This volume constitutes a series of invited chapters based on presentations given at an International Conference on the Sensory Biology of Aquatic Animals held June 24-28, 1985 at the Mote Marine Laboratory in Sarasota, Florida. The immediate purpose of the conference was to spark an exchange of ideas, concepts, and techniques among investigators concerned with the different sensory modalities employed by a wide variety of animal species in extracting information from the aquatic environment. By necessity, most investigators of sensory biology are specialists in one sensory system: different stimulus modalities require different methods of stimulus control and, generally, different animal models. Yet, it is clear that all sensory systems have principles in common, such as stimulus filtering by peripheral structures, tuning of receptor cells, signal-to-noise ratios, adaption and disadaptation, and effective dynamic range. Other features, such as hormonal and efferent neural control, circadian reorganization, and receptor recycling are known in some and not in other senses. The conference afforded an increased awareness of new discoveries in other sensory systems that has effectively inspired a fresh look by the various participants at their own area of specialization to see whether or not similar principles apply. This inspiration was found not only in theoretical issues, but equally in techniques and methods of approach. The myopy of sensory specialization was broken in one unexpected way by showing limitations of individual sense organs and their integration within each organism. For instance, studying vision, one generally chooses a visual animal as a model.

Sensory biology of aquatic animals

Fishes have an impressive complement of hydrodynamic and acoustic sensors, commonly referred to as the lateral-line and inner-ear sense organs The basic receptor elements are the hair cells, which detect the minute displacements imparted to their apical ciliary bundles (Fig 41a) The directional sensitivity of the individual receptor cells is indicated by the asymmetric position of the single kinocilium relative to the several rows of stereocilia Morphologically, the hair cells of the various sensory clusters are strikingly uniform Their diversity in function is determined mainly by the peripheral structures coupling the ciliary bundles to the physical world that the animals inhabit

Hydrodynamic and Acoustic Field Detection

Structural and functional evolution of the basal ganglia in vertebrates

The literature on fish hearing has increased significantly since our last critical review in 1973. The purpose of the current paper is to review the more recent literature and to identify those questions that need to be asked to develop a fuller understanding of the auditory capabilities and processing mechanisms of fishes. We conclude that while our understanding of fish hearing has increased substantially in the past years, there are still major gaps in what we know. In particular, the comparative functional literature is extremely limited, and we do not yet know whether different species, and particularly hearing specialists as compared to hearing nonspecialists, have fundamentally different auditory capabilities and mechanisms.

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Sound Detection and Processing by Fish: Critical Review and Major Research Questions (Part 1 of 2)

The lateral line system is the only sensory system unique to aquatic vertebrates, being restricted to fish and some aquatic amphibians. It contains at least two, and probably three, different receptor classes: ciliated mechanoreceptors, electroreceptors, and a largely unknown group of receptors called the vesicles of Savi, found only in certain elasmobranch fishes (Garman 1888; Coggi 1891; Norris 1932). While all these organs are innervated by branches of the lateral line nerve and may have common embryological origins (Coggi 1891; Norris 1932; Northcutt 1985), it is quite clear that ciliated mechanoreceptors and electroreceptors respond to different stimuli and that their CNS projections are segregated at least at the level of the brain stem (McCormick 1983; Northcutt 1985). Thus, the concept of a single lateral line system has given way to one of at least two different sensory systems, with emerging evidence of anatomical functional variability within each of these.

Diversity of Lateral Line Systems: Evolutionary and Functional Considerations

In cichlid, poecilid and centrarchid fishes luteinizing hormone releasing hormone (LHRH)-immunoreactive neurons are found in a cell group (nucleus olfactoretinalis) located at the transition between the ventral telencephalon and olfactory bulb. Processes of these neurons project to the contralateral retina, traveling along the border between the internal plexiform and internal nuclear layer, and probably terminating on amacrine or bipolar cells. Horseradish peroxidase (HRP) injected into the eye or optic nerve is transported retrogradely in the optic nerve to the contralateral nucleus olfactoretinalis where neuronal perikarya are labeled. Labeled processes leave this nucleus in a rostral direction and terminate in the olfactory bulb. The nucleus olfactoretinalis is present only in fishes, such as cichlids, poecilids and centrarchids, in which the olfactory bulbs border directly the telencephalic hemispheres. In cyprinid, silurid and notopterid fishes, in which the olfactory bulbs lie beneath the olfactory epithelium and are connected to the telencephalon via olfactory stalks, the nucleus olfactoretinalis or a comparable arrangement of LHRH-immunoreactive neurons is lacking. After retrograde transport of HRP in the optic nerve of these fishes no labeling of neurons in the telencephalon occurred. It is proposed that the nucleus olfactoretinalis anatomically and functionally interconnects and integrates parts of the olfactory and optic systems.

