So many toadfish, so little timea).

The oyster toadfish, Opsanus tau, has been a valuable biomedical model for a wide diversity of studies. However, its vocalization ability arguably has attracted the most attention, with numerous studies focusing on its ecology, behavior, and neurophysiology in regard to its sound production and reception. This paper reviews 30 years of research in my laboratory using this model to understand how aquatic animals detect, integrate, and respond to external environment cues. The dual vestibular and auditory role of the utricle is examined, and its ability to integrate multimodal input is discussed. Several suggestions for future research are provided, including in situ auditory recording, interjecting natural relevant ambient soundscapes into laboratory sound studies, adding transparency to the field of acoustic deterrents, and calls for fish bioacoustics teaching modules to be incorporated in K-12 curricula to excite and diversify the next generation of scientists.

Introduction to the special issue on fish bioacoustics: Hearing and sound communicationa).

Fish bioacoustics, or the study of fish hearing, sound production, and acoustic communication, was discussed as early as Aristotle. However, questions about how fishes hear were not really addressed until the early 20th century. Work on fish bioacoustics grew after World War II and considerably in the 21st century since investigators, regulators, and others realized that anthropogenic (human-generated sounds), which had primarily been of interest to workers on marine mammals, was likely to have a major impact on fishes (as well as on aquatic invertebrates). Moreover, passive acoustic monitoring of fishes, recording fish sounds in the field, has blossomed as a noninvasive technique for sampling abundance, distribution, and reproduction of various sonic fishes. The field is vital since fishes and aquatic invertebrates make up a major portion of the protein eaten by a signification portion of humans. To help better understand fish bioacoustics and engage it with issues of anthropogenic sound, this special issue of The Journal of the Acoustical Society of America (JASA) brings together papers that explore the breadth of the topic, from a historical perspective to the latest findings on the impact of anthropogenic sounds on fishes.

Sound frequencies detectable by grass (Ctenopharyngodon idella) and black (Mylopharyngodon pisceus) carp determined with auditory evoked potentialsa).

Grass (Ctenopharyngodon idella) and black (Mylopharyngodon piceus) carp were imported to the United States as biocontrol agents in aquaculture facilities; however, due to intentional stocking or accidental release, invasive populations have become established. Invasive bigheaded carps display negative phonotaxis to sound, suggesting acoustic deterrents as a potential management tool. Grass and black carp frequency detection in response to 0.2-6 kHz sound was evaluated using auditory evoked potentials (AEPs) and determined for sound pressure level (SPL) and particle acceleration level (PAL). AEPs were detectable in response to pure tone stimuli from 0.2 to 4 kHz. The lowest SPL thresholds for grass carp were at 0.3 kHz (87.0 ± 3.6 dB re 1 µPa SPLrms; mean ± standard deviation), and the lowest PAL thresholds were at 0.4 kHz (-65.8 ± 3.2 dB re 1 ms-2 PALrms). For black carp, the lowest SPL and PAL thresholds were at 0.3 kHz (93.4 ± 3.0 dB re 1 µPa SPLrms, -60.8 ± 1.5 dB re 1 ms-2 PALrms). These results indicate that the ranges of detectable frequencies for grass and black carp overlap with those reported for bigheaded carps. However, behavioral studies are needed to determine potential efficacy of acoustic deterrents for these fish.

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 3-D 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. In addition, 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.NEW & NOTEWORTHY The inner ear of teleost fishes is composed of three paired multimodal otolithic end organs, which are sensitive to vestibular and auditory inputs. Previous studies investigating inner ear functions have primarily focused on the effects of unimodal stimuli; therefore, it remains unclear how otolithic end organs simultaneously encode multiple stimuli. Here, we show that utricular afferents remain sensitive to behaviorally relevant acoustic stimuli during swimming.

The effect of biological and anthropogenic sound on the auditory sensitivity of oyster toadfish, Opsanus tau.

