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African weakly electric mormyrid fish show a high diversity of their electric organ discharge (EOD) both across and within genera. Thanks to a recently developed technique of artificial reproduction in mormyrid fish, we were able to perform hybridizations between different genera and within one genus (Campylomormyrus). The hybrids of intergenus hybridizations exhibited different degrees of reduced survival related to the phylogenetic distance of the parent species: hybrids of the crosses between C. rhynchophorus and its sister genus Gnathonemus survived and developed normally. Hybrids between C. rhynchophorus and a Mormyrus species (a more basal clade compared to Campylomormyrus s) survived up to 42days and developed many malformations, e.g., at the level of the unpaired fins. Hybrids between C. numenius and Hippopotamyrus pictus (a derived clade, only distantly related to Campylomormyrus) only survived for two days during embryological development. Eight different hybrid combinations among five Campylomormyrus species (C. tamandua, C. compressirostris, C. tshokwe, C. rhynchophorus, C. numenius) were performed. The aim of the hybridizations was to combine species with (1) either caudal or rostral position of the main stalk innervating the electrocytes in the electric organ and (2) short, median or long duration of their EOD. The hybrids, though they are still juveniles, show very interesting features concerning electrocyte geometry as well as EOD form and duration: the caudal position of the stalk is prevailing over the rostral position, and the penetration of the stalk is dominant over the non-penetrating feature (in the Campylomormyrus hybrids); in the hybrid between C. rhynchophorus and Gnathonemus petersii it is the opposite. When crossing species with long and short EODs, it is always the long duration EOD that is expressed in the hybrids. The F1-Hybrids of the cross C. tamandua×C. compressirostris are fertile: viable F2-fish could be obtained with artificial reproduction.

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

The weakly electric fish Gnathonemus petersii uses its electric sense to actively probe the environment. Its highly mobile chin appendage, the Schnauzenorgan, is rich in electroreceptors. Physical measurements have demonstrated the importance of the position of the Schnauzenorgan in funneling the fish's self-generated electric field. The present study focuses on the trigeminal motor pathway that controls Schnauzenorgan movement and on its trigeminal sensory innervation and central representation. The nerves entering the Schnauzenorgan are very large and contain both motor and sensory trigeminal components as well as an electrosensory pathway. With the use of neurotracer techniques, labeled Schnauzenorgan motoneurons were found throughout the ventral main body of the trigeminal motor nucleus but not among the population of larger motoneurons in its rostrodorsal region. The Schnauzenorgan receives no motor or sensory innervation from the facial nerve. There are many anastomoses between the peripheral electrosensory and trigeminal nerves, but these senses remain separate in the sensory ganglia and in their first central relays. Schnauzenorgan trigeminal primary afferent projections extend throughout the descending trigeminal sensory nuclei, and a few fibers enter the facial lobe. Although no labeled neurons could be identified in the brain as the trigeminal mesencephalic root, some Schnauzenorgan trigeminal afferents terminated in the trigeminal motor nucleus, suggesting a monosynaptic, possibly proprioceptive, pathway. In this first step toward understanding multimodal central representation of the Schnauzenorgan, no direct interconnections were found between the trigeminal sensory and electromotor command system, or the electrosensory and trigeminal motor command. The pathways linking perception to action remain to be studied.

The present study has explored the optomotor response in two species of mormyrid fish, Gnathonemus petersii and Brienomyrus niger. Both species tended to follow a black and white striped stimulus pattern under illumination levels of 6, 12, and 60 lx. The optomotor response ceased to occur under 540 lx. The behavioral data support earlier histological findings implicating the mormyrid retina in dim light vision.

