The Yaeyama Islands (八重山列島 ''Yaeyama-rettō'', also 八重山諸島 ''Yaeyama-shotō'', Yaeyama: ''Yaima'', Yonaguni: ''Daama'', Okinawan: ''Yeema'', Northern Ryukyuan: ''Yapema'') are an archipelago in the southwest of Okinawa ...

, including the

Mariana Islands The Mariana Islands (; also the Marianas; in Chamorro: ''Manislan Mariånas'') are a crescent-shaped archipelago comprising the summits of fifteen longitudinally oriented, mostly dormant volcanic mountains in the northwestern Pacific Ocean, betw ...

and

Caroline Islands The Caroline Islands (or the Carolines) are a widely scattered archipelago of tiny islands in the western Pacific Ocean, to the north of New Guinea. Politically, they are divided between the Federated States of Micronesia (FSM) in the centra ...

Biology

''E. boraborensis'' is normally found living within the body cavity of a sea cucumber which it enters through the anus. The leopard sea cucumber (''B. argus'') is favoured and the pineapple sea cucumber (''T. ananas'') is also used for this purpose. Competition between carapid fish occurs for the right to occupy a host. Two male adults were observed to fight when inside the sea cucumber until one was killed, other fish have been found with bite marks on its tail and one adult was found to have a juvenile silver pearlfish (''

Encheliophis homei The silver pearlfish, ''Encheliophis homei'', is a species of eel-like fish in the family Carapidae. This pearlfish lives inside the coelom of sea cucumbers such as ''Bohadschia argus'', '' Thelenota ananas '', and '' Stichopus chloronotus''. It ...

'') in its stomach. Whereas ''Encheliophis homei'' emerges at night from its host to forage, ''Encheliophis boraborensis'', with its small eyes and specialised mouthparts, may at least in part feed on its host's tissues. When ''E. boraborensis'' enters a host and finds a carapid fish already present, it makes a noise. The sounds emitted are regular pulses and the timing in males and females is different, so it is possible to tell the sex of the fish from the sounds it makes. If the fish already present is the closely related silver pearlfish, the sound is often reduced to a single longer pulse. When ''E. homei'' enters a sea cucumber already occupied by ''E. boraborensis'', the sound it emits is also often reduced to a single pulse, but in this case it is shorter than its normal call. Both fish are able to change their calls, adapting them to whichever species of fish they encounter inside their hosts.

References

{{Taxonbar, from=Q2036785 Carapidae Taxa named by Johann Jakob Kaup Fish described in 1856