Directional hearing and sound source localization by fishes

Comparison of acoustic particle acceleration detection capabilities in three shark species

Behavioural studies have shown that sharks are capable of directional orientation to sound. However, only one previous experiment addresses the physiological mechanisms of directional hearing in sharks. Here, we used a directional shaker table in combination with the auditory evoked potential (AEP) technique to understand the broadscale directional hearing capabilities in the New Zealand carpet shark (Cephaloscyllium isabellum), rig shark (Mustelus lenticulatus) and school shark (Galeorhinus galeus). The aim of this experiment was to test if sharks are more sensitive to vertical (z-axis) or head-to-tail (x-axis) accelerations, and whether there are any differences between species. Our results support previous findings, suggesting that shark ears can receive sounds from all directions. Acceleration detection bandwidth was narrowest for the carpet shark (40-200 Hz), and broader for rig and school sharks (40-800 Hz). Greatest sensitivity bands were 40-80 Hz for the carpet shark, 100-200 Hz for the rig and 80-100 Hz for the school shark. Our results indicate that there may be differences in directional hearing abilities among sharks. The bottom-dwelling carpet shark was equally sensitive to vertical and head-to-tail particle accelerations. In contrast, both benthopelagic rig and school sharks appeared to be more sensitive to vertical accelerations at frequencies up to 200 Hz. This is the first study to provide physiological evidence that sharks may differ in their directional hearing and sound localisation abilities. Further comparative physiological and behavioural studies in more species with different lifestyles, habitats and feeding strategies are needed to further explore the drivers for increased sensitivity to vertical accelerations among elasmobranchs.

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 continued importance of comparative auditory research to modern scientific discovery

A rich history of comparative research in the auditory field has afforded a synthetic view of sound information processing by ears and brains. Some organisms have proven to be powerful models for human hearing due to fundamental similarities (e.g., well-matched hearing ranges), while others feature intriguing differences (e.g., atympanic ears) that invite further study. Work across diverse "non-traditional" organisms, from small mammals to avians to amphibians and beyond, continues to propel auditory science forward, netting a variety of biomedical and technological advances along the way. In this brief review, limited primarily to tetrapod vertebrates, we discuss the continued importance of comparative studies in hearing research from the periphery to central nervous system with a focus on outstanding questions such as mechanisms for sound capture, peripheral and central processing of directional/spatial information, and non-canonical auditory processing, including efferent and hormonal effects.

Balance dysfunction in large yellow croaker in response to ocean acidification

Large yellow croaker (Larimichthys crocea) is a coastal-dwelling soniferous, commercially important fish species that is sensitive to sound. An understanding of how ocean acidification might affect its auditory system is therefore important for its long-term viability and management as a fisheries resource. We tested the effects of ocean acidification with four CO₂ treatments (440 ppm (control), 1000 ppm, 1800 ppm, and 3000 ppm) on the inner ear system of this species. After exposure to acidified water for 50 d, the impacts on the perimeter and mass of the sagitta, asteriscus, and lapillus otoliths were determined. In the acidified water treatments, the shape of sagittal otoliths became more irregular, and the surface became rougher. Similar sound frequency ranges triggered startle responses of fish in all treatments. In the highest CO₂ treatment (3000 ppm CO₂), significant asymmetry of the left and right lapillus perimeter and weight was apparent. Moreover, in the higher CO₂ treatments (1800 ppm and 3000 ppm CO₂), the fish were unable to maintain a balanced dorsal-up posture and tilted to one side. This result suggested that the balance functions of the inner ear might be affected by ocean acidification, which may threaten large yellow croaker individuals and populations. The molecular response to acidification was analyzed by RNA-Seq. The differentially expressed genes (DEGs) between right and left sensory epithelia of the utricle in each CO₂ treatment group were identified. In higher CO₂ concentration groups, nervous system function and regulation of bone mineralization pathways were enriched with DEGs. The comparative transcriptome analyses provide valuable molecular information about how the inner ear system responds to an acidified environment.

