Fish Hearing Research Articles

Auditory sensitivity of larval zebrafish (Danio rerio) measured using a behavioral prepulse inhibition assay

Zebrafish (Danio rerio) have become a valuable model for investigating the molecular genetics and development of the inner ear in vertebrates. In this study, we employed a prepulse inhibition (PPI) paradigm to assess hearing in larval wild-type (AB) zebrafish during early development at 5-6 days post-fertilization (d.p.f.). We measured the PPI of the acoustic startle response in zebrafish using a 1-dimensional shaker that simulated the particle motion component of sound along the fish's dorsoventral axis. The thresholds to startle-inducing stimuli were determined in 5-6 d.p.f. zebrafish, and their hearing sensitivity was then characterized using the thresholds of prepulse tone stimuli (90-1200 Hz) that inhibited the acoustic startle response to a reliable startle stimulus (820 Hz at 20 dB re. 1 m s(-2)). Hearing thresholds were defined as the minimum prepulse tone level required to significantly reduce the startle response probability compared with the baseline (no-prepulse) condition. Larval zebrafish showed greatest auditory sensitivity from 90 to 310 Hz with corresponding mean thresholds of -19 to -10 dB re. 1 m s(-2), respectively. Hearing thresholds of prepulse tones were considerably lower than previously predicted by startle response assays. The PPI assay was also used to investigate the relative contribution of the lateral line to the detection of acoustic stimuli. After aminoglycoside-induced neuromast hair-cell ablation, we found no difference in PPI thresholds between treated and control fish. We propose that this PPI assay can be used to screen for novel zebrafish hearing mutants and to investigate the ontogeny of hearing in zebrafish and other fishes.

Structure and Possible Functions of Constant-Frequency Calls in Ariopsis seemanni (Osteichthyes, Ariidae)

In the 1970s, Tavolga conducted a series of experiments in which he found behavioral evidence that the vocalizations of the catfish species Ariopsis felis may play a role in a coarse form of echolocation. Based on his findings, he postulated a similar function for the calls of closely related catfish species. Here, we describe the physical characteristics of the predominant call-type of Ariopsis seemanni. In two behavioral experiments, we further explore whether A. seemanni uses these calls for acoustic obstacle detection by testing the hypothesis that the call-emission rate of individual fish should increase when subjects are confronted with novel objects, as it is known from other vertebrate species that use pulse-type signals to actively probe the environment. Audio-video monitoring of the fish under different obstacle conditions did not reveal a systematic increase in the number of emitted calls in the presence of novel objects or in dependence on the proximity between individual fish and different objects. These negative findings in combination with our current understanding of directional hearing in fishes (which is a prerequisite for acoustic obstacle detection) make it highly unlikely that A. seemanni uses its calls for acoustic obstacle detection. We argue that the calls are more likely to play a role in intra- or interspecific communication (e.g. in school formation or predator deterrence) and present results from a preliminary Y-maze experiment that are indicative for a positive phonotaxis of A. seemanni towards the calls of conspecifics.

Hearing in Cichlid Fishes under Noise Conditions

BackgroundHearing thresholds of fishes are typically acquired under laboratory conditions. This does not reflect the situation in natural habitats, where ambient noise may mask their hearing sensitivities. In the current study we investigate hearing in terms of sound pressure (SPL) and particle acceleration levels (PAL) of two cichlid species within the naturally occurring range of noise levels. This enabled us to determine whether species with and without hearing specializations are differently affected by noise.Methodology/Principal FindingsWe investigated auditory sensitivities in the orange chromide Etroplus maculatus, which possesses anterior swim bladder extensions, and the slender lionhead cichlid Steatocranus tinanti, in which the swim bladder is much smaller and lacks extensions. E. maculatus was tested between 0.2 and 3kHz and S. tinanti between 0.1 and 0.5 kHz using the auditory evoked potential (AEP) recording technique. In both species, SPL and PAL audiograms were determined in the presence of quiet laboratory conditions (baseline) and continuous white noise of 110 and 130 dB RMS. Baseline thresholds showed greatest hearing sensitivity around 0.5 kHz (SPL) and 0.2 kHz (PAL) in E. maculatus and 0.2 kHz in S. tinanti. White noise of 110 dB elevated the thresholds by 0–11 dB (SPL) and 7–11 dB (PAL) in E. maculatus and by 1–2 dB (SPL) and by 1–4 dB (PAL) in S. tinanti. White noise of 130 dB elevated hearing thresholds by 13–29 dB (SPL) and 26–32 dB (PAL) in E. maculatus and 6–16 dB (SPL) and 6–19 dB (PAL) in S. tinanti.ConclusionsOur data showed for the first time for SPL and PAL thresholds that the specialized species was masked by different noise regimes at almost all frequencies, whereas the non-specialized species was much less affected. This indicates that noise can limit sound detection and acoustic orientation differently within a single fish family.

