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.

Transmembrane Channel-Like (Tmc) Subunits Contribute to Frequency Sensitivity in the Zebrafish Utricle

Information about dynamic head motion is conveyed by a central "striolar" zone of vestibular hair cells and afferent neurons in the inner ear. How vestibular hair cells are tuned to transduce dynamic stimuli at the molecular level is not well understood. Here we take advantage of the differential expression pattern of tmc1, tmc2a, and tmc2b, which encode channel subunits of the mechanotransduction complex in zebrafish vestibular hair cells. To test the role of various combinations of Tmc subunits in transducing dynamic head movements, we measured reflexive eye movements induced by high-frequency stimuli in single versus double tmc mutants. We found that Tmc2a function correlates with the broadest range of frequency sensitivity, whereas Tmc2b mainly contributes to lower-frequency responses. Tmc1, which is largely excluded from the striolar zone, plays a minor role in sensing lower-frequency stimuli. Our study suggests that the Tmc subunits impart functional differences to the mechanotransduction of dynamic stimuli.Significance Statement Information about dynamic head movements is transmitted by sensory receptors, known as hair cells, in the labyrinth of the inner ear. The sensitivity of hair cells to fast or slow movements of the head differs according to cell type. Whether the mechanotransduction complex that converts mechanical stimuli into electrical signals in hair cells participates in conveying frequency information is not clear. Here we find that the transmembrane channel-like 1/2 genes, which encode a central component of the complex, are differentially expressed in the utricle and contribute to frequency sensitivity in zebrafish.

Anatomical adventures in the fish auditory medullaa)

This paper provides an overview of my work on the central auditory system of fish. It focuses on my comparative analyses of a nucleus that receives input from the inner ear, the descending nucleus, and more specifically on that part of the descending nucleus supplied by the otolith end organs, the dorsal descending nucleus. I begin by summarizing my initial work on the bowfin, Amia calva, and go on to explain the importance of taking a comparative approach to understanding ancestral and specialized anatomical and putative functional characteristics of the dorsal descending nucleus in modern bony fishes, the teleosts.

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.

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.

Morphometric study of the vestibuloauditory organ of the African clawed frog, Xenopus laevis

Independent auditory end-organs appear first in amphibians in vertebrate phylogeny. In amphibians, sound detection is carried out by the amphibian papilla, basilar papilla and macula saccule. Amphibians inhabit distinct habitats and exhibit specific behaviours and sound frequency responses, so the amphibian vestibuloauditory system is an excellent model for considering the relationships between behaviour and physiological/anatomical vestibuloauditory properties. The African clawed frog, Xenopus laevis, lives in shallow water throughout its life and is thought to use sound in a higher frequency range compared with terrestrial anurans. In this study, the size of each vestibuloauditory end-organ and the distribution of ganglion cells in the vestibuloauditory ganglion were examined using haematoxylin and eosin staining and lectin histochemistry in Xenopus laevis. This study revealed that the size ratios among end-organs in Xenopus are similar to those in terrestrial anurans. Large and small cells were observed in the ganglion, but their distribution patterns are different from those in general terrestrial anurans. Lycopersicon esculentum lectin stained a large number of ganglion cells. Lectin-stained cells were found throughout the whole ganglion, but were especially abundant in the caudal part. These results suggested a unique distribution pattern of the vestibuloauditory ganglion cells in Xenopus.

Central Vestibular Tuning Arises from Patterned Convergence of Otolith Afferents

As sensory information moves through the brain, higher-order areas exhibit more complex tuning than lower areas. Though models predict that complexity arises via convergent inputs from neurons with diverse response properties, in most vertebrate systems, convergence has only been inferred rather than tested directly. Here, we measure sensory computations in zebrafish vestibular neurons across multiple axes in vivo. We establish that whole-cell physiological recordings reveal tuning of individual vestibular afferent inputs and their postsynaptic targets. Strong, sparse synaptic inputs can be distinguished by their amplitudes, permitting analysis of afferent convergence in vivo. An independent approach, serial-section electron microscopy, supports the inferred connectivity. We find that afferents with similar or differing preferred directions converge on central vestibular neurons, conferring more simple or complex tuning, respectively. Together, these results provide a direct, quantifiable demonstration of feedforward input convergence in vivo.