Many aquatic organisms use vocalizations for reproductive behavior; therefore, disruption of their soundscape could adversely affect their life history. Male oyster toadfish (Opsanus tau) establish nests in shallow waters during spring and attract female fish with boatwhistle vocalizations. Males exhibit high nest fidelity, making them susceptible to anthropogenic sound in coastal waters, which could mask their vocalizations and/or reduce auditory sensitivity levels. Additionally, the effect of self-generated boatwhistles on toadfish auditory sensitivity has yet to be addressed. To investigate the effect of sound exposure on toadfish auditory sensitivity, sound pressure and particle acceleration sensitivity curves were determined using auditory evoked potentials before and after (0-, 1-, 3-, 6- and 9-day) exposure to 1- or 12-h of continuous playbacks to ship engine sound or conspecific vocalization. Exposure to boatwhistles had no effect on auditory sensitivity. However, exposure to anthropogenic sound caused significant decreases in auditory sensitivity for at least 3 days, with shifts up to 8 dB SPL and 20 dB SPL immediately following 1- and 12-h anthropogenic exposure, respectively. Understanding the effect of self-generated and anthropogenic sound exposure on auditory sensitivity provides an insight into how soundscapes affect acoustic communication.

Reproductive state-dependent plasticity in the visual system of an African cichlid fish.

Visual communication is used widely across the animal kingdom to convey crucial information about an animals' identity, reproductive status, and sex. Although it is well-demonstrated that auditory and olfactory sensitivity can change with reproductive state, fewer studies have tested for plasticity in the visual system, a surprising detail since courtship and mate choice behaviors in many species are largely dependent on visual signals. Here, we tested for reproductive state-dependent plasticity in the eye of the cichlid fish Astatotilapia burtoni using behavioral, gene expression, neural activation, and electrophysiology techniques. Males court ovulated females more intensely than gravid females, and ovulated females were more responsive to male courtship behaviors than gravid females. Using electroretinography to measure visual sensitivity in dark-adapted fish, we revealed that gravid, reproductively-ready females have increased visual sensitivity at wavelengths associated with male courtship coloration compared to non-gravid females. After ovulation was hormonally induced, female's spectral sensitivity further increased compared to pre-injection measurements. This increased sensitivity after hormone injection was absent in non-gravid females and in males, suggesting an ovulation-triggered increase in visual sensitivity. Ovulated females had higher mRNA expression levels of reproductive neuromodulatory receptors (sex-steroids; gonadotropins) in the eye than nonovulated females, whereas males had similar expression levels independent of reproductive/social state. In addition, female mate choice-like behaviors positively correlated with expression of gonadotropin system receptors in the eye. Collectively, these data provide crucial evidence linking endocrine modulation of visual plasticity to mate choice behaviors in females.

Lateral line sensitivity in free-swimming toadfish Opsanus tau.

A longstanding question in aquatic animal sensory physiology is the impact of self-generated movement on lateral line sensitivity. One hypothesis is that efferent modulation of the sensory hair cells cancels self-generated noise and allows fish to sample their surroundings while swimming. In this study, microwire electrodes were chronically implanted into the anterior lateral line nerve of oyster toadfish and neural activity was monitored during forward movement. Fish were allowed to freely swim or were moved by a tethered sled. In all cases, neural activity increased during movement with no evidence of efferent modulation. The anterior lateral line of moving fish responded to a vibrating sphere or the tail oscillations of a robotic fish, indicating that the lateral line also remains sensitive to outside stimulus during self-generated movement. The results suggest that during normal swim speeds, lateral line neuromasts are not saturated and retain the ability to detect external stimuli without efferent modulation.

The effect of light stimuli on dark-adapted visual sensitivity in invasive silver carp Hypophthalmichthys molitrix and bighead carp H. nobilis.

