Otoliths are of interest to investigators from several disciplines including systematics, auditory neuroscience, and fisheries. However, there is often very little sharing of information or ideas about otoliths across disciplines despite similarities in the questions raised by different groups of investigators. A major purpose of this paper is to present otolith-related questions common to all disciplines and then demonstrate that the issues are not only similar but also that more frequent interactions would be mutually beneficial. Because otoliths evolved as part of the inner ear to serve the senses of balance and hearing, we first discuss the basic structure of the ear. We then raise several questions that deal with the structure and patterns of otolith morphology and how changes in otoliths with fish age affect hearing and balance. More specifically, we ask about the significance of otolith size and how this might affect ear function; the growth of otoliths and how hearing and balance may or may not change with growth; the significance of different otolith shapes with respect to ear function; the functional significance of otoliths that do not contact the complete sensory epithelium; and why teleost fishes have otoliths and not the otoconia found in virtually all other extant vertebrates.

The teleost family Sciaenidae, collectively known as the croakers and drums because of their propensity for making sound, includes roughly 70 genera and 270 species worldwide. Although many other groups of fish also communicate using sound, the sciaenids are unique in the diversity of their sound production mechanisms, variety of sounds produced, and structural variation in sound-detecting structures. This paper reviews the bioacoustics of sciaenid fishes, including mechanisms involved in the production and reception of sound, the types of sounds produced, and the functions of these sounds. We propose the hypothesis that the unusual diversity in the design of the structures associated with sound production and detection is correlated with a similar diversity in how these structures function. Production and detection of sound appear to be important aspects of sciaenid behavior. But despite the vast literature on sciaenid sound production, we know relatively little about the biological significance of their sounds. This lack of understanding leaves plenty of room for research by physiologists, bioacousticians, behavioral ecologists, and fisheries scientists.[Article] FIGURE 1.-Swim bladders of selected sciaenids. The shaded areas represent sonic muscles and the dashed horizontal lines the point at which the septum transversum merges with the body wall ventrally. The dashed vertical lines (partial or complete) divide the diagrams into left and right portions, each of which pertains to a separate species. Types IÀVI are explained in Table 2; scientific names not given below are in Table 1. The figure is reproduced from Chao (1986). First row.-A (type I): left side ¼ spot, right side ¼ spotted drum Equetus punctatus; B (type I): left side ¼ shorthead drum Larimus breviceps, right side ¼ banded drum L. fasciatus; C (type II): left side ¼ king weakfish Macrodon ancylodon, right side ¼ scalyfin corvina; D (type III): left side ¼ Peruvian banded croaker Paralonchurus peruanus, right side ¼ totoaba; E (type III): left side ¼ smalleye croaker Nebris microps, right side ¼ Atlantic croaker; F (type V): left side ¼ red drum, right side ¼ corvina Sciaena gilberti; G (type IV): left side ¼ spotfin croaker Roncador stearnsii, right side ¼ white croaker; H (type IV): black drum. Second row.-A (type IV): meagre Argyrosomus regius; B (type III): boe drum Pteroscion peli; C (type III): Angola croaker Miracorvina angolensis; D (type III): blackmouth croaker Pentheroscion mbizi; E (type III): longneck croaker (also known as flathead captainfish) Pseudotolithus typus; F (type III): cassava croaker (also known as captainfish) P. senegalensis. Third row.-A (type III): kathala croaker Kathala axillaries; B (type III): Chinese bahaba Bahaba taipingensis; C (type III): left side ¼ pama croaker Otolithoides pama, right side ¼ Plagioscion sp. from the Amazon River; D (type III): left side ¼ panna croaker Panna microdon, right side ¼ Boeseman croaker Boesemania microlepis; E (type IV): pawak croaker Pennahia pawak; F (type IV): left side ¼ pric...

