T. H. Bean, 1890
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 crustaceans and small fish. They typically reach lengths of up to 1 foot and are considered an example of deep-sea gigantism.
According to O'Day (1973) luminescent silhouetting may aid the fish in mating, spacing themselves out as they hunt, maintaining conspecific aggregations, warning potential predators of their own formidable size, or perhaps allowing them to escape from predators by temporarily blinding them. These functions, however, remain speculative. 
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.
As most fish species, the Pacific viperfish has several parasites. In 2018, Susumu Ohtsuka, Dhugal J. Lindsay and Kunihiko Izawa described a new genus and species of pennellid copepod, Protosarcotretes nishikawai, from a single ovigerous female infecting a Pacific viperfish collected from the deep-waters of Suruga Bay, Japan. According to the authors, the new genus had the most plesiomorphic states in the first to fourth legs of pennellid copepods.
- O'Day, W. T. (1973). Luminescent silhouetting in stomiatoid fishes. Contributions in Science, 246. Natural History Museum, Los Angeles County PDF.
- Ohtsuka, Susumu; Lindsay, Dhugal J.; Izawa, Kunihiko (2018). "A new genus and species of the family Pennellidae (Copepoda, Siphonostomatoida) infecting the Pacific viperfish Chauliodus macouni". Parasite. 25: 6. doi:10.1051/parasite/2018003. ISSN 1776-1042.
- "Chauliodus macouni". Integrated Taxonomic Information System. Retrieved 30 January 2006.
- Froese, Rainer and Pauly, Daniel, eds. (2020). "Chauliodus macouni" in FishBase. 10 2020 version.
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