Biodiversity assessment using passive acoustic recordings from off-reef location-Unsupervised learning to classify fish vocalization

We present the quantitative characterization of Grande Island's off-reef acoustic environment within the Zuari estuary during the pre-monsoon period. Passive acoustic recordings reveal prominent fish choruses. Detailed characteristics of the call employing oscillograms and individual fish call parameters of the segmented data include vocal groups such as Sciaenidae, Terapon theraps, and planktivorous as well as invertebrate sounds, e.g., snapping shrimp. We calculated biodiversity parameters (i) Acoustic Evenness Index (AEI), (ii) Acoustic Complexity Index (ACI), and mean sound pressure level (SPLrms) for three frequency bands such as full band (50-22 050 Hz), the low-frequency fish band (100-2000 Hz), and the high-frequency shrimp band (2000-20 000 Hz). Here, ACI and AEI metrics characterize the location's soundscape data effectively indicating increased biodiversity of fish species for both the low-frequency and high-frequency bands. Whereas variations for SPLrms are prominent for three frequency bands. Moreover, we employ unsupervised classification through a hybrid technique comprising principal component analysis (PCA) and K-means clustering for data features of four fish sound types. Employed PCA for dimensionality reduction and related K-means clustering successfully provides 96.20%, 76.81%, 100.00%, and 86.36% classification during the dominant fish chorus. Overall, classification performance (89.84%) is helpful in the real-time monitoring of the fish stocks in the ecosystem.

Ontogenetic development of inner ear hair cell organization in the New Zealand carpet shark Cephaloscyllium isabellum

Introduction

The inner ear hair cells of fishes can provide insight into the early evolution of vertebrate inner ear structure. Fishes represent some of the first vertebrates to evolve auditory capacity, and the same basic structure, the sensory hair cell, provides the fundament for auditory and vestibular function in jawed vertebrates. Despite holding critical basal position in the evolutionary tree of gnathostomes, relatively little is known about inner ear hair cells in elasmobranchs. Specifically, the extent of plasticity in hair cell organization throughout ontogeny among different sensory epithelia and the degree of variation between species is unknown.

Methods

In this study, we characterized the inner ear hair cells of the New Zealand carpet shark Cephaloscyllium isabellum throughout ontogeny by quantifying macular area, number of hair cells, hair cell density, and hair cell orientations in the inner ear maculae from a range of body sizes.

Results

Similar to other elasmobranchs and bony fishes, macular area and the number of hair cells increased throughout ontogeny in the otolith organs. The orientations of hair cells within each maculae also was consistent with the limited data on other elasmobranchs. However, contrary to expectation, the macula neglecta did not increase in area or hair cell number throughout ontogeny, and hair cell density did not change with body size in any maculae.

Discussion

These findings suggest there may be variation between elasmobranch species in ontogenetic development of hair cell organization that may be related to hearing capabilities throughout life.

Reproductive state modulates utricular auditory sensitivity in a vocal fish

The plainfin midshipman, Porichthys notatus, is a seasonally breeding vocal fish that relies on acoustic communication to mediate nocturnal reproductive behaviors. Reproductive females use their auditory senses to detect and localize "singing" males that produce multiharmonic advertisement (mate) calls during the breeding season. Previous work showed that the midshipman saccule, which is considered the primary end organ used for hearing in midshipman and most other fishes, exhibits reproductive state and hormone-dependent changes that enhance saccular auditory sensitivity. In contrast, the utricle was previously posited to serve primarily a vestibular function, but recent evidence in midshipman and related toadfish suggests that it may also serve an auditory function and aid in the detection of behaviorally relevant acoustic stimuli. Here, we characterized the auditory-evoked potentials recorded from utricular hair cells in reproductive and nonreproductive female midshipman in response to underwater sound to test the hypothesis that variation in reproductive state affects utricular auditory sensitivity. We show that utricular hair cells in reproductive females exhibit up to a sixfold increase in the utricular potential magnitude and have thresholds based on measures of particle acceleration (re: 1 ms-2) that are 7-10 dB lower than nonreproductive females across a broad range of frequencies, which include the dominant harmonics of male advertisement calls. This enhanced auditory sensitivity of the utricle likely plays an essential role in facilitating midshipman social and reproductive acoustic communication.NEW & NOTEWORTHY In many animals, vocal-acoustic communication is fundamental for facilitating social behaviors. For the vocal plainfin midshipman fish, the detection and localization of social acoustic signals are critical to the species' reproductive success. Here, we show that the utricle, an inner ear end organ often thought to primarily serve a vestibular function, serves an auditory function that is seasonally plastic and modulated by the animal's reproductive state effectively enhancing auditory sensitivity to courting male advertisement calls.

