''Stylephorus chordatus'', the tube-eye or thread-tail, is a deep-sea

ray-finned fish Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of sk ...

, the only species in the genus ''Stylephorus'' and family Stylephoridae. It is found in deep subtropical and tropical oceans around the world, living at depths during the day and making nightly vertical migrations to feed on

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

. It is an extremely elongated fish; although its body grows only to long, its pair of tail fin rays triple its length to about . Its eyes are tubular in shape, resembling a pair of binoculars. It has a tubular mouth through which it sucks seawater by enlarging its oral cavity to about 40 times its original size. It then expels the water through the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s, leaving behind the

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s on which it feeds. The phylogenetic position of the tube-eye has been controversial. It has been historically placed amongst

Lampriformes Lampriformes is an order (biology), order of Actinopterygii, ray-finned fish. Members are collectively called lamprids (which is more properly used for the Opah, Lampridae) or lampriforms, and unite such open-ocean and partially Deep sea, deep-s ...

, but a study involving

mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

and

nuclear DNA Nuclear DNA (nDNA), or nuclear deoxyribonucleic acid, is the DNA contained within each cell nucleus of a eukaryotic organism. It encodes for the majority of the genome in eukaryotes, with mitochondrial DNA and plastid DNA coding for the rest. ...

sequences analysis suggested ''Stylephorus'' is instead a close relative of the order

Gadiformes Gadiformes , also called the Anacanthini, are an order of ray-finned fish that include the cod, hakes, pollock, haddock, burbot, rocklings and moras, many of which are food fish of major commercial value. They are mostly marine fish found thr ...

(cods and hakes).

References

Gadiformes Monotypic fish genera Fish described in 1791 {{Gadiformes-stub