Structure and function in the saccule of the goldfish (Carassius auratus): a model of diversity in the non-amniote ear

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.

The Sound World of Zebrafish: A Critical Review of Hearing Assessment

Zebrafish, like all fish species, use sound to learn about their environment. Thus, human-generated (anthropogenic) sound added to the environment has the potential to disrupt the detection of biologically relevant sounds, alter behavior, impact fitness, and produce stress and other effects that can alter the well-being of animals. This review considers the bioacoustics of zebrafish in the laboratory with two goals. First, we discuss zebrafish hearing and the problems and issues that must be considered in any studies to get a clear understanding of hearing capabilities. Second, we focus on the potential effects of sounds in the tank environment and its impact on zebrafish physiology and health. To do this, we discuss underwater acoustics and the very specialized acoustics of fish tanks, in which zebrafish live and are studied. We consider what is known about zebrafish hearing and what is known about the potential impacts of tank acoustics on zebrafish and their well-being. We conclude with suggestions regarding the major gaps in what is known about zebrafish hearing as well as questions that must be explored to better understand how well zebrafish tolerate and deal with the acoustic world they live in within laboratories.

Convergence of Olfactory Inputs within the Central Nervous System of a Cartilaginous and a Bony Fish: An Anatomical Indicator of Olfactory Sensitivity

The volume of the olfactory bulbs (OBs) relative to the brain has been used previously as a proxy for olfactory capabilities in many vertebrate taxa, including fishes. Although this gross approach has predictive power, a more accurate assessment of the number of afferent olfactory inputs and the convergence of this information at the level of the telencephalon is critical to our understanding of the role of olfaction in the behaviour of fishes. In this study, we used transmission electron microscopy to assess the number of first-order axons within the olfactory nerve (ON) and the number of second-order axons in the olfactory peduncle (OP) in established model species within cartilaginous (brownbanded bamboo shark, Chiloscyllium punctatum [CP]) and bony (common goldfish, Carassius auratus [CA]) fishes. The total number of axons varied from a mean of 18.12 ± 7.50 million in the ON to a mean of 0.38 ± 0.21 million in the OP of CP, versus 0.48 ± 0.16 million in the ON and 0.09 ± 0.02 million in the OP of CA. This resulted in a convergence ratio of approximately 50:1 and 5:1, respectively, for these two species. Based on astroglial ensheathing, axon type (unmyelinated [UM] and myelinated [M]) and axon size, we found no differentiated tracts in the OP of CP, whereas a lateral and a medial tract (both of which could be subdivided into two bundles or areas) were identified for CA, as previously described. Linear regression analyses revealed significant differences not only in axon density between species and locations (nerves and peduncles), but also in axon type and axon diameter (p < 0.05). However, UM axon diameter was larger in the OPs than in the nerve in both species (p = 0.005), with no significant differences in UM axon diameter in the ON (p = 0.06) between species. This study provides an in-depth analysis of the neuroanatomical organisation of the ascending olfactory pathway in two fish taxa and a quantitative anatomical comparison of the summation of olfactory information. Our results support the assertion that relative OB volume is a good indicator of the level of olfactory input and thereby a proxy for olfactory capabilities.

Volumetric analysis and morphological assessment of the ascending olfactory pathway in an elasmobranch and a teleost using diceCT

The size (volume or mass) of the olfactory bulbs in relation to the whole brain has been used as a neuroanatomical proxy for olfactory capability in a range of vertebrates, including fishes. Here, we use diffusible iodine-based contrast-enhanced computed tomography (diceCT) to test the value of this novel bioimaging technique for generating accurate measurements of the relative volume of the main olfactory brain areas (olfactory bulbs, peduncles, and telencephalon) and to describe the morphological organisation of the ascending olfactory pathway in model fish species from two taxa, the brownbanded bamboo shark Chiloscyllium punctatum and the common goldfish Carassius auratus. We also describe the arrangement of primary projections to the olfactory bulb and secondary projections to the telencephalon in both species. Our results identified substantially larger olfactory bulbs and telencephalon in C. punctatum compared to C. auratus (comprising approximately 5.2% vs. 1.8%, and 51.8% vs. 11.8% of the total brain volume, respectively), reflecting differences between taxa, but also possibly in the role of olfaction in the sensory ecology of these species. We identified segregated primary projections to the bulbs, associated with a compartmentalised olfactory bulb in C. punctatum, which supports previous findings in elasmobranch fishes. DiceCT imaging has been crucial for visualising differences in the morphological organisation of the olfactory system of both model species. We consider comparative neuroanatomical studies between representative species of both elasmobranch and teleost fish groups are fundamental to further our understanding of the evolution of the olfactory system in early vertebrates and the neural basis of olfactory abilities.