Sensory epithelia of the fish inner ear in 3D: studied with high-resolution contrast enhanced microCT

IntroductionWhile a number of studies have illustrated and analyzed 3D models of inner ears in higher vertebrates, inner ears in fishes have rarely been investigated in 3D, especially with regard to the sensory epithelia of the end organs, the maculae. It has been suggested that the 3D curvature of these maculae may also play an important role in hearing abilities in fishes. We therefore set out to develop a fast and reliable approach for detailed 3D visualization of whole inner ears as well as maculae.ResultsHigh-resolution microCT imaging of black mollies Poecilia sp. (Poeciliidae, Teleostei) and Steatocranus tinanti (Cichlidae, Teleostei) stained with phosphotungstic acid (PTA) resulted in good tissue contrast, enabling us to perform a reliable 3D reconstruction of all three sensory maculae of the inner ears. Comparison with maculae that have been 3D reconstructed based on histological serial sections and phalloidin-stained maculae showed high congruence in overall shape of the maculae studied here.ConclusionsPTA staining and subsequent high-resolution contrast enhanced microCT imaging is a powerful method to obtain 3D models of fish inner ears and maculae in a fast and more reliable manner. Future studies investigating functional morphology, phylogenetic potential of inner ear features, or evolution of hearing and inner ear specialization in fishes may benefit from the use of 3D models of inner ears and maculae.

Angular Oscillation of Solid Scatterers in Response to Progressive Planar Acoustic Waves: Do Fish Otoliths Rock?

Fish can sense a wide variety of sounds by means of the otolith organs of the inner ear. Among the incompletely understood components of this process are the patterns of movement of the otoliths vis-à-vis fish head or whole-body movement. How complex are the motions? How does the otolith organ respond to sounds from different directions and frequencies? In the present work we examine the responses of a dense rigid scatterer (representing the otolith) suspended in an acoustic fluid to low-frequency planar progressive acoustic waves. A simple mechanical model, which predicts both translational and angular oscillation, is formulated. The responses of simple shapes (sphere and hemisphere) are analyzed with an acoustic finite element model. The hemispherical scatterer is found to oscillate both in the direction of the propagation of the progressive waves and also in the plane of the wavefront as a result of angular motion. The models predict that this characteristic will be shared by other irregularly-shaped scatterers, including fish otoliths, which could provide the fish hearing mechanisms with an additional component of oscillation and therefore one more source of acoustical cues.

Pheromones: Fish Fear Factor

Fish, like many other animals, panic when another individual is injured. Now, the chemical nature of a substance that mediates this reaction has been uncovered.

Behavioral Measure of Frequency Detection and Discrimination in the Zebrafish, Danio rerio

Behavioral tests of hearing in fish are relatively rare and are generally based upon aversive conditioning, with little data available for the positive reinforcement methods common in other vertebrates. Despite its increasing importance as an auditory model, no behavioral hearing measures have been conducted on zebrafish (Danio rerio), with only physiological hearing estimates available. In the current study, a new behavioral testing paradigm is developed to assess sound detection abilities of zebrafish and the effect of training frequency on hearing sensitivity. Zebrafish were trained to respond to either a 400 Hz or a 1000 Hz tone, and behavioral thresholds were then measured to tones from 200 to 1000 Hz. Significant threshold differences existed between the behavioral audiograms, with fish from each set most sensitive to their conditioned frequency. Furthermore, fish acoustically conditioned to 1000 Hz were most sensitive to the upper range of test frequencies (600-1000 Hz). This appears to be the first study utilizing a positive reinforcement behavioral assay for testing hearing in zebrafish and provides further evidence of fine-scale auditory filtering in fish.

