Acanthonus armatus, the bony-eared assfish, is a bathypelagic species of cusk-eel found in tropical and sub-tropical oceans at depths of from 1,171 to 4,415 metres (3,842 to 14,485 ft). It has been found as far north as Queen Charlotte Sound off British Columbia's coast. This species grows to a length of 37.5 centimetres (14.8 in) SL. It is the only known member of its genus. It holds the record for the smallest brain-to-body weight ratio of all vertebrates.