An order of oceanic, mostly deep-water fishes that are structurally diverse and rare; most of the 41 species are known from one or a few specimens. Thus, their scientific study has been hindered, the anatomy is imperfectly known, and the relationships are in dispute. Five of the 10 families and 11 of the 21 genera currently placed in the order have been described since World War II. There is no fossil record.