Sensorimotor Transformations in the Zebrafish Auditory System

Organisms use their sensory systems to acquire information from their environment and integrate this information to produce relevant behaviors. Nevertheless, how sensory information is converted into adequate motor patterns in the brain remains an open question. Here, we addressed this question using two-photon and light-sheet calcium imaging in intact, behaving zebrafish larvae. We monitored neural activity elicited by auditory stimuli while simultaneously recording tail movements. We observed a spatial organization of neural activity according to four different response profiles (frequency tuning curves), suggesting a low-dimensional representation of frequency information, maintained throughout the development of the larvae. Low frequencies (150-450 Hz) were locally processed in the hindbrain and elicited motor behaviors. In contrast, higher frequencies (900-1,000 Hz) rarely induced motor behaviors and were also represented in the midbrain. Finally, we found that the sensorimotor transformations in the zebrafish auditory system are a continuous and gradual process that involves the temporal integration of the sensory response in order to generate a motor behavior.

Morphology of the utricular otolith organ in the toadfish, Opsanus tau

The utricle provides the vestibular reflex pathways with the sensory codes of inertial acceleration of self-motion and head orientation with respect to gravity to control balance and equilibrium. Here we present an anatomical description of this structure in the adult oyster toadfish and establish a morphological basis for interpretation of subsequent functional studies. Light, scanning, and transmission electron microscopy techniques were applied to visualize the sensory epithelium at varying levels of detail, its neural innervation and its synaptic organization. Scanning electron microscopy was used to visualize otolith mass and morphological polarization patterns of hair cells. Afferent nerve fibers were visualized following labeling with biocytin, and light microscope images were used to make three-dimensional (3-D) reconstructions of individual labeled afferents to identify dendritic morphology with respect to epithelial location. Transmission electron micrographs were compiled to create a serial 3-D reconstruction of a labeled afferent over a segment of its dendritic field and to examine the cell-afferent synaptic contacts. Major observations are: a well-defined striola, medial and lateral extra-striolar regions with a zonal organization of hair bundles; prominent lacinia projecting laterally; dependence of hair cell density on macular location; narrow afferent dendritic fields that follow the hair bundle polarization; synaptic specializations issued by afferents are typically directed towards a limited number of 7-13 hair cells, but larger dendritic fields in the medial extra-striola can be associated with > 20 hair cells also; and hair cell synaptic bodies can be confined to only an individual afferent or can synapse upon several afferents.

Evidence of Cnidarians sensitivity to sound after exposure to low frequency underwater sources

Jellyfishes represent a group of species that play an important role in oceans, particularly as a food source for different taxa and as a predator of fish larvae and planktonic prey. The massive introduction of artificial sound sources in the oceans has become a concern to science and society. While we are only beginning to understand that non-hearing specialists like cephalopods can be affected by anthropogenic noises and regulation is underway to measure European water noise levels, we still don’t know yet if the impact of sound may be extended to other lower level taxa of the food web. Here we exposed two species of Mediterranean Scyphozoan medusa, Cotylorhiza tuberculata and Rhizostoma pulmo to a sweep of low frequency sounds. Scanning electron microscopy (SEM) revealed injuries in the statocyst sensory epithelium of both species after exposure to sound, that are consistent with the manifestation of a massive acoustic trauma observed in other species. The presence of acoustic trauma in marine species that are not hearing specialists, like medusa, shows the magnitude of the problem of noise pollution and the complexity of the task to determine threshold values that would help building up regulation to prevent permanent damage of the ecosystems.