Jumping out of trouble: evidence for a cognitive map in guppies (Poecilia reticulata)

Spatial cognitive abilities allow individuals to remember the location of resources such as food patches, predator hide-outs, or shelters. Animals typically incorporate learned spatial information or use external environmental cues to navigate their surroundings. A spectacular example of how some fishes move is through aerial jumping. For instance, fish that are trapped within isolated pools, cut off from the main body of water during dry periods, may jump over obstacles and direct their jumps to return to safe locations. However, what information such re-orientation behavior during jumping is based on remains enigmatic. Here we combine a lab and field experiment to test if guppies (Poecilia reticulata) incorporate learned spatial information and external environmental cues (visual and auditory) to determine where to jump. In a spatial memory assay we found that guppies were more likely to jump towards deeper areas, hence incorporating past spatial information to jump to safety. In a matched versus mismatched spatial cue experiment in the field, we found that animals only showed directed jumping when visual and auditory cues matched. We show that in unfamiliar entrapments guppies direct their jumps by combining visual and auditory cues, whereas in familiar entrapments they use a cognitive map. We hence conclude that jumping behavior is a goal-directed behavior, guided by different sources of information and involving important spatial cognitive skills.

Response of toadfish (Opsanus tau) utricular afferents to multimodal inputs

The inner ear of teleost fishes is composed of three paired multimodal otolithic end organs (saccule, utricle, and lagena), which encode auditory and vestibular inputs via the deflection of hair cells contained within the sensory epithelia of each organ. However, it remains unclear how the multimodal otolithic end organs of the teleost inner ear simultaneously integrate vestibular and auditory inputs. Therefore, microwire electrodes were chronically implanted using a 3D printed micromanipulator into the utricular nerve of oyster toadfish (Opsanus tau) to determine how utricular afferents respond to conspecific mate vocalizations termed boatwhistles (180 Hz fundamental frequency) during movement. Utricular afferents were recorded while fish were passively moved using a sled system along an underwater track at variable speeds (velocity: 4.0 - 12.5 cm/s; acceleration: 0.2 - 2.6 cm/s²) and while fish freely swam (velocity: 3.5 - 18.6 cm/s; acceleration: 0.8 - 29.8 cm/s²). Afferent fiber activities (spikes/s) increased in response to the onset of passive and active movements; however, afferent fibers differentially adapted to sustained movements. Additionally, utricular afferent fibers remained sensitive to playbacks of conspecific male boatwhistle vocalizations during both passive and active movements. Here, we demonstrate in alert toadfish that utricular afferents exhibit enhanced activity levels (spikes/s) in response to behaviorally-relevant acoustic stimuli during swimming.

Soundscapes as heard by invertebrates and fishes: Particle motion measurements on coral reefs

Coral reef soundscapes are increasingly studied for their ecological uses by invertebrates and fishes, for monitoring habitat quality, and to investigate effects of anthropogenic noise pollution. Few examinations of aquatic soundscapes have reported particle motion levels and variability, despite their relevance to invertebrates and fishes. In this study, ambient particle acceleration was quantified from orthogonal hydrophone arrays over several months at four coral reef sites, which varied in benthic habitat and fish communities. Time-averaged particle acceleration magnitudes were similar across axes, within 3 dB. Temporal trends of particle acceleration corresponded with those of sound pressure, and the strength of diel trends in both metrics significantly correlated with percent coral cover. Higher magnitude particle accelerations diverged further from pressure values, potentially representing sounds recorded in the near field. Particle acceleration levels were also reported for boat and example fish sounds. Comparisons with particle acceleration derived audiograms suggest the greatest capacity of invertebrates and fishes to detect soundscape components below 100 Hz, and poorer detectability of soundscapes by invertebrates compared to fishes. Based on these results, research foci are discussed for which reporting of particle motion is essential, versus those for which sound pressure may suffice.

Hearing without a tympanic ear

The ability to sense and localize sound is so advantageous for survival that it is difficult to understand the almost 100 million year gap separating the appearance of early tetrapods and the emergence of an impedance-matching tympanic middle ear - which we normally regard as a prerequisite for sensitive hearing on land - in their descendants. Recent studies of hearing in extant atympanate vertebrates have provided significant insights into the ancestral state(s) and the early evolution of the terrestrial tetrapod auditory system. These reveal a mechanism for sound pressure detection and directional hearing in 'earless' atympanate vertebrates that may be generalizable to all tetrapods, including the earliest terrestrial species. Here, we review the structure and function of vertebrate tympanic middle ears and highlight the multiple acquisition and loss events that characterize the complex evolutionary history of this important sensory structure. We describe extratympanic pathways for sound transmission to the inner ear and synthesize findings from recent studies to propose a general mechanism for hearing in 'earless' atympanate vertebrates. Finally, we integrate these studies with research on tympanate species that may also rely on extratympanic mechanisms for acoustic reception of infrasound (<20 Hz) and with studies on human bone conduction mechanisms of hearing.