From Blind Cave Fish to the Tappan Zee Bridge – A Tale of “Translational Neuroethology”

Event Abstract Back to Event From Blind Cave Fish to the Tappan Zee Bridge – A Tale of “Translational Neuroethology” Arthur N. Popper1* 1 University of Maryland, Department of Biology, United States Over the more than 45 years since I started working on fish hearing there has been a substantial change in the way that the public views what we scientists do and how we do it. In contrast to past years, society today expects that scientists will ask and answer questions of relatively immediate social value. Indeed, the word “translational” has entered the vocabulary of much of science. Thus, where we once thought only in terms of answering excellent basic science questions, the current, and growing, expectation is that research today will be reasonably closely tied to human needs. In this presentation I am going to talk about the research that has been the central theme of most of my scientific career – how, what, and why fish hear, and the evolution of vertebrate hearing. Indeed, my research started out with trying to understand what fish hear, and then continued to include questions on such topics as: the morphology and ultrastructure of the auditory system; development, regeneration, and evolution of sensory hair cells and the ear; the physiology of the inner ear; why animals hear; and the neuroethology of fish ultrasound detection. And along with way I digressed a bit into work on sharks and marine mammals, and also edited a few books. However, more recently I have “reinvented” myself so that while I retain my fundamental interest in basic auditory neuroethology, the specific questions I now ask enable me to make direct contributions to solving some quite interesting and important societal problems. These tend to focus on how the addition of very substantial and intense man-made sounds to the environment can affect animals, and how to evaluate and mitigate those effects. Indeed, my personal “translation” has become a great deal of fun, and has led me in new and exciting directions I would not have imagined even 10 years ago. Keywords: anthropogenic, bioacoustics, Ear, evolution, fish, Hearing, Sound Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Plenary Address (including special lectures) (Note, these individuals have already been invited) Topic: Sensory: Audition Citation: Popper AN (2012). From Blind Cave Fish to the Tappan Zee Bridge – A Tale of “Translational Neuroethology”. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00030 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Dr. Arthur N Popper, University of Maryland, Department of Biology, College Park, Maryland, 20742, United States, apopper@umd.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Arthur N Popper Google Arthur N Popper Google Scholar Arthur N Popper PubMed Arthur N Popper Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Effects of mid-frequency active sonar on hearing in fish

Caged fish were exposed to sound from mid-frequency active (MFA) transducers in a 5 × 5 planar array which simulated MFA sounds at received sound pressure levels of 210 dB SPL(re 1 μPa). The exposure sound consisted of a 2 s frequency sweep from 2.8 to 3.8 kHz followed by a 1 s tone at 3.3 kHz. The sound sequence was repeated every 25 s for five repetitions resulting in a cumulative sound exposure level (SEL(cum)) of 220 dB re 1 μPa(2) s. The cumulative exposure level did not affect the hearing sensitivity of rainbow trout, a species whose hearing range is lower than the frequencies in the presented MFA sound. In contrast, one cohort of channel catfish showed a statistically significant temporary threshold shift of 4-6 dB at 2300 Hz, but not at lower tested frequencies, whereas a second cohort showed no change. It is likely that this threshold shift resulted from the frequency spectrum of the MFA sound overlapping with the upper end of the hearing frequency range of the channel catfish. The observed threshold shifts in channel catfish recovered within 24 h. There was no mortality associated with the MFA sound exposure used in this test.

Acoustic Communication: Sound Advice from Piranhas

An elegant new study has correlated the generation of sound patterns in the red-bellied piranha (Pygocentrus nattereri) with three distinct behaviours.

