''Stylephorus chordatus'', the tube-eye or thread-tail, is a deep-sea fish, the only fish in the genus ''Stylephorus'' and family Stylephoridae. It is found in deep subtropical and tropical waters 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 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 ...

. 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 respiratory organ that many 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 respiration on land provided they are ...

s, leaving behind the copepods 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 of ray-finned fish. Members are collectively called lamprids (which is more properly used for the Lampridae) or lampriforms, and unite such open-ocean and partially deep-sea Teleostei as the crestfishes, oarfish, opah ...

, but a study involving mitochondrial 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. I ...

sequences analysis suggested ''Stylephorus'' is instead a close parent of the

Gadiformes Gadiformes are an order of ray-finned fish, also called the Anacanthini, that includes the cod. Many major food fish are in this order. They are found in marine waters throughout the world and the vast majority of the species are found in tempe ...

order (cods and hakes).

References

Stylephoridae Monotypic fish genera Fish described in 1791 {{Paracanthopterygii-stub