Pearlfish are marine fish in the

ray-finned fish 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 h ...

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

Carapidae. Pearlfishes inhabit the

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

waters of the

Atlantic 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 an ...

Indian Indian or Indians may refer to: Peoples South Asia * Indian people, people of Indian nationality, or people who have an Indian ancestor ** Non-resident Indian, a citizen of India who has temporarily emigrated to another country * South Asia ...

, and

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

s at depths to , along oceanic shelves and slopes. They are slender, elongated fish with no scales, translucent bodies, and dorsal fin rays which are shorter than their anal fin rays. Adults of most species live

symbiotically Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

inside various invertebrate hosts, and some live parasitically inside sea cucumbers. The

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e are free living.

Characteristics

Pearlfishes are slender, distinguished by having

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 c ...

rays that are shorter than their anal fin rays. They have

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 scattering of light. On a macroscopic scale (one in which the dimensions ...

, scaleless bodies reminiscent of

eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...

s. The largest pearlfish are about in length. They reproduce by laying oval-shaped eggs, about 1 mm in length.

Ecology

Pearlfish preserved emerging from anus of sea cucumber

Pearlfishes are unusual in that the adults of most species live inside various types of

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s. They typically live inside

clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shel ...

s, starfish, or

sea squirt 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" m ...

s, and are simply

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 ...

, not harming their hosts. However, some species are known to be parasitic on sea cucumbers, eating their gonads and living in their anal pores. Pearlfish usually live alone, or in pairs.The fish that lives in a sea cucumber anus
Australian Geographic, 8 August 2014 Regardless of the habits of the adults, the

e of pearlfish are free-living among the

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

. Pearlfish larvae can be distinguished by the presence of a long filament in front of their

s, sometimes with various appendages attached.

Genera

The genera are divided into three major groupings based on their level of symbiosis: * ''

Echiodon ''Echiodon'' is a genus of pearlfishes, with these currently recognized species: * ''Echiodon anchipterus'' J. T. Williams, 1984 (closefin pearlfish) * '' Echiodon atopus'' M. E. Anderson, 2005 * '' Echiodon coheni'' J. T. Williams, 1984 * '' ...

'' and '' Snyderidia'' - free-living * ''

Carapus ''Carapus'' is a genus of pearlfishes, with these currently recognized species: * ''Carapus acus'' ( Brünnich, 1768) (pearlfish) * ''Carapus bermudensis'' ( J. M. Jones, 1874) (Atlantic pearlfish) * ''Carapus dubius'' ( Putnam, 1874) (Pacific pe ...

'' and ''

Onuxodon ''Onuxodon'' is an Indo-Pacific genus of pearlfishes from the family Carapidae. The generic name is derived from the Greek ''onyx'' meaning "claw" and ''odon'' meaning "tooth", referring to the sharp fang like teeth of ''Onuxodon parvibrachium ...

'' - commensal * ''

Encheliophis ''Encheliophis'' is a genus of pearlfishes, with six currently recognized species: * '' Encheliophis boraborensis'' ( Kaup, 1856) (pinhead pearlfish) * '' Encheliophis chardewalli'' Parmentier, 2004 * '' Encheliophis gracilis'' ( Bleeker, 1856) ...

'' -

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

, fish in this group live in invertebrate hosts found in shallow-water

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...

communities such as bivalves, sea cucumbers, and starfish.

References

* * {{Taxonbar, from=Q1307559 Parasitic vertebrates