Exogenous material in the inner ear of the adult Port Jackson shark, Heterodontus portusjacksoni (Elasmbranchii).

Previous studies have suggested that the inner ear of some benthic species of elasmobranchs contain only exogenous material within their otoconial organs, a unique feature within vertebrates. However, these examinations have not accounted for the possibility of otoconial degeneration or used modern experimental methods to identify the materials present. Both of these issues are addressed in this study using inner ear samples from the adult Port Jackson shark, Heterodontus portusjacksoni. A comparison of the otoconial mass in fixed specimens over short and medium time scales reveals that over those timescales the degeneration of calcium carbonate-based otoconia does not occur and confirms that calcium carbonate-based otoconia are not found within the otoconial organs of H. portusjacksoni. Additionally, microanalysis of the chemical composition and ultrastructure of the otoconial mass using energy dispersive X-ray microanalysis and scanning electron microscopy confirms that the entire otoconial mass is comprised of exogenous silicon dioxide particles, bound within a carbon matrix. This exogenous material is suggested to play an equivalent role to the otoconia found in other species of elasmobranchs, and allows both hearing and vestibular control to occur in benthic sharks that spend their lives foraging within a sandy substrate.