Several species of Mormyrid weakly electric fish have a mobile chin protuberance that serves as a mobile antenna during prey detection, tracking behaviors, and foraging for food. It has been proposed that it constitutes a fovea of the electrosensory system. The distribution of the three types of receptor organs involved in active imaging of the local surroundings, prey detection, and passive electroreception, and their central projection to the electrosensory lobe (ELL), have been studied in Gnathonemus petersii. Density distributions were compared for different body regions. Primary afferent projections were labeled with biocytin or biotinylated dextrans. This showed that there is considerable central "over-representation" of the mandibular and nasal regions of the sensory surface involved in electrolocation, at the expense of the other body regions investigated. This over-representation is not a mere effect of the very high density of receptor organs in these areas, but is found to be due to central magnification. This magnification differs between the subclasses of electroreceptors, suggesting a functional segregation in the brain. We conclude that the chin protuberance and the nasal region are the regions of greatest sensitivity for the resistive, capacitive, and low-frequency characteristics of the environment, and are probably most important in prey detection, whereas other regions of the skin with a lesser resolution and sensitivity to phase distortion of the EOD, in particular the trunk, are probably designed for imaging larger, inanimate features of the environment. Our data support the hypothesis that the chin appendage and nasal region are functionally distinct electrosensory foveae.

Gnathonemus petersii and other African weakly electric fish (Mormyridae) produce electric signals in order to sense their environment, a process called active electrolocation. During active electrolocation electrical images of objects in the environment are cast onto a mosaic of electroreceptors, which are embedded in the animal's skin. In Gn. petersii and four other species of mormyrid fish, the densities of electroreceptor organs (mormyromasts and ampullary organs) were determined at various skin regions, including the chin, the nasal region and the back. In all species investigated, the highest mormyromast densities were found at the chin, followed by the nasal region. In addition to this comparative approach, we investigated the electroreceptive epidermis of Gn. petersii in more detail. At the moveable chin appendix, the Schnauzenorgan, scanning electron microscopy analysis helped to elucidate the mosaic-like distribution and the morphology of the electroreceptor organs and their embedding in the epidermis. Because the number of mormyromasts increased very little when fish got bigger, receptor organ densities decreased with standard length. Mormyromast density at the Schnauzenorgan, especially at its tip, highly exceeded that at all other skin regions. Along the Schnauzenorgan, receptor densities decreased exponentially from the tip towards the base. At the nasal region, mormyromast densities were more than three times higher compared to the rest of the fish's head and trunk. A similar distribution to that of the mormyromasts was found for the ampullary electroreceptor organs, which are used for passive electroreception. Our findings support the hypothesis that at least in Gn. petersii the Schnauzenorgan and the nasal region are electroreceptive foveae.

To elucidate the specific properties of retinae with grouped photoreceptors the neural pathways in the outer and inner plexiform layer were studied. Photoreceptor bundles in this species consist of more than 100 rods and up to 50 cones, and are usually regarded as functional units. Golgi impregnation in thick and thin sections and light microscopy were used to identify bipolar cell types linking photoreceptors to amacrine and/or ganglion cells. Nine different types were distinguished based on their dendritic morphology and the position of the axon terminal in the inner plexiform layer. Small cells have dendritic fields smaller than the diameter of a photoreceptor bundle and are contacted mostly by cones. The dendritic field size of bushy cells matches that of a photoreceptor bundle; they are contacted mainly by rods. Flat cells receive about equal input from rods and cones; their dendritic field size exceeds the bundle diameter. Within the three major classes there are subtypes addressing three sublaminae of the inner plexiform layer, the proximal On-centre region (sl b), the distal Off-centre region (sl a) and a central sublayer (sl c) probably with transient activity. These observations suggest that cone vision has a spatial acuity better than the “bundle grain”. In rod dominated vision the resolution matches that of the bundles; for this pathway, the hypothesis of the bundle as a functional unit is confirmed. The mesopic flat cell pathway has a resolution inferior to the “bundle grain”; it may therefore be dedicated to movement detection.

