Functional Morphology of the Inner Ear of Southern Bluefin Tuna, Thunnus maccoyii (Scombridae, Teleostei): Adaptations for Hearing Within a Pelagic Environment

ABSTRACT

The inner ears of teleosts are structurally diverse, reflecting the crucial role of audition in sound detection, balance, and social communication within each species’ ecological niche. However, very little is known about the inner ear morphology of large pelagic teleosts. This study investigates the functional morphology of the inner ears of southern bluefin tuna Thunnus maccoyii , using scanning electron microscopy and magnetic resonance imaging, and examines the auditory sensory epithelia (maculae) of the three otolithic end organs (saccule, lagena, and utricle). The results show the inner ears of T. maccoyii are enveloped in compact otic capsules, cartilage‐like semicircular canals, and an interaural angle (35° from the midline) between the left and right saccules. The hair cell types are found to be heterogeneously distributed, where hair cell types with longer kinocilia lie along the periphery of the maculae, while the hair cell type with shorter kinocilium lies along the center of the maculae. Six groups of hair cells with opposing orientations are found in the saccular macula, where each opposing orientation group is innervated by separate rami of the VIIIth cranial nerve. The mean hair cell densities of the saccule, lagena, and utricle are 20,400, 39,300, and 36,400 mm ⁻² , respectively. The lagenar and utricular maculae are only partially covered by the overlying otolith. These findings contribute to the understanding of morphological adaptations of inner ears in pelagic teleosts and highlight the need for further research on structure‐function relationships in the auditory system of these species.