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

From the morphospace to the soundscape: Exploring the diversity and functional morphology of the fish inner ear, with a focus on elasmobranchsa)

Fishes, including elasmobranchs (sharks, rays, and skates), present an astonishing diversity in inner ear morphologies; however, the functional significance of these variations and how they confer auditory capacity is yet to be resolved. The relationship between inner ear structure and hearing performance is unclear, partly because most of the morphological and biomechanical mechanisms that underlie the hearing functions are complex and poorly known. Here, we present advanced opportunities to document discontinuities in the macroevolutionary trends of a complex biological form, like the inner ear, and test hypotheses regarding what factors may be driving morphological diversity. Three-dimensional (3D) bioimaging, geometric morphometrics, and finite element analysis are methods that can be combined to interrogate the structure-to-function links in elasmobranch fish inner ears. In addition, open-source 3D morphology datasets, advances in phylogenetic comparative methods, and methods for the analysis of highly multidimensional shape data have leveraged these opportunities. Questions that can be explored with this toolkit are identified, the different methods are justified, and remaining challenges are highlighted as avenues for future work.

The Distinctive Forehead Cleft of the Risso's Dolphin (Grampus griseus) Hardly Affects Biosonar Beam Formation

The Risso's dolphin (Grampus griseus) has a distinctive vertical crease (or cleft) along the anterior surface of the forehead. Previous studies have speculated that the cleft may contribute to biosonar beam formation. To explore this, we constructed 2D finite element models based on computer tomography data of the head of a naturally deceased Risso's dolphin. The simulated acoustic near-field signals, far-field signals, and transmission beam patterns were compared to corresponding measurements from a live, echolocating Risso's dolphin. To investigate the effect of the cleft, we filled the cleft with neighboring soft tissues in our model, creating a hypothetical "cleftless" forehead, as found in other odontocetes. We compared the acoustic pressure field and the beam pattern between the clefted and cleftless cases. Our results suggest that the cleft plays an insignificant role in forehead biosonar sound propagation and far-field beam formation. Furthermore, the cleft was not responsible for the bimodal click spectrum recorded and reported from this species.

Numerical modeling of the impacts of acoustic stimulus on fish otoliths from two directions

Previous experiments have shown (1) evidence that exposure to high-intensity sounds (e.g., air-gun signals) may cause damage to the sensory hair cells of the fish ears and impair fish hearing and (2) evidence that in some circumstances such exposures cause minimal structural damage. The contradictory results regarding the damage accrued suggested that the angle of sound energy arrivals at the fish ears may play a part in the propensity of the sound to cause damage to sensory hair cells. To further study this and gain insight into specific details of the differential motion of the otolith relative to the sensory macula when incident sounds arrive from different directions, three-dimensional finite element models were constructed based on the micro-computed tomography imaging of the sagittal otoliths of the bight redfish (Centroberyx gerrardi). We used the models to study the response of fish sagittal otoliths to sounds arriving from horizontal and vertical directions. Sound pressure levels, relative displacement, acceleration, and shear stress of the otoliths and/or otolith-water boundary were calculated and compared. The results suggest that the angle of sound energy arrivals at the otoliths and the geometry of the otolith lead to different magnitudes of the differential motion between the macula and otoliths, with sound arriving in the vertical potentially creating more damage than the same sound arriving from the horizontal.

A sciaenid swim bladder with long skinny fingers produces sound with an unusual frequency spectrum

Swim bladders in sciaenid fishes function in hearing in some and sound production in almost all species. Sciaenid swim bladders vary from simple carrot-shaped to two-chambered to possessing various diverticula. Diverticula that terminate close to the ears improve hearing. Other unusual diverticula heading in a caudal direction have not been studied. The fresh-water Asian species Boesemania microlepis has an unusual swim bladder with a slightly restricted anterior region and 6 long-slender caudally-directed diverticula bilaterally. We hypothesized that these diverticula modify sound spectra. Evening advertisement calls consist of a series of multicycle tonal pulses, but the fundamental frequency and first several harmonics are missing or attenuated, and peak frequencies are high, varying between < 1–2 kHz. The fundamental frequency is reflected in the pulse repetition rate and in ripples on the frequency spectrum but not in the number of cycles within a pulse. We suggest that diverticula function as Helmholz absorbers turning the swim bladder into a high-pass filter responsible for the absence of low frequencies typically present in sciaenid calls. Further, we hypothesize that the multicycle pulses are driven by the stretched aponeuroses (flat tendons that connect the sonic muscles to the swim bladder) in this and other sciaenids.

Ontogenetic variation in the auditory sensitivity of black sea bass (Centropristis striata) and the implications of anthropogenic sound on behavior and communication

Black sea bass (Centropristis striata) is an important fish species in both commercial and recreational fisheries of southern New England and the mid-Atlantic Bight. Due to the intense urbanization of these waters, this species is subject to a wide range of anthropogenic noise pollution. Concerns that C. striata are negatively affected by pile driving and construction noise predominate in areas earmarked for energy development. However, as yet, the hearing range of C. striata is unknown, making it hard to evaluate potential risks. This study is a first step in understanding the effects of anthropogenic noise on C. striata by determining the auditory detection bandwidth and thresholds of this species using auditory evoked potentials, creating pressure and acceleration audiograms. These physiological tests were conducted on wild-caught C. striata in three size/age categories. Results showed that juvenile C. striata had the significantly lowest thresholds, with auditory sensitivity decreasing in the larger size classes. Furthermore, C.striata has fairly sensitive sound detection relative to other related species. Preliminary investigations into the mechanisms of their sound detection ability were undertaken with gross dissections and an opportunistic micro-computed tomography image to address the auditory structures including otoliths and swim bladder morphology. Crucially, the auditory detection bandwidth of C. striata, and their most sensitive frequencies, directly overlap with high-amplitude anthropogenic noise pollution such as shipping and underwater construction.

The effect of low frequency noise on the behaviour of juvenile Sparus aurata

Anthropogenic activities are causing increased noise levels in the marine environment. To date, few studies have been undertaken to investigate the effects of different noise frequencies on the behaviour of juvenile fish. In this study, the behavioural changes of juvenile gilthead seabream (Sparus aurata) are evaluated when exposed to white noise filtered in third-octave bands centred at 63, 125, 500, and 1000 Hz (sound pressure level, 140-150 dB re 1 μΡa) for 7 h. The group dispersion, motility, and swimming height of the fish were analysed before and during the acoustic emission. Dispersion of the fish was found to reduce immediately upon application of low frequency sound (63 and 125 Hz) with a return to control condition after 2 h (indicative of habituation), whereas at 1 kHz, dispersion increased after 2 h without any habituation. The motility decreased significantly at 63 Hz throughout the 7 h of sound exposure. The swimming height decreased significantly for all frequencies other than 125 Hz. The results of this study highlight significant variations in the behavioural responses of juvenile fish that could have consequences on their fitness and survival.