The cerebellum of mormyrid fish is of interest for its large size and unusual histology. The mormyrid cerebellum, as in all ray-finned fishes, has three subdivisions--valvula, corpus, and caudal lobe. The structures of the mormyrid valvula and corpus have been examined previously, but the structure of the mormyrid caudal lobe has not been studied. The mormyrid caudal lobe includes a posterior caudal lobe associated with the electrosense and an anterior caudal lobe associated with lateral line and eighth nerve senses. In this article we describe cellular elements of the posterior caudal lobe and of the eminentia granularis posterior (EGp) in the mormyrid fish Gnathonemus petersii. The EGp gives rise to the parallel fibers of the posterior caudal lobe. We used intracellular injection of biocytin, extracellular injection of biotinylated dextran amine, and immunohistochemistry with antibodies to gamma-aminobutyric acid, inositol triphosphate receptor I, calretinin, and Zebrin II. The histological structure of the posterior caudal lobe is markedly irregular in comparison to that of the corpus and the valvula, and a tight modular organization of cerebellar elements is less apparent here. Most Purkinje cell bodies are in the middle of the molecular region. Their dendrites are only roughly oriented in the sagittal plane, extend both ventrally and dorsally, and branch irregularly. Climbing fibers terminate only on smooth dendrites near the soma. Most Purkinje cell axons terminate locally on eurydendroid cells that project outside the cortex. The results provide an additional variant to the already large set of different cerebellar and cerebellum-like structures.

An examination of the permanent bony structures of the anal fin complex in the mormyrid fish, Gnathonemus petersii, revealed two new structural sexual dimorphisms: longer proximal pterygiophores and wider anal fin rays in males than in females. Both structures are thought to facilitate the male’s courtship-associated anal fin reflex. Adult male mormyrid fishes are characterized by a dorsally directed indentation of the posterior body wall (anal fin indentation). The expression of this indentation in males, presumably driven by anal fin musculature, was correlated with the fish’s gonadal state: large indentations were associated with high gonado-somatic indices and small indentations with low indices.

SUMMARY Afferent responses to the fish's own electric organ discharge were explored in the electrosensory lobe of the mormyrid fish Gnathonemus petersii. In order to understand the neural encoding of natural sensory images,responses were examined while objects of different conductivities were placed at different positions along the skin of the fish, i.e. at different points within, and also outside, peripheral receptive fields. The presence of an object in the fish's self-generated electric field produces local modulation of transcutaneous current density. Measurement of the local electric organ discharge shows that object images formed at the electroreceptive sensory surface have an opposing center-surround, `Mexican hat' profile. This is a pre-receptor phenomenon intrinsic to the physical nature of the sensory stimulus that takes place prior to neural lateral inhibition and is independent of such central inhibition. Stimulus intensity is encoded in the latency and number of action potentials in the response of primary afferent fibers. It is also reflected in changes in the amplitude and area of extracellular field potentials recorded in the deep granular layer of the electrosensory lobe. Since the object image consists of a redistribution of current density over the receptive surface,its presence is coded by change in the activity of receptors over an area much larger than the skin surface facing the object. We conclude that each receptor encodes information coming from the whole scene in a manner that may seem ambiguous when seen from a single point and that, in order to extract specific object features, the brain must process the electric image represented over the whole sensory surface.

In pairs of interacting Gnathonemus petersii, a mormyrid electric fish, dominant individuals were larger (longer body). Pairs of interacting fish of similar body size emitted more parallel displays at the onset of an interaction. Over the course of 10 min interactions, head butting increased and parallel display decreased. This decrease occurred primarily in pairs that contained a prior resident and intruder, as compared with two intruders. The patterns of electric organ discharge during parallel display, and the possible sensory modalities mediating this behaviour are discussed.