Centrifugal innervation of the retina by luteinizing hormone releasing hormone (LHRH)-immunoreactive telencephalic neurons in teleostean fishes.

LHRH systems in the brain of platyfish

The activities of single afferent fibers were recorded in the trunk lateral line nerve of the cichlid fishSarotherodon niloticus L. Using both electrophysiological recordings and neuroanatomical tracing techniques, the number, arrangement, and innervation of superficial (SNs) and canal (CNs) neuromasts were determined. Both, SNs and CNs, are innervated by several afferent fibers of different diameters and efferent fibers. The CNs and SNs are neuronally separated: afferent fibers which innervate both CNs and SNs were not found. Whereas the single CN is innervated by a separate set of afferent fibers, fibers innervating the SNs within rows often branched to reach all or several SNs. The SNs within a row were thus considered to form a functional unit. With the exception of SNs on the tail fin, functional units of neuromasts were in general topographically restricted to single scales. The majority of lateral line units had resting activity. On the basis of the time interval distribution of the resting activity, 4 types of units were classified: these were labelled irregular (type I), regular (type II), bimodal (type III) and silent (type IV). Type I was the most common type of resting activity (obtained in 47.8% of the recorded units). Units with this resting activity type were identified as afferents innervating either SNs or CNs. Units with resting activity of type II represented mostly afferents of CNs if their mean activity was high (around 40 imp/s). If the mean activity of this type was below 20 imp/s the units were unresponsive to local water movements and at least some were identified as efferent fibers. Resting activity of type III was found only in units originating from CNs. Only 4% of the units were silent (type IV). These units were often identified as injured neuromasts. Units originating from CNs show higher mean resting activity than those from SNs. For both SN and CN units, the mean discharge rate of the resting activity correlated with the sensitivity to stimulation for sinusoidal water movements. During stimulation of the neuromasts by sinusoidal water movements of small amplitude and different frequencies, the response characteristics of SN and CN units were determined by linear frequency analysis under steady state conditions. Most units responded linearly to small stimulus amplitudes. In this amplitude range the units' resting activity was modulated according to the stimulus frequency. Small stimulus amplitudes proportionally changed the amount of modulation but did not alter the phase of the response. CN and SN units that responded linearly produce differing frequency responses. Whereas CNs were most sensitive at frequencies of up to 200 Hz (center frequencies between 100 and 200 Hz), the center frequencies of SNs were distributed between 10 and 70 Hz with a maximum number at about 30 Hz. Bode plots for many CN and SN units indicated that the neuromasts were sensitive to the acceleration component of the water movement. The functional significance of the differences between the two types of lateral line neuromasts (SNs and CNs) were discussed.

Single unit activity in the peripheral lateral line system of the cichlid fish Sarotherodon niloticus L.

Centrifugal innervation of the retina in cichlid and poecilid fishes. A horseradish peroxidase study

Topography, morphology, and innervation of superficial neuromasts and canal neuromast in adult bony fishes ofSarotherodon niloticus (L.) were studied and compared by light and electron microscopical methods. Apart from certain other morphological differences the two neuromast types also differ in innervation. They can be distinguished by the number, course, and ending of the myelin sheath of their corresponding nerve fibers. All superficial neuromasts arranged in one row are interconnected by a so-called connecting strand. This tissue, unlike the epidermis, consists of tightly packed cells the external membranes of which are considerably meshed. The tissue of the connecting strand does not contain neuronal structures.

Heinrich Münz

Papers

Centrifugal innervation of the retina by luteinizing hormone releasing hormone (LHRH)-immunoreactive telencephalic neurons in teleostean fishes.

LHRH systems in the brain of platyfish

Single unit activity in the peripheral lateral line system of the cichlid fish Sarotherodon niloticus L.

Centrifugal innervation of the retina in cichlid and poecilid fishes. A horseradish peroxidase study

Morphology and innervation of the lateral line system inSarotherodon niloticus (L.) (cichlidae, teleostei)