The viperfish (Chauliodus sloani) is one of the nastiest-looking deep sea fish in existence, the kind of fish that H.R. Giger would design when he had a bad hangover. Growing up to twelve inches long, it has a long, thin, dark silver-blue body a little like that of a snake, and a proportionately enormous head and jaws with large eyes and extremely long, needle-like teeth that angle back from the lower jaw. The teeth of the viperfish are so long that they do not fit in its mouth, and in fact extend far enough back that it is in danger of poking its eyes out if it closes its mouth fully.
Viperfish live at huge depths in the Pacific ocean and Bering sea, though specimens have been found elsewhere. Their natural depth is up to 9000 feet below sea level in very cold water, though they have been known to swim up to within a few hundred feet of the surface at night. Like many deep sea creatures, they have a varied diet, feeding mainly on shrimp, plankton and other small fish, but occasionally catching a larger fish, which they are able to overpower and swallow with the help of large, hinged jaws and strong jaw muscles.
Viperfish are heavily equipped with photophores - patches of bioluminescent bacteria that help it to attract prey and sexual partners. Two rows of photophores extend along the back of its body, and it has a high concentration of photophores in its mouth. It also has a long, thin lure which extends from its dorsal fin. No viperfish has been photographed alive in its natural habitat (as far as I am aware) but many specimens have been recovered, and reports have been made by scientists who have observed these fish from the windows of deep sea exploration vessels. One such report from a bathyscaph noted that the viperfish seemed to hunt by hovering in the water at an angle of 45 degrees, dangling its lure in front of its open mouth.
Very little is really known about these fish, such as their lifespan (which scientists tentatively estimate to be 8 years) and mating habits (which is thought to involve external fertilization - i.e. sperm and eggs are both released into the water), and even the depth at which they prefer to live is unclear, with some sources stating that viperfish exist between 5000 to 9000 feet below the surface, and others giving their range as 500 - 2000 feet. Making a subjective judgement of my own about the relative authority of the different sources I came across, I would guess that some of the confusion arises from the fact that some sources have incorrectly given a metric depth in feet, and the rest may be due to the viperfish's nightly migration to shallower water.