Saffan®, previously known as CT1341, is an injectable steroid anesthetic for use in cats and monkeys. It is also suited to induce anesthesia in fishes. It has the convenient property to induce anesthesia of the central nervous system, and to leave the sensory system of the integument operational. The active constituents are alphaxalone and alphadolone. A dose of 24 mg/kg i.m. is sufficient for 2 to 3 hours narcosis in fish of several hundreds of grams. For small specimens administration via bath-water in doses of 2 to 4.8 mg/l for induction and half this dose for maintenance are advisable. In catfish, Ictalurus nebulosus and I. melas, of 100 g or more the induction time after i.m. injection with 21 mg/kg is about 20 min and independent of weight or temperature. The recovery time varies with temperature from 213 min at 13°C to 19 min at 19°C. In 1000 g catfish, Clarias gariepinus, the onset of surgical anesthesia occurs after 26 min on average at doses of 24 mg/kg at 24°C. In 5 g catfish, Kryptopterus bicirrhis, 4.8 mg/l bath water at 25°C makes the opercular movements disappear after 5 min. For 15 cm TL Gnathonemus petersii a good induction dose is 2 mg/l bathwater. After 20 minutes at 23°C surgical narcosis is reached.

Both routine electron microscopy and immunocytochemistry with protein A–gold were used to identify the cell types within the islet organs of four species of teleosts ( Osteoglossum bicirrhosum, Pantodon buchholzi, Notopterus chitala, and Gnathonemus petersii ) within Osteoglossomorpha, a subdivision with an ancient lineage. Four primary endocrine cell types, A, B, D, and F, were identified within the islets of the four species examined. The B- and D-cells were located mainly in the central core of the islet in the four species. In general, the A-cells were located at the islet periphery in all of the four species but in P. buchholzi and N. chitala they were also differently distributed toward the islet core. F-cells were present only at the islet periphery. Granules of B-cells in three species had a relatively homogeneous shape of the matrix core, but in O. bicirrhosum, the shape varied greatly. Variation in matrix shape of B-cell granules may indicate a different conformation of insulin molecules among at least some species of osteoglossomorphs, and this observation may have some taxonomic significance. Two somatostatin-containing (SST) D-cell types (D1 and DX) with granules of different shape were observed in all four species of osteoglossomorphs. The granules of the two D-cells immunostained either with anti-SST-25 and anti-SST-14 (D1-cells) or with anti-SST-34 (DX-cells). Immunocytochemistry confirmed that A-cells, containing glucagon-family peptides, and F-cells, containing peptides of the pancreatic polypeptide family, had granules of different shape. The cells of the islet organs of these osteoglossomorphs are more similar to those in more derived teleosts than they are to those of nonteleost actinopterygians.

The paraventricular organ of Gnathonemus petersii was investigated with light and electronmicroscopical techniques. It contains high concentrations of dopamine, noradrenaline and serotonin, but the synthesizing enzymes are not or hardly present. Consequently, the cerebrospinal fluid-contacting neurons might pick up their biogenic amines from the ventricular fluid. Dense subependymal axonal plexuses in the everted telencephalon probably release these substances into the ventricle. However, electronmicroscopical observations suggest release rather than uptake by the paraventricular organ. The possible significance of intraventricular release, transport and uptake of biogenic amines is discussed.

Male Gnathonemus petersii (Mormyridae) exhibit two structural, sexually dimorphic characters: anal-fin ray bone expansion and an indentation of the posterior ventral body wall (formerly described as anal-fin indentation). Females lack this bone expansion, but may show a slight indentation. Morphometric data on both characters were obtained from radiographs of 414 fish (males and females) ranging in size from 60 to 276 mm Ls. Both body wall indentation and bone expansion began to develop in males of about 120 mm Ls. At 160–180 mm, the sex ratio of fish with expansion to fish without expansion was 50 : 50. Androgens seem to affect the expression of both these sexual dimorphisms. Bone expansion may provide increased bone surface for muscle attachment and thus facilitate the anal-fin reflex during courtship behaviour.