Multiscale analysis on otolith structural features reveals differences in ontogenesis and sex in Merluccius merluccius in the western Adriatic Sea

Otolith biomineralization results from biochemical processes regulated by the interaction of internal (physiological) and external (environmental) factors which lead to morphological and ultrastructural variability at intra- and interspecific levels. The aim of this study was to conduct a multi-scale analysis of the sagittal otoliths of the Merlucius merlucius (European hake) from the western Adriatic Sea in order to correlate otolith features with fish ontogeny and sex. We show that otoliths of sexually undifferentiated (non-sexed) individuals having a fish body total length (TL) less than 15 cm had faster growth in length, width, area, perimeter, volume and weight and a higher amount of organic matrix compared with otoliths of sexually differentiated individuals (females and males) having a fish size range of 15-50 cm. Most importantly, with increasing fish TL, female saccular otoliths contained a higher number of protuberances and rougher surface compared with male specimens, which showed more uniform mean curvature density. The differences between females and males discovered in this study could be associated with fish hearing adaptation to reproductive behavioural strategies during the spawning season. The outcomes of this research provide insights on how size and sex-related variations in otolith features may be affected by fish ecological and behavioural patterns.

Fish hearing "specialization" - A re-valuation

Investigators working with fish bioacoustics used to refer to fishes that have a narrow hearing bandwidth and poor sensitivity as "hearing generalists" (or "non-specialists"), while fishes that could detect a wider hearing bandwidth and had greater sensitivity were referred to as specialists. However, as more was learned about fish hearing mechanism and capacities, these terms became hard to apply since it was clear there were gradations in hearing capabilities. Popper and Fay, in a paper in Hearing Research in 2011, proposed that these terms be dropped because of the gradation. While this was widely accepted by investigators, it is now apparent that the lack of relatively concise terminology for fish hearing capabilities makes it hard to discuss fish hearing. Thus, in this paper we resurrect the terms specialist and non-specialist but use them with modifiers to express the specific structure of function that is considered a specialization. Moreover, this resurrection recognizes that hearing specializations in fishes may not only be related to increased bandwidth and/or sensitivity, but to other, perhaps more important, aspects of hearing such as sound source localization, discrimination between sounds, and detection of sounds in the presence of masking signals.

Bone conduction pathways confer directional cues to salamanders

Sound and vibration are generated by mechanical disturbances within the environment, and the ability to detect and localize these acoustic cues is generally important for survival, as suggested by the early emergence of inherently directional otolithic ears in vertebrate evolutionary history. However, fossil evidence indicates that the water-adapted ear of early terrestrial tetrapods lacked specialized peripheral structures to transduce sound pressure (e.g. tympana). Therefore, early terrestrial hearing should have required nontympanic (or extratympanic) mechanisms for sound detection and localization. Here, we used atympanate salamanders to investigate the efficacy of extratympanic pathways to support directional hearing in air. We assessed peripheral encoding of directional acoustic information using directionally masked auditory brainstem response recordings. We used laser Doppler vibrometry to measure the velocity of sound pressure-induced head vibrations as a key extratympanic mechanism for aerial sound reception in atympanate species. We found that sound generates head vibrations that vary with the angle of the incident sound. This extratympanic pathway for hearing supports a figure-eight pattern of directional auditory sensitivity to airborne sound in the absence of a pressure-transducing tympanic ear.

Noise Waveforms within Seabed Vibrations and Their Associated Evanescent Sound Fields

While the effects of sound pressures in water have been studied extensively, very much less work has been done on seabed vibrations. Our previous work used finite element modeling to interpret the results of field trials, studying propagation through graded seabeds as excited by impulsive energy applied to a point. A new simulation has successfully replicated further features of the original observations, and more field work has addressed other questions. We have concentrated on the water-particle motion near the seabed, as this is well known to be critical for benthic species. The evanescent pressure sound fields set up as the impulsive vibration energy passes are expected to be important for the local species, such as crabs and flatfish. By comparison with effects occurring away from boundaries, these seismic interface waves create vigorous water-particle motion but proportionately less sound pressure. This comparative increase ratio exceeds 12 for unconsolidated sediment areas, as typically used for piling operations.