Non-physical barriers, including the use of underwater strobe lights alone or paired with sound or bubbles, are being considered as a means to prevent the upstream migration of invasive silver carp Hypophthalmichthys molitrix and bighead carp H. nobilis. To optimize potential optical deterrents, it is necessary to understand the visual sensitivity of the fishes. Dark-adapted H. molitrix and H. nobilis were found to possess broad visual sensitivity between 470 to 620 nm with peak spectral sensitivity at 540 nm for H. molitrix and 560 nm in H. nobilis. To assess the effect of a strobe light on vision, dark-adapted H. molitrix, H. nobilis and common carp Cyprinus carpio, were exposed to three different 5 s trains (100, 200, or 500 ms on-off flashes) of white light and the recovery of visual sensitivity was determined by measuring the b-wave amplitude of the electroretinogram (ERG). For all species, the longest recoveries were observed in response to the 500 ms flash trains (H. molitrix mean ± SE = 702.0 ± 89.8 s; H. nobilis 648.0 ± 116.0 s; C. carpio 480 ± 180.0 s). The results suggest that strobe lights can temporarily depress visual sensitivity, which may render optical barriers less effective.

Impacts of broadband sound on silver (Hypophthalmichthys molitrix) and bighead (H. nobilis) carp hearing thresholds determined using auditory evoked potential audiometry.

Invasive silver (Hypophthalmichthys molitrix) and bighead (H. nobilis) carp, collectively referred to as bigheaded carps, threaten aquatic ecosystems of the Upper Midwestern USA. Due to the extensive ecological impacts associated with these species, prevention of their further range expansion is the aim for fisheries management. Recent behavioral studies indicate bigheaded carps are deterred by acoustic barriers and exhibit negative phonotaxis in response to anthropogenic sound sources (≥ 150 dB re 1 µPa). However, the impact of long-term exposure to these sounds on the hearing capabilities of bigheaded carps has not been well documented. In this study, the auditory evoked potential (AEP) technique was used to determine auditory thresholds among bigheaded carps before and after exposure to high intensity (155.7 ± 4.7 dB re 1 µPa SPLrms; - 8.0 ± 4.7 dB re 1 ms-2 PALrms; mean ± SD) broadband sound. Fish were exposed to sound for 30 min or 24 h and AEP measurements were taken at three time points: immediately after exposure, 48 h, or 96 h later. Results indicate that silver and bighead carp experience temporary threshold shifts (TTSs) in frequency detection following sound exposure with the magnitude and length of TTS correlated with exposure duration. The findings from this study will be used to increase the long-term efficacy of acoustical deterrent measures aimed at preventing further range expansion of bigheaded carps.

Potential role of the anterior lateral line in sound localization in toadfish (Opsanus tau).

Male oyster toadfish (Opsanus tau) acoustically attract females to nesting sites using a boatwhistle call. The rapid speed of sound underwater combined with the close proximity of the otolithic organs makes inner ear interaural time differences an unlikely mechanism to localize sound. To determine the role that the mechanosensory lateral line may play in sound localization, microwire electrodes were bilaterally implanted into the anterior lateral line nerve to record neural responses to vibrational stimuli. Highest spike rates and strongest phase-locking occurred at distances close to the fish and decreased as the stimulus was moved further from the fish. Bilateral anterior lateral line neuromasts displayed differential directional sensitivity to incoming vibrational stimuli, which suggests the potential for the lateral line to be used for sound localization in the near field. The present study also demonstrates that the spatially separated neuromasts of the toadfish may provide sufficient time delays between sensory organs for determining sound localization cues. Multimodal sensory input processing through both the inner ear (far field) and lateral line (near field) may allow for effective sound localization in fish.

Multimodal Sensory Input in the Utricle and Lateral Line of the Toadfish, Opsanus tau.