Acoustically induced streaming flows near a model cod otolith and their potential implications for fish hearing

The ears of fishes are remarkable sensors for the small acoustic disturbances associated with underwater sound. For example, each ear of the Atlantic cod (Gadus morhua) has three dense bony bodies (otoliths) surrounded by fluid and tissue, and detects sounds at frequencies from 30 to 500 Hz. Atlantic cod have also been shown to localize sounds. However, how their ears perform these functions is not fully understood. Steady streaming, or time-independent, flows near a 350% scale model Atlantic cod otolith immersed in a viscous fluid were studied to determine if these fluid flows contain acoustically relevant information that could be detected by the ear's sensory hair cells. The otolith was oscillated sinusoidally at various orientations at frequencies of 8-24 Hz, corresponding to an actual frequency range of 280-830 Hz. Phase-locked particle pathline visualizations of the resulting flows give velocity, vorticity, and rate of strain fields over a single plane of this mainly two-dimensional flow. Although the streaming flows contain acoustically relevant information, the displacements due to these flows are likely too small to explain Atlantic cod hearing abilities near threshold. The results, however, may suggest a possible mechanism for detection of ultrasound in some fish species.

Experimental investigation of the Krysl–Cranford–Schilt model for the otolith vibration of a teleost fish.

The Krysl–Cranford–Schilt model [Schilt, et al., J. Acoust. Soc. Am. 127, 1754 (2010)] for the response of a teleost otoith to sound makes some unexpected predictions, which, if true, would have significant implications for our understanding of the mechanisms of hearing in fish. It has been believed for over 60 years that the otoliths function like the inertial mass of an accelerometer, which, in the absence of attachments to the skull (or a swimbladder), would vibrate back and forth in the direction the acoustic particle velocity but with a smaller amplitude. The KCS model, however, predicts that the otolith motion is highly dependent on both the frequency and direction of the incident sound wave and involves significant rocking motion as well as translation. If this is true, then the particle-motion-vector models for directional hearing in fish, which have held sway for over 40 years are incorrect. The validity of this model is experimentally tested in a direct and simple manner. An otolith is suspended in a sphere of low-shear-modulus tissue phantom in a large acoustic test tank. The motion of the acoustically excited otolith is measured using an ultrasonic vibrometer.

Auditory evoked potential audiometry in fish.

A recent survey lists more than 100 papers utilizing the auditory evoked potential (AEP) recording technique for studying hearing in fish. More than 90% of these AEP-studies were published after Kenyon et al. introduced a non-invasive electrophysiological approach in 1998 allowing rapid evaluation of hearing and repeated testing of animals. Applying and further developing the AEP-technique enabled the investigation of a wide range of scientific questions. First, it was possible to describe and compare basic hearing abilities in a large number of species. Subsequently, the ontogenetic development of hearing as well as the influence of various accessory hearing structures (Weberian ossicles and swimbladder) were studied. The technique was also successfully utilized to analyze the temporal resolution ability of the auditory system. The AEP-technique is suitable for studying threshold shifts after exposure to (TTS) and in the presence of (masking) various noise types (white, ambient, and anthropogenic). Comparison of AEP-audiograms with sound spectra, along with the analysis of AEPs in response to conspecific sounds, enabled us to assess the ability to communicate acoustically in general, and during ontogeny and in the presence of noise in particular. Finally, various factors potentially influencing hearing such as temperature, albinism, and cave dwelling were investigated. [Work supported by the FWF.]

How do albino fish hear?

Pigmentation disorders such as albinism are occasionally associated with hearing impairments in mammals. Therefore, we wanted to investigate whether such a phenomenon also exists in non-mammalian vertebrates. We measured the hearing abilities of normally pigmented and albinotic specimens of two catfish species, the European wels Silurus glanis (Siluridae) and the South American bronze catfish Corydoras aeneus (Callichthyidae). The non-invasive auditory evoked potential (AEP) recording technique was utilized to determine hearing thresholds at 10 frequencies from 0.05 to 5 kHz. Neither auditory sensitivity nor shape of AEP waveforms differed between normally pigmented and albinotic specimens at any frequency tested in both species. Silurus glanis and C. aeneus showed the best hearing between 0.3 and 1 kHz; the lowest thresholds were 78.4 dB at 0.5 kHz in S. glanis (pigmented), 75 dB at 1 kHz in S. glanis (albinotic), 77.6 dB at 0.5 kHz in C. aeneus (pigmented) and 76.9 dB at 1 kHz in C. aeneus (albinotic). This study indicates no association between albinism and hearing ability. Perhaps because of the lack of melanin in the fish inner ear, hearing in fishes is less likely to be affected by albinism than in mammals.

