Polymixia is the only extant genus of the order Polymixiiformes and family Polymixiidae. It contains 12 species, all of which live in deepwater marine environments. They are found in tropical and subtropical waters of the Atlantic, Indian and western Pacific Oceans. They are bottom-dwelling fish, found down to about 800 m (2,600 ft). Most are relatively small fish, although one species is over 40 cm (16 in) in length. They can be considered "living fossils" due to being the only surviving members of the once-diverse order Polymixiiformes.

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