Telescopefish are small, deep-sea aulopiform fish comprising the small family Giganturidae. The two known species are within the genus Gigantura. Though rarely captured, they are found in cold, deep tropical to subtropical waters worldwide.
The common name of these fish is related to their bizarre, tubular eyes. The genus name Gigantura refers to the Gigantes, a race of giants in Greek mythology—coupled with the suffix oura, meaning 'tail', thus Gigantura refers to the greatly elongated, ribbon-like lower half of the tailfin that may comprise over half of the total body length.
The Giganturidae are slender, slightly tapered fish with large heads dominated by large, forward-pointing, telescoping eyes with large lenses. Their heads end in short, pointed snouts. The highly extensile mouth is lined with sharp, slightly recurved and depressible teeth and it extends well past the eyes. The body lacks scales, but is covered in easily abraded, silvery guanine, which imparts a greenish to purplish iridescence in life. The gas bladder is absent and the stomach is highly distensible.
The transparent fins are spineless; the deeply forked and hypocercal caudal fin is most striking, with the lower lobe extended to a length exceeding that of the body. The pectoral fins are large (about 30–42 rays), situated above the gill opening, and inserted horizontally. The anal fin (about 8–14 rays) and single dorsal fin (about 16–19 rays) are both situated far back of the head. The pelvic fins and adipose fin are absent.
Also absent are the premaxilla, orbitosphenoid, parietal, symplectic, posttemporal, and supratemporal bones, the gill rakers, and the branchiostegal rays. The loss of these structures is attributed to neoteny; that is, the retention of larval characteristics.
Gigantura indica is the larger of the two species at about 20.3 cm standard length (a measurement excluding the caudal fin). However, Gigantura chuni (at about 15.6 cm standard length) is slightly more robust in build.