Frequency coding of particle motion by saccular afferents of a teleost fish

The saccule is known to play an important role in hearing in fishes. In this study we investigated spatial frequency selectivity of single saccular afferents in a teleost fish (the sleeper goby, Dormitator latifrons) to acoustic particle motion at 50-400 Hz. Saccular afferents have similar distributions of best sensitivity (-90.0 to -54.8 dB re. 1 g, mean +/- s.d.=-81.1+/-8.0 dB) and characteristic frequencies (<or=50-400 Hz, median=80 Hz) along the longitudinal, side-to-side and dorsoventral axes of fish. They were lowpass, bandpass or broadly tuned to low frequencies with Q(50%) at 15 dB above threshold in ranges from 0.28 to 3.30 (1.46+/-0.71), 0.18 to 2.54 (1.36+/-0.78), and 0.41 to 4.26 (2.25+/-0.1.12) along the three axes, showing slightly greater frequency tuning in the vertical axis than horizontal axes. At supra threshold we found tuning plasticity, i.e. best frequencies of saccular afferents shifted to high frequencies as stimulus level increased. Isolevel rate-frequency curves were asymmetrical with shallow slopes at the low-frequency edge and steep slopes at the high-frequency edge. Saccular afferents of the sleeper goby have similar capabilities of coding particle motion frequencies in the three orthogonal axes. Results from this and other studies suggest that (1) the saccule is the major hearing organ in this species, (2) the saccule of this species is capable of encoding sound frequencies in three dimensional space, and (3) saccular afferents in fishes without accessory auditory structures exhibit similar frequency selectivity in response to particle motion.

Year-round variability of ambient noise in temperate freshwater habitats and its implications for fishes

Changes in habitat acoustics over the year can potentially affect fish hearing and orientation to sound, especially in temperate climates. This is the first study where year-round changes in ambient noise in aquatic habitats were assessed. Seven different European fresh-water habitats were chosen for this study. Sound pressure level (SPL) and spectral composition of the ambient noise varied in both quiet stagnant habitats (lakes, backwaters) and in flowing habitats (streams, rivers). Linear equivalent SPL (L(Leq, 60s)) tended to be lower in stagnant habitats (means: 91.6-111.7 dB) than in flowing habitats (means: 111.2-133.4 dB). The changes in SPL were smallest in the river (means: 4.2-4.4 dB, maxima: 8.5-10.1 dB), whereas significantly higher values were measured in stagnant habitats and the stream (means: 9.9-14.9 dB, maxima: 25.1-30.9 dB). The spectral compositions of the ambient noise determined at different times of the year were highly correlated to each other at the river sites (mean cross-correlation coefficients: 0.85 and 0.94) and were weaker or not correlated at the other study sites (means: 0.24-0.76). The changes in ambient noise spectra were negatively correlated to changes in SPL, indicating that large changes in SPLs were accompanied by large changes in spectral composition and vice versa. Comparison of these ecoacoustical data with a preceding study (Amoser and Ladich in J Exp Biol 208:3533-3542, 2005) indicates that the auditory sensitivity in hearing specialists is affected by changes in ambient noise levels and spectra throughout a year and that this effect tends to be more pronounced in stagnant waters and the stream than at river sites. On the other hand, absolute noise levels result in a higher degree of masking in flowing waters.

Ontogenetic development of auditory sensitivity and sound production in the squeaker catfish Synodontis schoutedeni