Fishy Hearing: A Short Biography of Arthur N. Popper, PhD

Biologist Dr. Arthur Popper's career spans decades, from his early work on comparative inner ear morphology in fishes to his recent interest in how underwater noise impacts aquatic vertebrates. Along the way Dr. Popper's research subjects span at least 19 vertebrate taxa, from lamprey to lungfish to humans, and he's had a profound influence in the field of fish bioacoustics. This brief biography describes some of Dr. Popper's many contributions to fish hearing research and highlights both some of his major discoveries and some of the biological mysteries he has yet to solve.

Relationship Between Hair Cell Loss and Hearing Loss in Fishes

Exposure to intense sound or ototoxic chemicals can damage the auditory hair cells of vertebrates, resulting in hearing loss. Although the relationship between such hair cell damage and auditory function is fairly established for terrestrial vertebrates, there are limited data available to understand this relationship in fishes. Although investigators have measured either the morphological damage of the inner ear or the functional deficits in the hearing of fishes, very few have directly measured both in an attempt to find a relationship between the two. Those studies that have examined both auditory hair cell damage in the inner ear and the resulting hearing loss in fishes are reviewed here. In general, there is a significant linear relationship between the number of hair cells lost and the severity of hearing threshold shifts, although this varies between species and different hair cell-damaging stimuli. After trauma to the fish ear, auditory hair cells are able to regenerate to control level densities. With this regeneration also comes a restoration of hearing. Thus there is also a significant relationship between hair cell recovery and hearing recovery in fishes.

Chemical Ototoxicity of the Fish Inner Ear and Lateral Line

Hair cell-driven mechanosensory systems are crucial for successful execution of a number of behaviors in fishes, and have emerged as good models for exploring questions relevant to human hearing. This review focuses on ototoxic effects in the inner ear and lateral line system of fishes. We specifically examine studies where chemical ototoxins such as aminoglycoside antibiotics have been employed as tools to disable the lateral line. Lateral line ablation results in alterations to feeding behavior and orientation to water current in a variety of species. However, neither behavior is abolished in the presence of additional sensory cues, supporting the hypothesis that many fish behaviors are driven by multisensory integration. Within biomedical research, the larval zebrafish lateral line has become an important model system for understanding signaling mechanisms that contribute to hair cell death and for developing novel pharmacological therapies that protect hair cells from ototoxic damage. Furthermore, given that fishes robustly regenerate damaged hair cells, ototoxin studies in fishes have broadened our understanding of the molecular and genetic events in an innately regenerative system, offering potential targets for mammalian hair cell regeneration. Collectively, studies of fish mechanosensory systems have yielded insight into fish behavior and in mechanisms of hair cell death, protection, and regeneration.

Ontogenetic development of the auditory sensory organ in zebrafish (Danio rerio): changes in hearing sensitivity and related morphology

Zebrafish (Danio rerio) is an important model organism in hearing research. However, data on the hearing sensitivity of zebrafish vary across different reports. In the present study, the hearing sensitivity of zebrafish was examined by analysing the auditory evoked potentials (AEPs) over a range of total lengths (TLs) from 12 to 46 mm. Morphological changes in the hair cells (HCs) of the saccule (the main auditory end organ) and their synapses with primary auditory neurons were investigated. The AEPs were detected up to a much higher frequency limit (12 kHz) than previously reported. No significant difference in the frequency response range was observed across the TL range examined. However, the AEP thresholds demonstrated both developmental improvement and age-related loss of hearing sensitivity. The changes in hearing sensitivity were roughly consistent with the morphological changes in the saccule including (1) the number and density of HCs, (2) the organization of stereocilia, and (3) the quantity of a main ribbon protein, Ribeye b. The results of this study established a clear baseline for the hearing ability of zebrafish and revealed that the changes in the saccule contribute to the observed changes in TL (age)-related hearing sensitivity.

