Ontogenetic change in predicted acoustic pressure sensitivity in larval red drum (Sciaenops ocellatus)

Detecting acoustic pressure can improve a fish's survival and fitness through increased sensitivity to environmental sounds. Pressure detection results from interactions between the swim bladder and otoliths. In larval fishes, those interactions change rapidly as growth and development alter bladder dimensions and otolith-bladder distance. We used computed tomography imagery of lab-reared larval red drum (Sciaenops ocellatus) in a finite-element model to assess ontogenetic changes in acoustic pressure sensitivity in response to a plane wave at frequencies within the frequency range of hearing by fishes. We compared the acceleration at points on the sagitta, asteriscus and lapillus when the bladder was air filled with results from models using a water-filled bladder. For larvae of 8.5-18 mm in standard length, the air-filled bladder amplified simulated otolith motion by a factor of 54-3485 times that of a water-filled bladder at 100 Hz. Otolith-bladder distance increased with standard length, which decreased modeled amplification. The concomitant rapid increase in bladder volume partially compensated for the effect of increasing otolith-bladder distance. Calculated resonant frequency of the bladders was between 8750 and 4250 Hz, and resonant frequency decreased with increasing bladder volume. There was a relatively flat frequency dependence of these effects in the audible frequency range, but we found a small increase in amplification with increasing excitation frequency. Using idealized geometry, we found that the larval vertebrae and ribs have negligible influence on bladder motion. Our results help clarify the auditory consequences of ontogenetic changes in bladder morphology and otolith-bladder relationships during larval stages.