The retina of the weakly electric fish Gnathonemus petersii is a so-called grouped retina where photoreceptors are bundled. These bundles are regarded as functional units and this type of retinal specialization is uniquely found in teleosts. To understand how this anatomical organization influences visual information processing we investigated the morphology and distribution of retinal ganglion cells (GCs) and the response properties of retinal afferents terminating in the major retinorecipient area, the optic tectum. GCs were classified based on their dendritic morphology (dendritic field diameters 280 μm: giant GCs). Within these classes subtypes were distinguished based on the ramification patterns of the dendrites in the sublaminae of the inner plexiform layer. Properties of presumed optic nerve terminals were investigated in the optic tectum using extracellular recordings. Physiological classes could be observed based on their response to visual stimuli (on; off; on-off, and fast units). Receptive field sizes and spatiotemporal properties were classified and the topographical representation of the visual space was mapped in the tectum. Gratings of low spatial frequencies were best responded to and followed up to high temporal frequencies (>30 Hz). Most of the recorded units were directionally selective. No evidence of distorted topographies in the tectum was found, i.e., no overrepresentation of the retina was seen in the tectum opticum. The grouped retina of G. petersii seems to be optimized for the detection of large, fast objects in an environment of low optical quality.

Corollary discharge signals associated with the motor command that elicits the electric organ discharge are prominent in the electrosensory lobe of mormyrid fish (Gnathonemus petersii). Central pathways and structures that convey these signals from the motor command nucleus to the electrosensory lobe are known anatomically, but these structures and their contributions to the various corollary discharge phenomena have not been examined physiologically. This study examines one such structure, the mesencephalic command associated nucleus (MCA).

Institute for Zoology, Department of Neuroethology/Sensory Ecology, University of Bonn, 53115 Bonn,North Rhine-Westphalia, GermanyABSTRACT The nocturnally active weakly electric fishGnathonemus petersii is known to employ active electro-location for the detection of objects and for orientation inits environment. The fish emits pulse-type electric signalswith an electric organ and perceives these signals withmore than 3,000 epidermal electroreceptor organs, themormyromasts, which are distributed over the animal’sskin surface. In this study, we measured the metricdimensions of the mormyromasts from different bodyregions to find structural and functional specialization ofthe various body parts. We focused on the two fovealregions of G. petersii, which are located at the elongatedand movable chin (the Schnauzenorgan; SO) and at thenasal region (NR), the skin region between the mouthand the nares. These two foveal regions were compared tothe dorsal part (back) of the fish, which contains typicalnonfoveal mormyromasts. While the gross anatomy of themormyromasts from all skin regions is similar, the metricdimensions of the main substructures differed. The mor-myromasts at the SO are the smallest and contain thesmallest receptor cells. In addition, the number of recep-tor cells per organ is lowest at the SO. In contrast, at theback the biggest receptor organs with the highest amountof receptor cells per organ occur. The mormyromasts atthe NR are in several respects intermediate betweenthose from the back and the SO. However, mormyromastsat the NR are longer than those at all other skin regions,the canal leading from the receptor pore to the innerchambers were the longest and the overlaying epidermallayers are the thickest. These results show that mormyro-masts and the epidermis they are embedded in at bothfoveal regions differ specifically from those found on therest of the body. The morphological specializations lead tofunctional specialization of the two foveae. J. Morphol.273:629–638, 2012.

Abstract. Newly imported, adult weakly discharging electric fish Gnathonemus petersii , were examined for sex differences in the waveform of their electric organ discharge and in external morphology. The discharge waveform of gonadally ripe subjects are quantitatively described at three stages corresponding to the Nigerian pre-rainy, rainy breeding, and post-rainy season in 1988. A sex difference was found in external morphology, with males displaying a distinct indentation at the dorsal margin of the anal fin, regardless of season. Pronounced sex differences in electric organ discharges were found only in fish imported during the Nigerian rainy season, and only in two phase of the tetraphasic discharge, with males exhibiting longer durations of phases 2 and 3. These sex differences were reflected in the Fourier transformation, with male discharges exhibiting lower peak power spectral frequencies than those of females. No sex differences were found in total duration of the electric discharge. Correlations between the electric discharge measures and fish length, as well as between the discharge measures themselves, revealed relationships which varied in both direction and degree as a function of sex and/or season. These results indicate that the duration of individual phases, rather than the entire duration of the electric organ discharge, function in the communication of sex differences in this mormyrid species. Previous work has shown that phase durations of the electric discharge are steroid-dependent and therefore indicative of physiological states of the animal. Thus, the kinds of information conveyed during breeding and non-breeding seasons may be quite different, given great differences in gonadal steroid levels. These findings explain previously unsuccessful attempts to find sex differences as well as a report of a sex difference in a direction opposite to that shown here in G. petersii.

