Interspecific Variations of Inner Ear Structure in the Deep‐Sea Fish Family Melamphaidae

Abstract

Inner ear structures are compared among three major genera of the deep-sea fish family Melamphaidae (bigscales and ridgeheads). Substantial interspecific variation is found in the saccular otoliths, including the presence of a unique otolithic "spur" in the genera Melamphaes and Poromitra. The variation in the saccular otolith is correlated with an increase in the number of hair bundle orientation groups on the sensory epithelia from the genera Scopelogadus to Poromitra to Melamphaes. The diverse structural variations found in the saccule may reflect the evolutionary history of these species. The sensory hair cell bundles in this family have the most variable shapes yet encountered in fish ears. In the saccule, most of the hair bundles are 15-20 μm high, an exceptional height for fish otolithic end organs. These bundles have large numbers of stereovilli, including some that reach the length of the kinocilium. In the utricle, the striolar region separates into two unusually shaped areas that have not been described in any other vertebrates. The brains in all species have a relatively small olfactory bulb and optic tectum, as well as an enlarged posterior cerebellar region that is likely to be involved in inner ear and lateral line (octavolateral) functions. Data from melamphaids support the hypothesis that specialized anatomical structures are found in the ears of some (if not most) deep-sea fishes, presumably enhancing their hearing sensitivity.