The utricular otolith and the mechanosensory lateral line of the toadfish, Opsanus tau, were investigated for sensitivity to multimodal sensory input by recording neural activity from free swimming fish. The utricle was sensitive to horizontal body movement, and displayed broad sensitivity to low frequency (80-200 Hz) sound. The lateral line was sensitive to water currents, swimming, prey movements, and sound with maximal sensitivity at 100 Hz. Both systems showed directional sensitivity to pure tones and toadfish vocalizations, indicating potential for sound localization. Thus, toadfish possess two hair cell based sensory systems that integrate information from disparate sources. However, swimming movements or predation strikes can saturate each system and it is unclear the effect that self-generated movement has on sensitivity. It is hypothesized that the toadfish's strategy of short distance swim movements allows it to sample the acoustical environment while static. Further study is needed to determine the integration of the two systems and if they are able to segregate and/or integrate multimodal sensory input.

Directional sound sensitivity in utricular afferents in the toadfish Opsanus tau.

The inner ear of fishes contains three paired otolithic end organs, the saccule, lagena and utricle, which function as biological accelerometers. The saccule is the largest otolith in most fishes and much of our current understanding on auditory function in this diverse group of vertebrates is derived from anatomical and neurophysiological studies on this end organ. In contrast, less is known about how the utricle contributes to auditory functions. In this study, chronically implanted electrodes were used, along with neural telemetry or tethers to record primary afferent responses from the utricular nerve in free-ranging and naturally behaving oyster toadfish Opsanus tau Linnaeus. The hypothesis was that the utricle plays a role in detecting underwater sounds, including conspecific vocalizations, and exhibits directional sensitivity. Utricular afferents responded best to low frequency (80-200 Hz) pure tones and to playbacks of conspecific boatwhistles and grunts (80-180 Hz fundamental frequency), with the majority of the units (∼75%) displaying a clear, directional response, which may allow the utricle to contribute to sound detection and localization during social interactions. Responses were well within the sound intensity levels of toadfish vocalization (approximately 140 SPL dBrms re. 1 µPa with fibers sensitive to thresholds of approximately 120 SPL dBrms re. 1 µPa). Neurons were also stimulated by self-generated body movements such as opercular movements and swimming. This study is the first to investigate underwater sound-evoked response properties of primary afferents from the utricle of an unrestrained/unanesthetized free-swimming teleost fish. These data provide experimental evidence that the utricle has an auditory function, and can contribute to directional hearing to facilitate sound localization.

Disruptive communication: stealth signaling in the toadfish.

Male oyster toadfish, Opsanus tau, produce long duration (250 to 650 ms) sexual advertisement calls or 'boatwhistles' during the breeding season. When males are in close proximity, the fishes alternate the production of boatwhistles with other males to avoid call overlap. However, males can also produce a number of different sounds, including a single, short duration pulse or 'grunt' (~100 ms). The vocalizations of competing males were recorded in situ with multiple hydrophones to examine intraspecific interactions. These short grunts were emitted almost exclusively during the boatwhistle of a conspecific male. The fundamental frequency (or pulse repetition rate) of the boatwhistles were modified by this disruptive grunt, 'jamming' the signal and decreasing its frequency. The disruptive grunt specifically targeted the second stage or tonal portion of the boatwhistle, believed to be the primary acoustic attractant for females, and its brevity and precision may allow its emitter to remain undetectable. While the acoustic repertoire of teleost fishes may be less diverse compared with terrestrial species, the disruptive grunts indicate fish have the capacity for complex acoustic interactions.

Anterior lateral line nerve encoding to tones and play-back vocalisations in free-swimming oyster toadfish, Opsanus tau.

In the underwater environment, sound propagates both as a pressure wave and as particle motion, with particle motion dominating close to the source. At the receptor level, the fish ear and the neuromast hair cells act as displacement detectors, and both are potentially stimulated by the particle motion component of sound. The encoding of the anterior lateral line nerve to acoustic stimuli in freely behaving oyster toadfish, Opsanus tau, was examined. Nerve sensitivity and directional responses were determined using spike rate and vector strength analysis, a measure of phase-locking of spike times to the stimulus waveform. All units showed greatest sensitivity to 100 Hz stimulus. While sensitivity was independent of stimuli orientation, the neuron's ability to phase-lock was correlated with stimuli origin. Two different types of units were classified, type 1 (tonic), and type 2 (phasic). The type 1 fibres were further classified into two sub-types based on their frequency response (type 1-1 and type 1-2), which was hypothesised to be related to canal (type 1-1) and superficial (type 1-2) neuromast innervation. Lateral line units also exhibited sensitivity and phase locking to boatwhistle vocalisations, with greatest spike rates exhibited at the onset of the call. These results provide direct evidence that oyster toadfish can use their lateral line to detect behaviourally relevant acoustic stimuli, which could provide a sensory pathway to aid in sound source localisation.