Weakly electric fish emit electric signals with an electric organ in their tail and perceive them with cutaneous electroreceptor organs. During active electrolocation, objects are detected and analyzed because they distort the self-produced electrical field and project electric images onto the electroreceptive skin surface. To optimize electrical imaging of the environment, the fish perform certain electromotor and locomotor behaviors (probing motor acts). For fine spatial analysis, the mormyrid Gnathonemus petersii focuses electric images of objects onto two ‘foveal’ skin regions: the ‘Schnauzenorgan’ and the ‘nasal region.’ Sudden emergences of novel stimuli evoke the so-called novelty responses (electromotor) and Schnauzenorgan responses (movements of the chin).

Most of the information about the anatomy of the fish's cranial nerves was collected in the first two decades of this century. Experimental analysis of the VIIth, IXth, and Xth cranial nerves by modern tract tracing techniques started about 20 years ago. Several species have been investigated to date, including one species of Agnatha (Myxinoidea), two species of elasmo-branchs, and species of some orders of Teleostei like Cyprinidae, Siluriformes, Perciformes, and Gadidae. The Sensory and motor nuclei of the VIIth, IXth, and Xth cranial nerves of Gnathonemus petersii were studied by anterograde and retrograde axoplasmatic transport of horseradish peroxidase and cobaltous lysine complex. The sensory nuclei form a continuous column of cells in the brain stem extending caudal to the obex. The rostral one-fourth of this column is occupied by the overlapping terminals of the VIIth and IXth nerves. The vagus nerve has 5 roots. The first 4 of these innervate the gills and the fifth supplies viscera. Afferents from the gills terminate ipsilaterally rostral to the obex in topographic order and their terminal fields overlap. Viscerosensory fibers terminate ipsilaterally in the obex region and bilaterally in the commissural nucleus of Cajal. The facial motor nucleus is located rostral to the sensory nucleus. Facial motoneurons have pear-shaped and multipolar perikarya. Their axons form a rostrally directed knee before leaving the brain. The motoneurons of the IXth and Xth nerves have a common cell column. The vagal motoneurons form a periventricular, a medial, and an intermediate cell group rostral to the obex. In the obex region and also caudal to it, a lateral and a caudal group can be distinguished. Vagal motoneurons show a topographic arrangement that is similar to that of the sensory vagal projections. The majority of motoneurons have pear-shaped perikarya and ventrolaterally oriented dendrites. In the caudal nucleus the dendrites extend dorsally and overlap the terminals of sensory fibers. The axons form a dorsolaterally directed arch before joining the sensory roots. Since G. petersii uses its electrosensory system primarily for detection of food, its gustatory system is less developed than in other fishes, which possess a large number of taste buds. © 1992 Wiley-Liss, Inc.

Gnathonemus petersii discriminates between ohmic and capacitive objects. To investigate the sensory basis of this discrimination we recorded from primary afferents that innervate either A or B mormyromast sensory cells. Modified and natural electric organ discharges were used as stimuli. In both A and B fibres frequencies below the peak-power frequency (3.8 to 4.5 kHz) of the electric organ discharge caused minimal first-spike latencies and a maximum number of spikes. A fibres did not discriminate phase-shifted stimuli, whereas B fibres responded significantly with a decrease in first-spike latency if the phase shift was only — 1°. In both A and B fibres an amplitude increase caused a decrease in spike latency and an increase in spike number; an amplitude decrease had the reverse effect. If stimulated with quasi-natural electric organ discharges distorted by capacitive objects, the responses of A fibres decreased with increasing signal distortion. In contrast, the responses of B fibres increased until amplitude effects began to dominate. Gnathonemus may use the physiological differences between A and B fibres to detect and discriminate between capacitive and purely ohmic objects.

