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Sternoptychidae (hatchetfish; superorder Protacanthopterygii, order Salmoniformes) A small family of marine fish that have a deep, short, and very compressed body, the outline of the belly resembling the blade of a hatchet. The mouth is large and directed upward. Many photophores are present on the flanks of Argyropelecus lychnus (silvery hatchetfish), 6 cm. There are about 27 species, distributed world-wide at depths of 100–4000 m.