Telescopefish are small, deep-sea aulopiform fish comprising the small family Giganturidae. There are just two known species, both within the genus Gigatura. Though rarely captured, they are found in cold, deep tropical to subtropical waters worldwide.
These fish are understandably named for their bizarre, tubular eyes. The genus name Gigantura is in reference to the Gigantes, a race of giants in Greek mythology””coupled with the suffix oura, meaning “tail”, thus Gigantura refers to the ribbon-like lower half of the tailfin.
The Giganturidae are slender, slightly tapered fish with large heads dominated by large, forward-pointing telescoping eyes with large lenses. The head ends in a short, pointed snout. 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.
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 (ca. 30″“42 rays), situated above the gill opening, and inserted horizontally. The anal fin (ca. 8″“14 rays) and single dorsal fin (ca. 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 in terms of length at 8 in (20.3 cm) standard length (a measurement excluding the caudal fin). However, Gigantura chuni (at 6.14 in (15.6 cm) standard length is slightly more robust in build.
Telescopefish are presumed to be solitary, active predators, frequenting the mesopelagic to bathypelagic zones of the water column, from 1640.42 ft (500 m) down to 1.86 mi (3000 m). By using their tubular, large-lensed eyes””which are adapted for optimal binocular light collection, at the expense of lateral vision””Telescopefish are likely able to spy their prey’s weak bioluminescence from a distance, as well as (by looking skyward) resolve the outlined silhouettes of prey against the gloom above. Their eyes may also help Telescopefish to better judge distance of prey; these visual adaptations are typical of deep-sea fish (cf. barrel-eye, tube-eye). Common prey items include Bristlemouths, Lanternfish, and Barbeled Dragonfish. Owing to the Telescopefishes’ highly extensile jaws and distensible stomachs, they are able to swallow prey larger than themselves; this is also a common adaptation to life in the lean depths (cf. Sabertooth fish, Black Seadevil).
Much less is known of their reproductive habits. They are presumed to be non-guarding pelagic spawners, releasing eggs and sperm indiscriminately into the water. The fertilized eggs are buoyant and become incorporated into the zooplankton, wherein they and the larvae remain””likely at much shallower depths than the adults””until metamorphosis into juvenile or adult form.