Sciaenid fishes (Family Sciaenidae) could potentially serve as models for understanding the relationship between structure and function in the teleost auditory system, as they show a broad range of variation in not only the structure of the ear but also in the relationship between the ear and swim bladder. In this study, scanning electron microscopy (SEM) was used to investigate inner ear ultrastructure of the Atlantic croaker (Micropogonias undulatus), spotted seatrout (Cynoscion nebulosus), kingfish (Menticirrhusamericanus) and spot (Leiostomus xanthurus). These species reflect the diversity of otolith and swim bladder morphology in sciaenids. The distribution of different hair cell bundle types, as well as hair cell orientation patterns on the saccular and lagenar maculae of these fishes were similar to one another. The rostral ends of the saccular sensory epithelia (maculae) were highly expanded in a dorsal-ventral direction in the Atlantic croaker and spotted seatrout as compared to the kingfish and spot. Also, ciliary bundles of the saccular maculae contained more stereocilia in the Atlantic croaker and spotted seatrout as compared with kingfish and spot. The shapes of the lagenar maculae were similar in all four species. In the Atlantic croaker and spotted seatrout lagenar maculae, the number of stereocilia per bundle was greater than those for the kingfish and spot. Given that saccular macula shape and numbers of stereocilia per bundle correlate with swim bladder proximity to the ear in the studied species, it is possible that inner ear ultrastructure could be indicative of auditory ability in fishes.

We investigated how morphological differences in the auditory periphery of teleost fishes may relate to hearing capabilities. Two species of western Atlantic sciaenids were examined: weakfish (Cynoscion regalis, Block and Schneider) and spot (Leiostomus xanthurus, Lacepede). These species differ in the anatomical relationship between the swim bladder and the inner ear. In weakfish, the swim bladder has a pair of anterior horns that terminate close to the ear, while there are no extensions of the swim bladder in spot. Thus, the swim bladder in spot terminates at a greater distance from the ear when compared to weakfish. With the use of the auditory brainstem response technique, Cynoscion regalis were found to detect frequencies up to 2000 Hz, while Leiostomus xanthurus detected up to 700 Hz. There were, however, no significant interspecific differences in auditory sensitivity for stimuli between 200 and 700 Hz. These data support the hypothesis that the swim bladder can potentially expand the frequency range of detection.

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.

. Eigenmannia, a genus of weakly electric fish, exhibits a specialized behavior known as the jamming avoidance response (JAR). The JAR results in a categorical difference between Eigenmannia that are in groups of conspecifics and those that are alone. Fish in groups exhibit the JAR behavior and thereby experience ongoing, global synchronous 20-to 50-Hz electrosensory oscillations, whereas solitary fish do not. Although previous work has shown that these ongoing signals do not significantly degrade electrosensory behavior, these oscillations nevertheless elicit short-term synaptic depression in midbrain circuits. Because short-term synaptic depression can have profound effects on the transmission of information through synapses, we examined the differences in intracellularly recorded responses of midbrain neurons in awake, behaving fish to moving electrosensory images under electrosensory conditions that mimic solitary fish and fish in groups. In solitary conditions, moving objects elicited Gaussian or sinusoidal postsynaptic potentials (PSPs) that commonly exhibited preferential responses to a direction of motion. Surprisingly, when the same stimulus was presented in the presence of the global oscillations, directional selectivity was increased in all neurons tested. The magnitudes of the differences in PSP amplitude for preferred and nonpreferred directions were correlated with a measure of short-term synaptic depression in both conditions. The electrosensory consequences of the JAR appear to result in an enhancement of the representation of direction of motion in midbrain neurons. The data also support a role for short-term synaptic depression in the generation and modulation of directional responses.

Wave-type weakly electric fish such as Eigenmannia produce continuous sinusoidal electric fields. When conspecifics are in close proximity, interaction of these electric fields can produce deficits in electrosensory function. We examined a neural correlate of such jamming at the level of the midbrain. Previous results indicate that neurons in the dorsal layers of the torus semicircularis can (1) respond to jamming signals, (2) respond to moving electrosensory stimuli, and (3) receive convergent information from the four sensory maps of the electrosensory lateral line lobe (ELL). In this study we recorded the intracellular responses of both tuberous and ampullary neurons to moving objects. Robust Gaussian-shaped or sinusoidal responses with half-height durations between 55 ms and 581 ms were seen in both modalities. The addition of ongoing global signals with temporal-frequencies of 5 Hz attenuated the responses to the moving object by 5 dB or more. In contrast, the responses to the moving object were not attenuated by the addition of signals with temporal frequencies of 20 Hz or greater. This occurred in both the ampullary and tuberous systems, despite the fact that the ampullary afferents to the torus originate in a single ELL map whereas the tuberous afferents emerge from three maps.

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.