The present paper describes the location, morphology, ultrastructure and immunocytochemical properties of neurons in the brain of the mormyrid fish Gnathonemus petersii, that appear to be unspecifically immunoreactive to a number of secondary or tertiary antibodies used in immunohistochemical procedures, including rabbit-anti-mouse immunoglobulins (IGGs), rabbit peroxidase-anti-peroxidase IGGs, and rabbit-anti-sheep or sheep peroxidase-anti-peroxidase IGGs. Unspecifically immunoreactive (UI) cells have typically neuronal morphological and ultrastructural characteristics, and occur at four specific locations in the mormyrid brain. A small rhombencephalic group is located rostrolateral to the efferent octaval nucleus, between the fasciculus longitudinalis medialis and the decussation of the lateral lemniscus. A mesencephalic cluster of cells is located in the dorsal midbrain tegmentum against the tractus telencephalo-mesencephalicus. In addition, dispersed UI neurons were observed in the nucleus lateralis of the torus semicircularis and in the preoptic region above the optic chiasm. Remarkably, UI cells are clearly present in a substantial number of brains investigated, but not detectable in others. The present findings point to a curious pitfall in chemical neuroanatomy, the functional significance of which is unknown at present. In several previous studies using the brain of G. petersii, UI cells were abusively included in the description of monoaminergic cell groups. Similar cells have until now not been reported in other vertebrate brains.

Immunocytochemistry has demonstrated unexpected heterogeneity among cerebellar Purkinje cells. For example, monoclonal antibody Mab anti-zebrin II reveals parasagittal bands of immunoreactive Purkinje cells in the mammalian cerebellum, but reveals a non-sagittal cerebellar compartmentation pattern in goldfish and gymnotiform fish. The present paper investigates the cerebellar compartmentation pattern, as reflected in the zebrin II distribution, in two other teleosts, the electric mormyrid fish Gnathonemus petersii with its large and regularly built gigantocerebellum, and the electrosensory osteoglossomorph teleost Xenomystis nigri, by using light as well as electron microscopic immunohistochemical techniques. Zebrin II is expressed only in Purkinje cells, where it is present in the cytoplasm of all neuronal compartments, including spines, distal and proximal dendrites, the cell body, and the initial part, as well as terminal boutons of the axon. Other types of cerebellar neurons, including the eurydendroid projection neurons, are zebrin II-negative. In Gnathonemus, zebrin II-positive Purkinje cells are present in the large caudolateral part of the valvula, in lobes C2, C3, and C4 of the corpus, and in the anterior as well as the posterior part of the caudal cerebellar lobe. Zebrin II-negative Purkinje cells are present in a continuous region encompassing the rostromedial part of the valvula, the lobus transitorius, lobe C1 and the ventral part of lobe C2, and in a small, lateral zone of the posterior part of the caudal lobe. In Xenomystis, all Purkinje cells, including those in the medial valvula and the posterior part of the caudal lobe, appear to react with mab anti-zebrin II. This more widespread distribution may be due to the presence of a second antigenic polypeptide in this species. On the basis of the present findings, it is concluded that the mormyrid lobus transitorius, lobe C1, and the ventral part of lobe C2 probably belong to the valvula, while the corpus is restricted to the dorsal part of lobe C2, lobe C3, and lobe C4. The functional significance of zebrin II expression for different subsets of teleostean Purkinje cells remains unclear, since comparisons of different teleosts reveal no general correlation with particular afferent or efferent connections, nor with special morphological features such as a dendritic palisade pattern or different arrangements of the Purkinje cell bodies. A comparison between mammals and teleosts suggests that a distinct parasagittal cerebellar zonation in teleosts is absent, and the major part of the teleostean cerebellum may be considered as a single (midsagittal) cerebellar zone, with about the same width as one mammalian parasagittal zone.

