Acanthonus armatus, a deep-sea teleost fish with a minute brain and large ears.

Abstract

Acanthonus armatus, a deep-water benthopelagic fish, has, per unit body weight, the smallest brain and largest semicircular canals of any known teleost and possibly any vertebrate. Pertinent areas of the brainstem and the cerebellum are large; this observation suggests that the fish's lateral line and vestibular senses are particularly acute. The huge cranial cavity also contains heavy saccular otoliths, which may indicate that the fish is sensitive to low-frequency sound. Brain size and specialization are consistent with an apparent pattern of low energy requirement, hovering and slow movement over the deep-sea floor, and consumption of small benthic prey in a dark environment.