Chauliodus

A viperfish is any species of marine fish in the genus Chauliodus. Viperfish are characterized by long, needle-like teeth and hinged lower jaws. A typical viperfish grows to lengths of 30 to 60 cm (12 to 23.5 in). Viperfish stay near lower depths (250–5,000 feet [80–1,520 m]) in the daytime and shallower depths at night, primarily in tropical and temperate waters. Viperfish are believed to attack prey after luring them within range with light-producing organs called photophores, which are located along the ventral sides of its body, and with a prominent photophore at the end of a long spine in the dorsal fin reminiscent of the illicium of the unrelated deepsea anglerfishes. The viperfish flashes this natural light on and off, at the same time moving its dorsal spine around like a fishing rod and hanging completely still in the water. It also uses the light producing organ to communicate to potential mates and rivals.

Viperfish vary in color from green, silver, to black. A viperfish uses its fang-like teeth to immobilize prey and would not be able to close its mouth because of their length, if it were not able to fold and curve them behind its head. The first vertebra behind the head of the viperfish absorbs the shock of biting prey. As with other deepsea fish, they are able to undergo long periods with scarcely any food.

Viperfish are believed to live from 30 to 40 years in the wild, but in captivity they rarely live more than a few hours. Some species of dolphins and sharks are known to prey upon viperfish. Scientists believe they can swim at a speed of two body lengths per second, but this is not yet an official speed.

Although it may look like it is covered in scales, it in fact is covered by a thick, transparent coating of unknown substance. Extremely large, fang-like teeth give the fish a slightly protruded lower jaw which makes catching prey easy for this deep-sea predator. The viperfish is lined with three different types of photophores which some speculate is used to lure in unsuspecting prey  They have microscopic spheres without a pigment layer that are scattered over the dorsal side, large spheres with a pigment coat, reflectors, and lens, and finally, large, bell-shaped organs with a pigment coat, reflectors, and lens that are grouped together in rows along the dorsal surface. Photophores can also be seen along the ventral and lateral surface of the fish.

Habitat
Because viperfish live in bathypelagic environments inaccessible to humans, very little is known about their habits. They are found from a depth of 1000 to 4000 meters with an average temperature of 4 degrees Celsius. Viperfish are thought to engage in a daily vertical migration, as they have been observed in the mesopelagic region during the night, which lies directly above the bathypelagic region. More direct observation is needed to confirm this assumption, however.

Feeding
Viperfish do not appear to have any preferred prey: stomach contents of captured individuals have contained lanternfish, bristlemouthes, and other fish, suggesting that they attack and swallow whatever random prey they encounter. With their dark coloration, they can appear invisible and have been observed to remain motionless for hours ready to strike on unsuspecting prey. Prey is captured and killed by being pierced by the viperfish's long teeth as the prey is grasped in the viperfish's mouth: subdued prey is then swallowed whole. The photophores along the viperfish's belly are thought to help lure prey closer to it, though further observation is needed to confirm this hypothesis.

Species
There are currently nine, extant, recognized species in this genus: At least two more species are recognized from Late Miocene-aged fossils:
 * Chauliodus barbatus  Garman, 1899
 * Chauliodus danae  Regan & Trewavas, 1929  (Dana viperfish)
 * Chauliodus dentatus  Garman, 1899
 * Chauliodus macouni  T. H. Bean, 1890  (Pacific viperfish)
 * Chauliodus minimus  Parin & Novikova, 1974
 * Chauliodus pammelas  Alcock, 1892
 * Chauliodus schmidti  Ege, 1948
 * Chauliodus sloani  Bloch & J. G. Schneider, 1801  (Sloane's viperfish)
 * Chauliodus vasnetzovi  Novikova, 1972
 * Chauliodus eximus,  (Jordan, 1925), originally Eostomias eximus, from Late Miocene California
 * Chauliodus testa,  Nazarkin, 2014, from the Late Miocene of Western Sakhalin Island