The connexin 30.3 of zebrafish homologue of human connexin 26 may play similar role in the inner ear

The intercellular gap junction channels formed by connexins (CXs) are important for recycling potassium ions in the inner ear. CXs are encoded by a family of the CX gene, such as GJB2, and the mechanism leading to mutant connexin-associated diseases, including hearing loss, remains to be elucidated. In this study, using bioinformatics, we found that two zebrafish cx genes, cx27.5 and cx30.3, are likely homologous to human and mouse GJB2. During embryogenesis, zebrafish cx27.5 was rarely expressed at 1.5-3 h post-fertilization (hpf), but a relatively high level of cx27.5 expression was detected from 6 to 96 hpf. However, zebrafish cx30.3 transcripts were hardly detected until 9 hpf. The temporal experiment was conducted in whole larvae. Both cx27.5 and cx30.3 transcripts were revealed significantly in the inner ear by reverse transcription polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization (WISH). In the HeLa cell model, we found that zebrafish Cx27.5 was distributed intracellularly in the cytoplasm, whereas Cx30.3 was localized in the plasma membrane of HeLa cells stably expressing Cx proteins. The expression pattern of zebrafish Cx30.3 in HeLa cells was more similar to that of cells expressing human CX26 than Cx27.5. In addition, we found that Cx30.3 was localized in the cell membrane of hair cells within the inner ear by immunohistochemistry (IHC), suggesting that zebrafish cx30.3 might play an essential role in the development of the inner ear, in the same manner as human GJB2. We then performed morpholino knockdown studies in zebrafish embryos to elucidate the physiological functions of Cx30.3. The zebrafish cx30.3 morphants exhibited wild-type-like and heart edema phenotypes with smaller inner ears at 72 hpf. Based on these results, we suggest that the zebrafish Cx30.3 and mammalian CX26 may play alike roles in the inner ear. Thus, zebrafish can potentially serve as a model for studying hearing loss disorders that result from human CX26 mutations.

Patterns of saccular afferent innervation in sciaenids

In this study, saccular afferent arborization patterns in Atlantic croaker Micropogonias undulatus, red drum Sciaenops ocellatus and spot Leiostomus xanthurus were characterized. Leiostomus xanthurus showed the simplest configuration while M. undulatus displayed the most complex. In addition, hair-cell densities at sites sampled along the rostro-caudal axis of the saccular epithelia correlated with the observed patterns of arborization.

Plasticity in ion channel expression underlies variation in hearing during reproductive cycles

Sensory plasticity related to reproductive state, hormonal profiles, and experience is widespread among vertebrates, including humans. Improvements in audio-vocal coupling that heighten the detection of conspecifics are part of the reproductive strategy of many nonmammalian vertebrates. Although seasonal changes in hearing are known, molecular mechanisms determining this form of adult sensory plasticity remain elusive. Among both nonmammals and mammals, large-conductance, calcium-activated potassium (BK) channels underlie a primary outward current having a predominant influence on frequency tuning in auditory hair cells. We now report an example from fish showing that increased BK channel abundance can improve an individual's ability to hear vocalizations during the breeding season. Pharmacological manipulations targeting BK channels, together with measures of BK transcript abundance, can explain the seasonal enhancement of auditory hair cell sensitivity to the frequency content of calls. Plasticity in ion channel expression is a simple, evolutionarily labile solution for sculpting sensory bandwidth to maximize the detection of conspecific signals during reproductive cycles.

Structural and functional effects of acoustic exposure in goldfish: evidence for tonotopy in the teleost saccule

BACKGROUND

Mammalian and avian auditory hair cells display tonotopic mapping of frequency along the length of the cochlea and basilar papilla. It is not known whether the auditory hair cells of fishes possess a similar tonotopic organization in the saccule, which is thought to be the primary auditory receptor in teleosts. To investigate this question, we determined the location of hair cell damage in the saccules of goldfish (Carassius auratus) following exposure to specific frequencies. Subjects were divided into six groups of six fish each (five treatment groups plus control). The treatment groups were each exposed to one of five tones: 100, 400, 800, 2000, and 4000 Hz at 176 dB re 1 μPa root mean squared (RMS) for 48 hours. The saccules of each fish were dissected and labeled with phalloidin in order to visualize hair cell bundles. The hair cell bundles were counted at 19 specific locations in each saccule to determine the extent and location of hair cell damage. In addition to quantification of anatomical injury, hearing tests (using auditory evoked potentials) were performed on each fish immediately following sound exposure. Threshold shifts were calculated by subtracting control thresholds from post-sound exposure thresholds.

