Many deep-sea fishes have sensory adaptations for living at great depths with very limited light. While such adaptations are best known in the visual system, it is likely that there are also adaptations in the auditory system that enable deep-sea fishes to use the "auditory scene." However, there are few data on the inner ear of deep-sea fishes. The purpose of this study was to add to those data. Since deep-sea fishes are rarely taken alive, this study was done through comparative anatomical investigations. Three families were chosen from two major deep-sea fish fauna: benthopelagic and mesopelagic.

    In Antimora rostrata (family Moridae, deep-sea cods), the inner ear structure and its coupling to the swim bladder were analyzed and compared with similar systems found in shallow-water fishes. Part of the membrane labyrinth is thick and rigid. The elaborate structure of the saccular epithelium and the close contact between the ear and swim bladder suggests enhanced hearing sensitivity. 

    In the family Melamphaidae (bigscales and ridgeheads), five species from three genera show broad interspecific variation in the saccular otolith shapes, including having a long otolithic "stalk" in two genera. The presence of this "stalk" corresponds with a gradual change in the saccular maculae. A special type of ciliary bundle on the saccule may have enhanced sensitivity to bundle displacements.

    Ears were compared between six species of Macrouridae (grenadiers and rattails) that live at different depths. The saccule/lagena size ratio seems to increase with depth, especially between a mesopelagic and a benthopelagic species in the genus Nezumia, in which the benthopelagic species has an enlarged saccule associated with sound production. 

    These findings support the hypothesis that some deep-sea fishes have evolved specializations for inner ear function. While it is not possible to test hearing in deep-sea fishes, the various adaptations found suggest that at least some such species have evolved specialized structures to enable them to use sound in the deep-sea. Some features in the ears of deep-sea fishes that have never been seen in the ears of other vertebrates, which further reveals the structural diversity of fish inner ears in general.