BackgroundSurveys of ontogenetic development of hearing and sound production in fish are scarce, and the ontogenetic development of acoustic communication has been investigated in only two fish species so far. Studies on the labyrinth fish Trichopsis vittata and the toadfish Halobatrachus didactylus show that the ability to detect conspecific sounds develops during growth. In otophysine fish, which are characterized by Weberian ossicles and improved hearing sensitivities, the ontogenetic development of sound communication has never been investigated. We analysed the ontogeny of the auditory sensitivity and vocalizations in the mochokid catfish Synodontis schoutedeni. Mochokid catfishes of the genus Synodontis are commonly called squeakers because they produce broadband stridulation sounds during abduction and adduction of pectoral fin spines. Fish from six different size groups - from 22 mm standard length to 126 mm - were studied. Hearing thresholds were measured between 50 Hz and 6 kHz using the auditory evoked potentials recording technique; stridulation sounds were recorded and their sound pressure levels determined. Finally, absolute sound power spectra were compared to auditory sensitivity curves within each size group.ResultsThe smallest juveniles showed the poorest hearing abilities of all size groups between 50 and 1,000 Hz and highest hearing sensitivity at 5 and 6 kHz. The duration of abduction and adduction sounds and the pulse period increased and sound pressure level (in animals smaller than 58 mm) increased, while the dominant frequency of sounds decreased with size in animals larger than 37 mm. Comparisons between audiograms and sound spectra revealed that the most sensitive frequencies correlate with the dominant frequencies of stridulation sounds in all S. schoutedeni size groups and that all specimens are able to detect sounds of all size groups.ConclusionsThis study on the squeaker catfish S. schoutedeni is the first to demonstrate that absolute hearing sensitivity changes during ontogeny in an otophysine fish. This contrasts with prior studies on two cypriniform fish species in which no such change could be observed. Furthermore, S. schoutedeni can detect conspecific sounds at all stages of development, again contrasting with prior findings in fishes.

Rethinking sound detection by fishes

In this paper we reconsider the designation of fishes as being either “hearing specialists” or “hearing generalists,” and recommend dropping the terms. We argue that this classification is only vaguely and variously defined in the literature, and that these terms often have unclear and different meaning to different investigators. Furthermore, we make the argument that the ancestral, and most common, mode of hearing in fishes involves sensitivity to acoustic particle motion via direct inertial stimulation of the otolith organ(s). Moreover, any possible pressure sensitivity is the result of the presence of an air bubble (e.g., the swim bladder), and that hearing sensitivity may be enhanced by the fish having a specific connection between the inner ear to a bubble of air. There are data showing that some fish species have a sensitivity to both pressure and motion that is frequency dependent. Thus such species could not possibly be termed as either hearing “generalists” or specialists,” and many more species probably could be classified in this way as well. Furthermore, we propose that the term “specialization” be reserved for cases in which a species has some kind of morphological connection or close continuity between the inner ear and an air bubble that affects behavioral sensitivity to sound pressure (i.e., an otophysic connection).

Possible role of steady streaming flows in fish hearing.

Over the last five years, the authors have studied the time-independent portion, or the steady streaming, of flows near model otoliths and determined that these flows encode both the direction and frequency of incident sounds. This talk summarizes these findings and discusses whether the fish ear can detect the displacements due to these flows. Although the results for a model cod otolith indicate that information relating to the direction of the local sound field is indeed contained within the flow patterns in the vicinity of the sensory macula, the threshold for the detectability of these directional cues appears to be much higher than published thresholds for directionalization in fish. There is some evidence, however, that the observed thresholds may have been underestimated. While these studies indicate that steady streaming flows may be insufficient to explain directionalization in fish, the results suggest that steady streaming may provide a mechanism for fish to hear ultrasound. [Work supported by ONR.]

Fish hearing and bioacoustics since 1973.

In 1973, we wrote a review in J. Acoust. Soc. Am. that summarized our view of fish hearing and bioacoustics. This presentation looks back on the the major questions we had at the time and how the field has dealt with them, or not, to the present. Some of the issues identified included: (1) the roles of the various otolith organs for hearing; (2) lateral line function and its relation to hearing; (3) sound source localization; and (4) how sound stimulation works for fishes without obvious specializations for pressure reception. There have been major advances in these and other aspects of fish sound reception, but in every case, there are significant unanswered questions. For example, it is still controversial whether the utricle plays a role in hearing in most species, and the functional role of the lagena remains an enigma. However, we now have a much better understanding of lateral line stimulation and function, and we now understand that it is not an “accessory auditory organ,” as was widely assumed initially. Many questions of sound localization are now thought to be answered theoretically, and in some cases empirically, but many critical and fundamental experiments on localization remain to be done.