The Pacific viperfish, Chauliodus macouni, is a predatory fish that lives in the abyssal depths of the deep sea. In daytime it can be found from 200–5000 m below the ocean surface. At night it swims up into shallower depths of less than 200m where food is more plentiful. Pacific viperfish will mostly eat crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s and small fish. They typically reach lengths of up to 1 foot and are considered an example of deep-sea gigantism. It is believed that its first dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

's photophore

Photophore

A photophore is a light-emitting organ which appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors...

s are used to attract prey.

Pacific viperfish can be characterized by its large mouth, long fang-like teeth and long dorsal fin ray (as much as half its body length). They are iridescent dark silver-blue color in life with pale fins. They can also be a light black color with blue fins.