RESULTS

All sound-exposed fish exhibited significant hair cell and hearing loss following sound exposure. The location of hair cell loss varied along the length of the saccule in a graded manner with the frequency of sound exposure, with lower and higher frequencies damaging the more caudal and rostral regions of the saccule, respectively. Similarly, fish exposed to lower frequency tones exhibited greater threshold shifts at lower frequencies, while high-frequency tone exposure led to hearing loss at higher frequencies. In general, both hair cell and hearing loss declined as a function of increasing frequency of exposure tone, and there was a significant linear relationship between hair cell loss and hearing loss.

CONCLUSIONS

The pattern of hair cell loss as a function of exposure tone frequency and saccular rostral-caudal location is similar to the pattern of hearing loss as a function of exposure tone frequency and hearing threshold frequency. This data suggest that the frequency analysis ability of goldfish is at least partially driven by peripheral tonotopy in the saccule.

Differential ablation of sensory receptors underlies ototoxin-induced shifts in auditory thresholds of the goldfish (Carassius auratus)

In recent years, fish models have become popular for investigations of ototoxic agents. However, the vast majority of such studies have focused on anatomical changes in lateral line hair cells after drug administration. Using the goldfish (Carassius auratus), we confirm that the acquisition of auditory evoked potentials offers a rapid and non-invasive method for quantifying ototoxin-induced changes in hearing sensitivity. Gentamicin (100 mg ml(-1)) was the drug of choice as it is a well-studied human ototoxin. Auditory threshold elevation was observed between 300 and 600 Hz and was accompanied by significant reductions in hair cell ciliary bundle densities in specific regions of the utricle and saccule. The correlations between structure and function suggest that differential susceptibility of sensory hair cells to acute gentamicin treatment underlies the frequency-specific elevation of auditory thresholds. We propose that fish auditory systems should be used alongside the lateral line, for the assessment of ototoxicity in new-developed drugs.

The ears of butterflyfishes (Chaetodontidae): 'hearing generalists' on noisy coral reefs?

Analysis of the morphology of all three otolithic organs (sacculus, lagena and utriculus), including macula shape, hair cell morphology, density, orientation pattern, otolith morphology and the spatial relationships of the swimbladder and ear, reveals that butterflyfishes in the genera Chaetodon (which has anterior swimbladder horns) and Forcipiger (which lacks anterior swimbladder horns) both demonstrate the ear morphology typical of teleosts that lack otophysic connections, fishes that have traditionally been considered to be 'hearing generalists'.

Physiological dimensions of ototoxic responses in a model fish species

Pharmaceutical agents known to be toxic to the human auditory system also impair sensory hair cells of teleosts, and this supports the use of fish models for the screening of such compounds. However, previous investigations have focused almost exclusively on anatomical changes after drug administration without assessing macro-level physiological effects. Using the goldfish (Carassius auratus), we demonstrate that the acquisition of auditory evoked potentials offers a rapid and non-invasive means for tracking ototoxin-induced shifts in auditory thresholds. Gentamicin (100mg/mL) was the agent of choice as it is an extensively-studied human ototoxin. Significant shifts (p<0.05) in hearing sensitivity were observed between 300 Hz and 600 Hz and these shifts depended on acoustic pressure, but not particle motion. This differential elevation of auditory thresholds may be caused by impairment of specific populations of auditory sensory hair cells.

Myosin VI and VIIa distribution among inner ear epithelia in diverse fishes

Unconventional myosins are critical motor proteins in the vertebrate inner ear. Mutations in any one of at least six different myosins can lead to human hereditary deafness, but the precise functions of these proteins in the ear are unknown. This study uses a comparative approach to better understand the role of myosins VI and VIIa in vertebrate ears by examining protein distribution for these two myosins in the ears of evolutionarily diverse fishes and the aquatic clawed toad Xenopus laevis. Both myosins are expressed in the inner ears of all species examined in this study. Myo7a localizes to hair cells, particularly the actin-rich hair bundle, in all species studied. Myo6 also localizes to hair cells, but its distribution differs between species and end organs. Myo6 is found in hair bundles of most fish and frog epithelia examined here but not in anterior and posterior utricular hair bundles of American shad. These results show that myo7a distribution is highly conserved in diverse vertebrates and suggest functional conservation as well. The finding of myo6 in fish and Xenopus hair bundles, however, suggests a novel role for this protein in anamniotic hair cells. The lack of myo6 in specific American shad utricular hair bundles indicates a unique quality of these cells among fishes, perhaps relating to ultrasound detection capability that is found in this species.