Vocal production complexity correlates with neural instructions in the oyster toadfish (Opsanus tau).

Sound communication is fundamental to many social interactions and essential to courtship and agonistic behaviours in many vertebrates. The swimbladder and associated muscles in batrachoidid fishes (midshipman and toadfish) is a unique vertebrate sound production system, wherein fundamental frequencies are determined directly by the firing rate of a vocal-acoustic neural network that drives the contraction frequency of superfast swimbladder muscles. The oyster toadfish boatwhistle call starts with an irregular sound waveform that could be an emergent property of the peripheral nonlinear sound-producing system or reflect complex encoding in the central nervous system. Here, we demonstrate that the start of the boatwhistle is indicative of a chaotic strange attractor, and tested whether its origin lies in the peripheral sound-producing system or in the vocal motor network. We recorded sound and swimbladder muscle activity in awake, freely behaving toadfish during motor nerve stimulation, and recorded sound, motor nerve and muscle activity during spontaneous grunts. The results show that rhythmic motor volleys do not cause complex sound signals. However, arrhythmic recruitment of swimbladder muscle during spontaneous grunts correlates with complex sounds. This supports the hypothesis that the irregular start of the boatwhistle is encoded in the vocal pre-motor neural network, and not caused by peripheral interactions with the sound-producing system. We suggest that sound production system demands across vocal tetrapods have selected for muscles and motorneurons adapted for speed, which can execute complex neural instructions into equivalently complex vocalisations.

The effects of intensive trapping on invasive round goby densities.

The round goby (Neogobius melanostomus) is an invasive benthic fish first introduced to the Laurentian Great Lakes in 1990 that has negatively impacted native fishes through increased competition for food and habitat, aggressive interactions, and egg predation. While complete eradication of the round goby is currently not possible, intensive trapping in designated areas during spawning seasons could potentially protect critical native fish spawning habitats. Baited minnow traps were spaced 10 meters apart in shallow water along a 100-meter stretch of shoreline within the Duluth-Superior Harbor during the round goby breeding period (June to October) with captured round gobies removed from interior traps (N = 10) every 48 hours. These traps were bracketed by two pairs of reference traps deployed weekly for 48 hours, from which round gobies were also tagged and released. The number of round gobies captured in the interior traps declined by 67% compared to reference traps over the course of the study, with extended periods of no captures. The tagged round gobies showed high site affinity, with 82.8% of tagged fish recaptured at the previous release site. The results indicate that even at open water sites, which allow natural migration of round gobies into the area, extensive trapping could reduce local population numbers.

Individual, temporal, and population-level variations in circulating 11-ketotestosterone and 17beta-estradiol concentrations in the oyster toadfish Opsanus tau.

Sex steroid hormones are important for reproduction in all vertebrates, but few studies examine inter-individual, temporal, and population-level variations, as well as environmental influences on circulating steroid levels within the same species. In this study we analyzed plasma 11-ketotoestosterone (11-KT) and 17beta-estradiol (E(2)) levels in the oyster toadfish to test for 1) individual and temporal variations by serially sampling the same individuals during the reproductive and post-reproductive period, 2) variations in steroid levels among toadfish obtained from different sources or maintained under different holding conditions, and 3) correlations with environmental parameters. Results from serial sampling showed marked inter-individual variations in male 11-KT levels in two separate groups of toadfish, but no temporal differences from June to September. Females also showed inter-individual variations in E(2) concentrations, but most had elevated levels late in the reproductive season coincident with oocyte growth prior to winter quiescence. E(2) concentration, but not 11-KT, was positively correlated with water temperature, and negatively correlated with daylength and lunar phase. Maricultured toadfish held under constant conditions had elevated levels of E(2) and 11-KT that should be considered when using these fish for experimentation. This study provides important comparative information on the relationship between individual variations in steroid levels, and how they relate to physiological and environmental correlates in a model marine teleost.

