Species of the deep-sea stomiid fish genus Malacosteus are readily diagnosed within the Malacosteinae by the presence of a single round nostril on each side of the snout, palatine bones without teeth, and a single large tear-shaped accessory light organ positioned below the anterior margin of the eye. In contrast to the many recent investigations into the biochemical basis of the red-shifted bioluminescence produced by the accessory light organ, Malacosteus has received little taxonomic scrutiny in the last 75 years. Four nominal species of Malacosteus have been described: M. niger, M. indicus, M. choristodactylus, and M. danae. Two species are considered valid: M. niger and a new species known from 182 specimens captured in temperate and sub-Antarctic waters of the southern hemisphere, the tropical Indian Ocean, and waters of the Indo-Australian Archipelago. Diagnosed primarily on the basis of postorbital photophore size and serial photophore counts, the new species differs further from its only known congener in having relatively small jaws and fleshy orbit.

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