Myosin VI is required for structural integrity of the apical surface of sensory hair cells in zebrafish

Unconventional myosins have been associated with hearing loss in humans, mice, and zebrafish. Mutations in myosin VI cause both recessive and dominant forms of nonsyndromic deafness in humans and deafness in Snell's waltzer mice associated with abnormal fusion of hair cell stereocilia. Although myosin VI has been implicated in diverse cellular processes such as vesicle trafficking and epithelial morphogenesis, the role of this protein in the sensory hair cells remains unclear. To investigate the function of myosin VI in zebrafish, we cloned and examined the expression pattern of myosin VI, which is duplicated in the zebrafish genome. One duplicate, myo6a, is expressed in a ubiquitous pattern during early development and at later stages, and is highly expressed in the brain, gut, and kidney. myo6b, on the other hand, is predominantly expressed in the sensory epithelium of the ear and lateral line at all developmental stages examined. Both molecules have different splice variants expressed in these tissues. Using a candidate gene approach, we show that myo6b is satellite, a gene responsible for auditory/vestibular defects in zebrafish larvae. Examination of hair cells in satellite mutants revealed that stereociliary bundles are irregular and disorganized. At the ultrastructural level, we observed that the apical surface of satellite mutant hair cells abnormally protrudes above the epithelium and the membrane near the base of the stereocilia is raised. At later stages, stereocilia fused together. We conclude that zebrafish myo6b is required for maintaining the integrity of the apical surface of hair cells, suggesting a conserved role for myosin VI in regulation of actin-based interactions with the plasma membrane.

Masked auditory thresholds in sciaenid fishes: a comparative study

Western Atlantic sciaenids comprise a taxonomically diverse teleost family with significant variations in the relationship between the swim bladder and the otic capsule. In this study, the auditory brainstem response (ABR) was used to test the hypothesis that fishes with different peripheral auditory structures (black drum, Pogonias chromis and Atlantic croaker, Micropogonias undulatus) show differences in frequency selectivity. In a black drum the swim bladder is relatively distant from the otic capsule while the swim bladder in Atlantic croaker possesses anteriorly-directed diverticulas that terminate relatively near the otic capsule. Signals were pure tones in the frequency range, 100 Hz to 1.5 kHz, and thresholds were determined both with and without the presence of simultaneous white noise at two intensity levels (124 dB and 136 dB, re: 1 microPa). At the 124 dB level of white noise background, both the black drum and Atlantic croaker showed similar changes in auditory sensitivity. However, in the presence of the 136 dB white noise masker, black drum showed significantly greater shifts in auditory thresholds between 300 and 600 Hz. The results indicate that the two species differ in frequency selectivity since the Atlantic croaker was less susceptible to auditory threshold shifts, particularly at the higher level of masking. This difference may be linked to peripheral auditory mechanisms.

Form and function in the unique inner ear of a teleost: The silver perch (Bairdiella chrysoura)

Members of the teleost family Sciaenidae show significant variation in inner ear and swim bladder morphology as well as in the relationship between the swim bladder and the inner ear. In the silver perch (Bairdiella chrysoura), a Stellifer-group sciaenid, both the saccular and utricular otoliths are enlarged relative to those in other teleosts. Additionally, its swim bladder is two-chambered, and the anterior chamber surrounds the otic capsule and terminates lateral to the saccules. Structure and function of the auditory system of the silver perch were explored by using gross dissections, scanning electron microscopy, CT scan reconstruction, and auditory brainstem response approach. Several morphological specializations of the auditory system of the silver perch were found, including expansion of the utricular and lagenar otoliths, close proximity between the saccules and the utricles, deeply grooved sulci on the saccular otoliths, two-planar saccular sensory epithelia, and a unique orientation pattern of sensory hair cell ciliary bundles on the saccular sensory epithelium. It was determined that the silver perch can detect up to 4 kHz, with lowest auditory thresholds between 600 Hz and 1 kHz. Audition in the silver perch is comparable to that in the goldfish (Carassius auratus), a hearing "specialist." The morphological specializations of the inner ear and swim bladder of the silver perch may be linked to its enhanced hearing capabilities. The findings of this study support the proposal that sciaenids are excellent model species for investigating structure-function relations in the teleost auditory system.