Seasonal and Daily Patterns of the Mating Calls of the Oyster Toadfish, Opsanus tau.

Acoustic communication is vital across many taxa for mating behavior, defense, and social interactions. Male oyster toadfish, Opsanus tau, produce courtship calls, or "boatwhistles," characterized by an initial broadband segment (30-50 ms) and a longer tone-like second part (200-650 ms) during mating season. Male calls were monitored continuously with an in situ SoundTrap hydrophone that was deployed in Eel Pond, Woods Hole, Massachusetts, during the 2015 mating season. At least 10 vocalizing males were positively identified by their unique acoustic signatures. This resident population was tracked throughout the season, with several individuals tracked for extended periods of time (72 hours). Toadfish began calling in mid-May when water temperature reached 14.6 °C with these early-season "precursor" boatwhistles that were shorter in duration and contained less distinct tonal segments compared to calls later in the season. The resident toadfish stopped calling in mid-August, when water temperature was about 25.5 °C. The pulse repetition rate of the tonal part of the call was significantly related to ambient water temperature during both short-term (hourly) and long-term (weekly) monitoring. This was the first study to monitor individuals in the same population of oyster toadfish in situ continuously throughout the mating season.

Reexamining the frequency range of hearing in silver (Hypophthalmichthys molitrix) and bighead (H. nobilis) carp.

Silver (Hypophthalmichthys molitrix) and bighead (H. nobilis) carp (collectively bigheaded carp) are invasive fish that threaten aquatic ecosystems in the upper Midwest United States and the Laurentian Great Lakes. Controlling bigheaded carp is a priority of fisheries managers and one area of focus involves developing acoustic deterrents to prevent upstream migration. For an acoustic deterrent to be effective however, the hearing ability of bigheaded carp must be characterized. A previous study showed that bigheaded carp detected sound up to 3 kHz but this range is narrower than what has been reported for other ostariophysans. Therefore, silver and bighead carp frequency detection was evaluated in response to 100 Hz to 9 kHz using auditory evoked potentials (AEPs). AEPs were recorded from 100 Hz to 5 kHz. The lowest thresholds were at 500 Hz for both species (silver carp threshold: 80.6 ± 3.29 dB re 1 µPa SPLrms, bighead carp threshold: 90.5 ± 5.75 dB re 1 µPa SPLrms; mean ± SD). These results provide fisheries managers with better insight on effective acoustic stimuli for deterrent systems, however, to fully determine bigheaded carp hearing abilities, these results need to be compared with behavioral assessments.

The effect of motorboat sound on Australian snapper Pagrus auratus inside and outside a marine reserve.

Human-generated sound affects hearing, movement, and communication in both aquatic and terrestrial animals, but direct natural underwater behavioral observations are lacking. Baited underwater video (BUV) were deployed in near shore waters adjacent to Goat Island in the Cape Rodney-Okakari Point Marine Reserve (protected) or outside the reserve approximately four km south in Mathesons Bay (open), New Zealand to determine the natural behavior of Australian snapper Pagrus auratus exposed to motorboat sound. BUVs worked effectively at bringing fish into video range to assess the effects of sound. The snapper inhabiting the protected area showed no behavioral response to motorboat transits; however, fish in the open zones either scattered from the video frame or decreased feeding activity during boat presence. Our study suggests that motorboat sound, a common source of anthropogenic activity in the marine environment can affect fish behavior differently depending on the status of their habitat (protected versus open).