The inner ear of the Port Jackson shark, Heterodontus portusjacksoni: morphometric analysis using bioimaging and phalloidin staining

The inner ear plays a crucial role in detecting sound and maintaining balance in elasmobranchs, yet its morphology and function remain poorly understudied compared to teleosts. This study provides the first detailed morphometric analysis of the inner ear in the Port Jackson shark Heterodontus portusjacksoni using micro-computed tomography (µCT), fine dissection, and phalloidin labelling of macular hair cells. Measurements of key structures, including the saccule, lagena, utricle, and semicircular canals, are obtained from individuals across different life stages. Hair cell density and orientation are quantified using phalloidin staining and confocal microscopy to assess potential auditory and vestibular specialisations. The macula neglecta, a structure associated with directional hearing in elasmobranchs but not previously identified in this species, is present but relatively small, with low hair cell density, consistent with the benthic lifestyle of H. portusjacksoni. The findings suggest that this species relies more on substrate-borne vibrations and spatial stability than on acute directional hearing, aligning with its ecology in wave-exposed, rocky reef habitats. These findings contribute to the understanding of sensory adaptations in elasmobranchs and highlight the need for further research on the impacts of anthropogenic sound on elasmobranch auditory systems.