Substrate vibrations and their potential effects upon fishes and invertebrates

This paper reviews the nature of substrate vibration within aquatic environments where seismic interface waves may travel along the surface of the substrate, generating high levels of particle motion. There are, however, few data on the ambient levels of particle motion close to the seabed and within the substrates of lakes and rivers. Nor is there information on the levels and the characteristics of the particle motion generated by anthropogenic sources in and on the substrate, which may have major effects upon fishes and invertebrates, all of which primarily detect particle motion. We therefore consider how to monitor substrate vibration and describe the information gained from modeling it. Unlike most acoustic modeling, we treat the substrate as a solid. Furthermore, we use a model where the substrate stiffness increases with depth but makes use of a wave that propagates with little or no dispersion. This shows the presence of higher levels of particle motion than those predicted from the acoustic pressures, and we consider the possible effects of substrate vibration upon fishes and invertebrates. We suggest that research is needed to examine the actual nature of substrate vibration and its effects upon aquatic animals.

Sound detection by Atlantic cod: An overview

The Atlantic cod (Gadus morhua) is among the commercially most important fish species in the world. Since sound plays such an important role in the lives of Atlantic cod and its related species, understanding of their bioacoustics is of great importance. Moreover, since cod are amenable to studies of hearing, especially in open bodies of water, they have the potential to become a "model species" for investigations of fish hearing. To serve as the basis for future studies, and to bring together what is now known about cod hearing, this paper reviews the literature to date. While there is some discussion of other species in the paper, the focus is upon what is already known about cod hearing, and what now needs to be known. An additional focus is on what knowledge of cod hearing tells about hearing in fishes in general.

Ammonia exposure impairs lateral-line hair cells and mechanotransduction in zebrafish embryos

Ammonia (including NH₃ and NH₄⁺) is a major pollutant of freshwater environments. However, the toxic effects of ammonia on the early stages of fish are not fully understood, and little is known about the effects on the sensory system. In this study, we hypothesized that ammonia exposure can cause adverse effects on embryonic development and impair the lateral line system of fish. Zebrafish embryos were exposed to high-ammonia water (10, 15, 20, 25, and 30 mM NH₄Cl; pH 7.0) for 96 h (0-96 h post-fertilization). The body length, heart rate, and otic vesicle size had significantly decreased with ≥15 mM NH₄Cl, while the number and function of lateral-line hair cells had decreased with ≥10 mM NH₄Cl. The mechanoelectrical transduction (MET) channel-mediated Ca²⁺ influx was measured with a scanning ion-selective microelectrode technique to reveal the function of hair cells. We found that NH₄⁺ (≥5 mM NH₄Cl) entered hair cells and suppressed the Ca²⁺ influx of hair cells. Neomycin and La³⁺ (MET channel blockers) suppressed NH₄⁺ influx, suggesting that NH₄⁺ enters hair cells via MET channels in hair bundles. In conclusion, this study showed that ammonia exposure (≥10 mM NH₄Cl) can cause adverse effects in zebrafish embryos, and lateral-line hair cells are sensitive to ammonia exposure.

Sablefish (Anoplopoma fimbria) produce high frequency rasp sounds with frequency modulation

Sablefish sounds, named rasps, were recorded at two captive facilities in British Columbia and Washington State. Rasps consisted of highly variable broadband trains of 2 to 336 ticks that lasted between 74 and 10 500 ms. The 260 rasps that were measured contained frequencies between 344 and 34 000 Hz with an average peak frequency of 3409 Hz. The frequency structure of ticks within rasps was highly variable and included both positive and negative trends. This finding makes sablefish one of the few deep-sea fish for which sounds have been validated and described. The documentation of sablefish sounds will enable the use of passive acoustic monitoring methods in fisheries and ecological studies of this commercially important deep-sea fish.

