The tube-eye or thread-tail, Stylephorus chordatus, is a deep-sea Lampriformes 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. It is an extremely elongated fish: although its body grows only to 28 centimetres (11 in) long, it has a pair of tail fin rays that triple its length to about 90 centimetres (35 in). 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 forty times its original size. It then expels the water through the gills, leaving behind the copepods on which it feeds.
The phylogenetic position of the tube-eye amongst Lampriformes has been challenged by an study involving molecular mitochondrial and nuclear DNA sequences analysis. It suggested that Stylephorus is not a Lampriformes at all, but a close parent of the Gadiformes order (cods and hakes).