Abstract

Fundamental knowledge regarding the early development of vertebrate hearing can be gained from research on fish auditory systems. Although elasmobranchs hold a crucial basal position in the evolution of vertebrates, there is comparably little information available regarding their auditory systems compared to that of bony fishes. Although the neurological system is highly malleable, little is known about how the many sensory epithelia in the inner ear of elasmobranchs change over the course of a lifetime. In the saccule, lagena, utricle, and macula neglecta of school sharks (Galeorhinus galeus) of various body sizes, we measured macular area, the quantity of sensory hair cells, hair cell density, and hair cell orientations using immunohistochemistry and fluorescence microscopy. Macular area and the total number of hair cells significantly increased in all maculae throughout ontogeny, although hair cell density showed a parallel ontogenetic decline (excluding the utricle). The four maculae also differed significantly in terms of macular area, hair cell quantity, and hair cell density. However, the orientation patterns of hair cells did not differ between people or alter as the body grew. These results demonstrate morphological alterations that may have ramifications for hearing ability over ontogeny and are among the most thorough characterizations of the inner ear sensory epithelia in an elasmobranch.

Keywords: Inner ear maculae; Sharks; Ontogeny