Group behavior and tolerance of Eurasian minnow (Phoxinus phoxinus) in response to tones of differing pulse repetition rate

Behavioral guidance systems are commonly used in freshwater fish conservation. The biological relevance of sound to fish and recorded responses to human-generated noise supports the viability of the use of acoustics as an effective stimulus in such technologies. Relatively little information exists on the long-term responses and recovery of fish to repeated acoustic exposures. In a controlled laboratory study, the response and tolerance of Eurasian minnow (Phoxinus phoxinus) shoals to tonal signals (150 Hz of 1 s pulse duration) differing only in temporal characteristics ("continuous," "slow," "intermediate," or "fast" pulse repetition rate) were investigated. In comparison to independent control groups, fish increased their mean group swimming speed, decreased inter-individual distance, and became more aligned in response to the onset of all four acoustic treatments. The magnitude of response, and time taken to develop a tolerance to a treatment differed according to pulse repetition rate. Groups were found to have the greatest and longest lasting response to tone sequences tested in this study when they were pulsed at an intermediate rate of 0.2 s^-1. This study illustrates the importance of understanding the response of fish to acoustic signals, and will assist toward the development of longer-term effective acoustic guidance systems.

Acidified water impairs the lateral line system of zebrafish embryos

Acidification of freshwater ecosystems is recognized as a global environmental problem. However, the influence of acidic water on the early stages of freshwater fish is still unclear. This study focused on the sublethal effects of acidic water on the lateral line system of zebrafish embryos. Zebrafish embryos were exposed to water at different pH values (pH 4, 5, 7, 9, and 10) for 96 (0-96 h post-fertilization (hpf)) and 48 h (48∼96 hpf). The survival rate, body length, and heart rate significantly decreased in pH 4-exposed embryos during the 96-h incubation. The number of lateral-line neuromasts and the size of otic vesicles/otoliths also decreased in pH 4-exposed embryos subjected to 96- and 48-h incubations. The number of neuromasts decreased in pH 5-exposed embryos during the 96-h incubation. Alkaline water (pH 9 and 10) did not influence embryonic development but suppressed the hatching process. The mechanotransducer channel-mediated Ca²⁺ influx was measured to reveal the function of lateral line hair cells. The Ca²⁺ influx of hair cells decreased in pH 5-exposed embryos subjected to the 48-h incubation, and both the number and Ca²⁺ influx of hair cells had decreased in pH 5-exposed embryos after 96 h of incubation. In addition, the number and function of hair cells were suppressed in H⁺-ATPase- or GCM2-knockdown embryos, which partially lost the ability to secrete acid into the ambient water. In conclusion, this study suggests that lateral line hair cells are sensitive to an acidic environment, and freshwater acidification could be a threat to the early stages of fishes.

The importance of underwater sounds to gadoid fishes

The codfish family includes more than 500 species that vary greatly in their abundance in areas like the North Sea and are widely fished. Gadoids (codfish) gather at particular locations to spawn, where they exhibit complex reproductive behavior with visual and acoustic displays. Calls have been described from seven species, including the Atlantic cod and haddock. They vocalize by means of a specialized apparatus, consisting of rapidly contracting striated muscles (the drumming muscles) attached to the gas-filled swim bladder. Several gadoids, such as the ling and the Greenland cod, possess drumming muscles and are likely to make sounds. Non-vocal gadoids, such as the poor cod, lack these muscles. It is suggested that the sonic apparatus was present in the early species of the gadoids, with some species having lost their sonic ability. Interestingly, silent gadoids are mainly small schooling fishes. Gadoid species are most sensitive to sounds from 30 to 500 Hz. Gadoid hearing can be masked by ambient sound but also by anthropogenic sounds, which may therefore adversely affect their reproduction, with potential effects upon discrete local stocks. Listening for gadoid sounds provides a reliable, non-invasive way of locating spawning sites, which can enhance the protection of reproducing fish from human impacts.

Auditory evoked potentials of the plainfin midshipman fish (Porichthys notatus): implications for directional hearing

The plainfin midshipman (Porichthys notatus) is an acoustically communicative teleost fish. Here, we evaluated auditory evoked potentials (AEPs) in reproductive female midshipman exposed to tones at or near dominant frequencies of the male midshipman advertisement call. An initial series of experiments characterized AEPs at behaviorally relevant suprathreshold sound levels (130-140 dB SPL re. 1 µPa). AEPs decreased in magnitude with increasing stimulus frequency and featured a stereotyped component at twice the stimulus frequency. Recording electrode position was varied systematically and found to affect AEP magnitude and phase characteristics. Later experiments employed stimuli of a single frequency to evaluate contributions of the saccule to the AEP, with particular attention to the effects of sound source azimuth on AEP amplitude. Unilateral excision of saccular otoliths (sagittae) decreased AEP amplitude; unexpectedly, decreases differed for right versus left otolith excision. A final set of experiments manipulated the sound pressure-responsive swim bladder. Swim bladder excision further reduced the magnitude of AEP responses, effectively eliminating responses at the standard test intensity (130 dB SPL) in some animals. Higher-intensity stimulation yielded response minima at forward azimuths ipsilateral to the excised sagitta, but average cross-azimuth modulation generally remained slight. Collectively, the data underscore that electrode position is an essential variable to control in fish AEP studies and suggest that in female midshipman: (1) the saccule contributes to the AEP, but its directionality as indexed by the AEP is limited, (2) a left-right auditory asymmetry may exist and (3) the swim bladder provides gain in auditory sensitivity that may be important for advertisement call detection and phonotaxis.