Development of form and function in peripheral auditory structures of the zebrafish (Danio rerio)

Investigations of the development of auditory form and function have, with a few exceptions, thus far been largely restricted to birds and mammals, making it difficult to postulate evolutionary hypotheses. Teleost fishes represent useful models for developmental investigations of the auditory system due to their often extensive period of posthatching development and the diversity of auditory specializations in this group. Using the auditory brainstem response and morphological techniques we investigated the development of auditory form and function in zebrafish (Danio rerio) ranging in size from 10 to 45 mm total length. We found no difference in auditory sensitivity, response latency, or response amplitude with development, but we did find an expansion of maximum detectable frequency from 200 Hz at 10 mm to 4000 Hz at 45 mm TL. The expansion of frequency range coincided with the development of Weberian ossicles in zebrafish, suggesting that changes in hearing ability in this species are driven more by development of auxiliary specializations than by the ear itself. We propose a model for the development of zebrafish hearing wherein the Weberian ossicles gradually increase the range of frequencies available to the inner ear, much as middle ear development increases frequency range in mammals.

Afferent innervation patterns of the saccule in pigeons

The innervation patterns of vestibular saccular afferents were quantitatively investigated in pigeons using biotinylated dextran amine as a neural tracer and three-dimensional computer reconstruction. Type I hair cells were found throughout a large portion of the macula, with the highest density observed in the striola. Type II hair cells were located throughout the macula, with the highest density in the extrastriola. Three classes of afferent innervation patterns were observed, including calyx, dimorph, and bouton units, with 137 afferents being anatomically reconstructed and used for quantitative comparisons. Calyx afferents were located primarily in the striola, innervated a number of type I hair cells, and had small innervation areas. Most calyx afferent terminal fields were oriented parallel to the anterior-posterior axis and the morphological polarization reversal line. Dimorph afferents were located throughout the macula, contained fewer type I hair cells in a calyceal terminal than calyx afferents and had medium sized innervation areas. Bouton afferents were restricted to the extrastriola, with multi-branching fibers and large innervation areas. Most of the dimorph and bouton afferents had innervation fields that were oriented dorso-ventrally but were parallel to the neighboring reversal line. The organizational morphology of the saccule was found to be distinctly different from that of the avian utricle or lagena otolith organs and appears to represent a receptor organ undergoing evolutionary adaptation toward sensing linear motion in terrestrial and aerial species.

Temporal encoding for auditory computation: physiology of primary afferent neurons in sound-producing fish

Many fish rely on sounds for communication, yet the peripheral structures containing the hair cells are simple, without the morphological specializations that facilitate frequency analysis in the mammalian cochlea. Despite this, neurons in the midbrain of sound-producing fish (Pollimyrus) have complex receptive fields, extracting features from courtship sounds. Here we present an analysis of the initial encoding of sounds by the primary afferents and demonstrate that the representation of sound undergoes a substantial transformation as it ascends to the midbrain. Afferents were isolated as they coursed from the sacculus through the medulla. Tones (100 Hz-1.2 kHz) elicited synchronized spikes [vector strength (VS) >0.9] on each stimulus cycle [coefficient of variation (CV) <1.1], with little spike rate adaptation. Most afferents (67%) were spontaneously active and began synchronizing 10 dB below rate threshold. Rate thresholds for the most sensitive afferents (65 dB) were close to behavioral thresholds. The distribution of characteristic frequencies and best sensitivities was matched to the spectrum of sounds of this species and to its audiogram. Three clusters of afferents were identified, one including afferents that generated spike bursts and had v-shaped response areas (bursters), and two others that included entrained afferents with broad response areas (entrained types I and II). All afferents encoded the timing of clicks within click trains with time-locked spikes, and none showed selectivity for interclick intervals. Understanding the computations that yield complex receptive fields is an essential goal for auditory neuroscience, and these data on primary encoding advance this goal, allowing a comparison of inputs with feature-extracting midbrain neurons.

