Abstract:
The basal hearing mechanism in all fish is through particle acceleration detection by the inner ear. Pressure sensitivity occurs through ancillary hearing structures, which typically increases the fishes hearing sensitivity or expands their hearing range. Unlike some species of fish, elasmobranchs are not known to possess any pressure sensitive mechanisms. However, elasmobranchs have a unique hearing anatomy, where they have a direct connection to the water column through the endolymphatic duct that connects to the poorly defined macula neglecta. It is thought that this unique anatomy may provide some form of pressure detection in sharks helping contribute to increased auditory sensitivity. For that reason, this study set out to define the auditory abilities of two species of sharks, the carpet shark (Cephaloscyllium isabella) and school shark (Galeorhinus galeus) as well as determine if any ancillary hearing structures are contributing to shark hearing. This was done using the auditory evoked potential (AEP) technique to different stimuli, a particle acceleration stimulus (shaker table), and a mixed stimulus of particle motion and pressure (underwater speaker). Both shark species displayed typical AEP responses to both the shaker and speaker stimuli. In response to the shaker stimulus C. isabella and G. galeus responded between 40-200 Hz, however when exposed to the speaker stimulus the frequency bandwidth the frequency bandwidth was significantly wider 80-1000 Hz. Blocking the endolymphatic pores/ the underlying parietal fossa with a metal plate to inhibit the macula neglecta resulted in a significant decrease in frequency bandwidth (80-600 Hz) when exposed to the speaker stimulus. The results of this study suggest that the macula neglecta is acting an ancillary hearing mechanism allowing them to detect sound pressure to expand their auditory bandwidth. This result would be the first to classify the macula neglecta as a pressure sensitive endorgan as well as present the first electrophysiological evidence that elasmobranchs can detect acoustic pressure.