Auditory evoked potentials of utricular hair cells in the plainfin midshipman, Porichthys notatus

The plainfin midshipman, Porichthys notatus, is a soniferous marine teleost fish that generates acoustic signals for intraspecific social communication. Nocturnally active males and females rely on their auditory sense to detect and locate vocally active conspecifics during social behaviors. Previous work showed that the midshipman inner ear saccule and lagena are highly adapted to detect and encode socially relevant acoustic stimuli, but the auditory sensitivity and function of the midshipman utricle remain largely unknown. Here, we characterized the auditory evoked potentials from hair cells in the utricle of non-reproductive type I males and tested the hypothesis that the midshipman utricle is sensitive to behaviorally relevant acoustic stimuli. Hair cell potentials were recorded from the rostral, medial and caudal regions of the utricle in response to pure tone stimuli presented by an underwater speaker. We show that the utricle is highly sensitive to particle motion stimuli produced by an underwater speaker positioned in the horizontal plane. Utricular potentials were recorded across a broad range of frequencies with lowest particle acceleration (dB re. 1 m s-2) thresholds occurring at 105 Hz (lowest frequency tested; mean threshold -32 dB re. 1 m s-2) and highest thresholds at 605-1005 Hz (mean threshold range -5 to -4 dB re. 1 m s-2). The high gain and broadband frequency sensitivity of the utricle suggest that it likely serves a primary auditory function and is well suited to detect conspecific vocalizations including broadband agonistic signals and the multiharmonic advertisement calls produced by reproductive type I males.

Swim bladder enhances lagenar sensitivity to sound pressure and higher frequencies in female plainfin midshipman (Porichthys notatus)

The plainfin midshipman fish (Porichthys notatus) is an established model for investigating acoustic communication because the reproductive success of this species is dependent on the production and reception of social acoustic signals. Previous work showed that female midshipman have swim bladders with rostral horn-like extensions that project close to the saccule and lagena, while nesting (type I) males lack such rostral swim bladder extensions. The relative close proximity of the swim bladder to the lagena should increase auditory sensitivity to sound pressure and higher frequencies. Here, we test the hypothesis that the swim bladder of female midshipman enhances lagenar sensitivity to sound pressure and higher frequencies. Evoked potentials were recorded from auditory hair cell receptors in the lagena in reproductive females with intact (control condition) and removed (treated condition) swim bladders while pure tone stimuli (85-1005 Hz) were presented by an underwater speaker. Females with intact swim bladders had auditory thresholds 3-6 dB lower than females without swim bladders over a range of frequencies from 85 to 405 Hz. At frequencies from 545 to 1005 Hz, only females with intact swim bladders had measurable auditory thresholds (150-153 dB re. 1 µPa). The higher percentage of evoked lagenar potentials recorded in control females at frequencies >505 Hz indicates that the swim bladder extends the bandwidth of detectable frequencies. These findings reveal that the swim bladders in female midshipman can enhance lagenar sensitivity to sound pressure and higher frequencies, which may be important for the detection of behaviorally relevant social signals.

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.

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.

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.

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.

Lagenar potentials of the vocal plainfin midshipman fish, Porichthys notatus

The plainfin midshipman fish (Porichthys notatus) is a species of marine teleost that produces acoustic signals that are important for mediating social behavior. The auditory sensitivity of the saccule is well established in this species, but the sensitivity and function of the midshipman's putative auditory lagena are unknown. Here, we characterize the auditory-evoked potentials from hair cells in the lagena of reproductive type I males to determine the frequency response and auditory sensitivity of the lagena to behaviorally relevant acoustic stimuli. Lagenar potentials were recorded from the caudal and medial region of the lagena, while acoustic stimuli were presented by an underwater speaker. Our results indicate that the midshipman lagena has a similar low-frequency sensitivity to that of the midshipman saccule based on sound pressure and acceleration (re: 1 µPa and 1 ms^-2, respectively), but the thresholds of the lagena were higher across all frequencies tested. The relatively high auditory thresholds of the lagena may be important for encoding high levels of behaviorally relevant acoustic stimuli when close to  a sound source.

Directional hearing and sound source localization by fishes

Directional hearing may enable fishes to seek out prey, avoid predators, find mates, and detect important spatial cues. Early sound localization experiments gave negative results, and it was thought unlikely that fishes utilized the same direction-finding mechanisms as terrestrial vertebrates. However, fishes swim towards underwater sound sources, and some can discriminate between sounds from different directions and distances. The otolith organs of the inner ear detect the particle motion components of sound, acting as vector detectors through the presence of sensory hair cells with differing orientation. However, many questions remain on inner ear functioning. There are problems in understanding the actual mechanisms involved in determining sound direction and distance. Moreover, very little is still known about the ability of fishes to locate sound sources in three-dimensional space. Do fishes swim directly towards a source, or instead "sample" sound levels while moving towards the source? To what extent do fishes utilize other senses and especially vision in locating the source? Further behavioral studies of free-swimming fishes are required to provide better understanding of how fishes might actually locate sound sources. In addition, more experiments are required on the auditory mechanism that fishes may utilize.

The potential for the anterior lateral line to function for 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.