Morphology and cell type heterogeneities of the inner ear epithelia in adult and juvenile zebrafish (Danio rerio)

Although the zebrafish has become an important model for genetic analysis of the vertebrate auditory system, a comprehensive description of the zebrafish ear has been provided for embryonic and larval development only (Haddon and Lewis [1996] J. Comp. Neurol. 365:113). Here we describe the development of sensory maculae in juvenile fish and the morphology of the adult zebrafish ear. This description was obtained via three-dimensional reconstruction of serial sections and confocal microscopy of immunolabeled preparations and includes the Weberian ossicles and fluid spaces. Phalloidin staining, which labels actin filaments of stereocilia, was used to delineate the sensory epithelia, to visualize the distribution of hair cells, to estimate their density in different areas of the maculae, and to perform hair cell counts. Morphology of ciliary bundles in different regions of the lagena, saccule, utricle, macula neglecta, and cristae was characterized with an anti-acetylated tubulin antibody and by phalloidin staining. We have identified two antibodies characterized by region-specific staining patterns in the inner ear epithelia. Zn-1 antibody staining largely correlates with the presence of short-bundle hair cells in the peripheral regions of sensory epithelia. Zn-4 antibody, on the other hand, labels a zone of epithelial cells surrounding the sensory maculae. These analyses extend previous observations of cell-type heterogeneity in both sensory and nonsensory epithelia of the fish ear.

Efferent innervation in the goldfish saccule examined by acetylcholinesterase histochemistry

In contrast to the abundance of information available regarding the anatomy and physiology of afferents within the goldfish saccule, the efferent system of this auditory endorgan has been scarcely studied morphologically. In this study, acetylcholinesterase histochemistry with diaminobenzidine enhancement was used to describe the morphology of efferents. Under light microscopy, labeled fibers appeared in the distal portion of the saccular nerve, penetrated the basement membrane and formed a horizontal mesh-like plexus near the base of hair cells. Many vertical branchlets with terminal swellings protruded upward toward hair cells from the plexus. Under electron microscopy, dense extracellular labeling was present around efferent terminals, which often formed clusters on hair cells. Labeling was also present around unmyelinated fibers of passage within the sensory epithelium and the distal saccular nerve. These fibers contained coarse microtubules and small vesicles, and often ran in a bundle with other similar fibers. Based on their position within the epithelium, histochemistry and ultrastructural characteristics, these fibers were concluded to be efferents. These fibers became myelinated and unlabeled in the proximal saccular nerve. These results suggest that acetylcholinesterase can be a marker of entire distal unmyelinated portions of efferent fibers and demonstrated abundant efferent innervation in the goldfish saccule.

Evolution of hearing in vertebrates: the inner ears and processing

This paper considers aspects of the evolution of the vertebrate auditory system from an 'ichthyocentric' perspective. It is argued that all vertebrate auditory systems are required to do certain basic tasks including acoustic feature discrimination, sound source localization, frequency analysis, and auditory scene analysis, among others. These sorts of capabilities arose very early in the evolution of the vertebrates and have been modified by selection in different species. In some cases the same structures have been involved in detection and analysis throughout the vertebrates, while in other cases the mechanism by which the same type of analysis takes place may have changed.

Hair cell heterogeneity and ultrasonic hearing: recent advances in understanding fish hearing

The past decade has seen a wealth of new data on the auditory capabilities and mechanisms of fishes. We now have a significantly better appreciation of the structure and function of the auditory system in fishes with regard to their peripheral and central anatomy, physiology, behaviour, sound source localization and hearing capabilities. This paper deals with two of the newest of these findings, hair cell heterogeneity and the detection of ultrasound. As a result of this recent work, we now know that fishes have several different types of sensory hair cells in both the ear and lateral line and there is a growing body of evidence to suggest that these hair cell types arose very early in the evolution of the octavolateralis system. There is also some evidence to suggest that the differences in the hair cell types have functional implications for the way the ear and lateral line of fishes detect and process stimuli. Behavioural studies have shown that, whereas most fishes can only detect sound to 1-3 kHz, several species of the genus Alosa (Clupeiformes, i.e. herrings and their relatives) can detect sounds up to 180 kHz (or even higher). It is suggested that this capability evolved so that these fishes can detect one of their major predators, echolocating dolphins. The mechanism for ultrasound detection remains obscure, though it is hypothesized that the highly derived utricle of the inner ear in these species is involved.