Sexually dimorphic swim bladder extensions enhance the auditory sensitivity of female plainfin midshipman fish, Porichthys notatus

The plainfin midshipman fish, Porichthys notatus, is a seasonally breeding, nocturnal marine teleost fish that produces acoustic signals for intraspecific social communication. Females rely on audition to detect and locate 'singing' males that produce multiharmonic advertisement calls in the shallow-water, intertidal breeding environments. Previous work showed that females possess sexually dimorphic, horn-like rostral swim bladder extensions that extend toward the primary auditory end organs, the saccule and lagena. Here, we tested the hypothesis that the rostral swim bladder extensions in females increase auditory sensitivity to sound pressure and higher frequencies, which potentially could enhance mate detection and localization in shallow-water habitats. We recorded the auditory evoked potentials that originated from hair cell receptors in the saccule of control females with intact swim bladders and compared them with those from treated females (swim bladders removed) and type I males (intact swim bladders lacking rostral extensions). Saccular potentials were recorded from hair cell populations in vivo while behaviorally relevant pure-tone stimuli (75-1005 Hz) were presented by an underwater speaker. The results indicate that control females were approximately 5-11 dB re. 1 µPa more sensitive to sound pressure than treated females and type I males at the frequencies tested. A higher percentage of the evoked saccular potentials were recorded from control females at frequencies >305 Hz than from treated females and type I males. This enhanced sensitivity in females to sound pressure and higher frequencies may facilitate the acquisition of auditory information needed for conspecific localization and mate choice decisions during the breeding season.

An overview of fish bioacoustics and the impacts of anthropogenic sounds on fishes

Fishes use a variety of sensory systems to learn about their environments and to communicate. Of the various senses, hearing plays a particularly important role for fishes in providing information, often from great distances, from all around these animals. This information is in all three spatial dimensions, often overcoming the limitations of other senses such as vision, touch, taste and smell. Sound is used for communication between fishes, mating behaviour, the detection of prey and predators, orientation and migration and habitat selection. Thus, anything that interferes with the ability of a fish to detect and respond to biologically relevant sounds can decrease survival and fitness of individuals and populations. Since the onset of the Industrial Revolution, there has been a growing increase in the noise that humans put into the water. These anthropogenic sounds are from a wide range of sources that include shipping, sonars, construction activities (e.g., wind farms, harbours), trawling, dredging and exploration for oil and gas. Anthropogenic sounds may be sufficiently intense to result in death or mortal injury. However, anthropogenic sounds at lower levels may result in temporary hearing impairment, physiological changes including stress effects, changes in behaviour or the masking of biologically important sounds. The intent of this paper is to review the potential effects of anthropogenic sounds upon fishes, the potential consequences for populations and ecosystems and the need to develop sound exposure criteria and relevant regulations. However, assuming that many readers may not have a background in fish bioacoustics, the paper first provides information on underwater acoustics, with a focus on introducing the very important concept of particle motion, the primary acoustic stimulus for all fishes, including elasmobranchs. The paper then provides background material on fish hearing, sound production and acoustic behaviour. This is followed by an overview of what is known about effects of anthropogenic sounds on fishes and considers the current guidelines and criteria being used world-wide to assess potential effects on fishes. Most importantly, the paper provides the most complete summary of the effects of anthropogenic noise on fishes to date. It is also made clear that there are currently so many information gaps that it is almost impossible to reach clear conclusions on the nature and levels of anthropogenic sounds that have potential to cause changes in animal behaviour, or even result in physical harm. Further research is required on the responses of a range of fish species to different sound sources, under different conditions. There is a need both to examine the immediate effects of sound exposure and the longer-term effects, in terms of fitness and likely impacts upon populations.