The African weakly electric fish Gnathonemus petersii is well known for its electrosensory capabilities. These animals can detect and distinguish objects through active electrolocation in complete darkness. Because of their nocturnal lifestyle, a low contribution of vision for orientation and object detection has been expected. However, as we show in this review, the retina of G. petersii is highly specialized with hundreds of rods and tens of cones grouped together in bundles in a complex way, ensheathed by a tapetum lucidum. The structure of the bundles goes beyond what would be expected if only photon catch was supposed to be increased. During daytime, the structure of these "macro-receptors" changes dramatically depending on retinomotor movements. During the day, the rods and cones are located in different compartments of the bundle, separated by a narrow canal in the form of a "bottle neck". Investigations on cell structure and neurochemistry in the retina indicate a general organization that is simpler in terms of bipolar and ganglion cell diversity than in tetrachromatic species such as goldfish, yet similar in terms of neurochemical differentiation of amacrine cells. In both respects, the inner retina of the elephantnose fish bears the greatest similarity to catfish and some deep-sea fish retinae. Neuronal circuits and bundle structure give hints of possible adaptations for contrast and/or movement detection. Behavioral experiments suggest that, in contrast to the vision specialists Lepomis gibbosus, pattern detection of G. petersii is not affected by higher spatial frequencies. A pattern of low spatial frequencies, however, was equally well detected by G. petersii and L. gibbosus. Optomotor response experiments indicate that motion vision is important for Gnathonemus, narrowing down the search for the functional specialization of the Gnathonemus retina and providing a starting point for work on multisensory integration in these fish.

Mormyrids are one of two groups of teleost fishes known to have evolved electroreception, and the concomitant neuroanatomical changes have confounded the interpretation of many of their brain areas in a comparative context, e.g., the diencephalon, where different sensory systems are processed and relayed. Recently, cerebellar and retinal connections of the diencephalon in mormyrids were reported. The present study reports on the telencephalic and tectal connections, specifically in Gnathonemus petersii, as these data are critical for an accurate interpretation of diencephalic nuclei in teleosts. Injections of horseradish peroxidase into the telencephalon retrogradely labeled neurons ipsilaterally in various thalamic, preglomerular, and tuberal nuclei, the nucleus of the locus coeruleus (also contralaterally), the superior raphe, and portions of the nucleus lateralis valvulae. Telencephalic injections anterogradely labeled the dorsal preglomerular and the dorsal tegmental nuclei bilaterally. Injections into the optic tectum retrogradely labeled neurons bilaterally in the central zone of area dorsalis telencephali and ipsilaterally in the torus longitudinalis, various thalamic, pretectal, and tegmental nuclei, some nuclei in the torus semicircularis, the nucleus of the locus coeruleus, the nucleus isthmi and the superior reticular formation, basal cells in the ipsilateral valvula cerebelli, and eurydendroid cells in the contralateral lobe C4 of the corpus cerebelli. Weaker contralateral projections were also observed to arise from the ventromedial thalamus and various pretectal and tegmental nuclei, and from the locus coeruleus and superior reticular formation. Tectal injections anterogradely labeled various pretectal nuclei bilaterally, as well as ipsilaterally the dorsal preglomerular and dorsal posterior thalamic nuclei, some nuclei in the torus semicircularis, the dorsal tegmental nucleus, nucleus isthmi, and, again bilaterally, the superior reticular formation. A comparison of retinal, cerebellar, tectal, and telencephalic connections in Gnathonemus with those in nonelectrosensory teleosts reveals several points: (1) the visual area of the diencephalon is highly reduced in Gnathonemus, (2) the interconnections between the preglomerular area and telencephalon in Gnathonemus are unusually well developed compared to those in other teleosts, and (3) two of the three corpopetal diencephalic nuclei are homologues of the central and dorsal periventricular pretectum in other teleosts. The third is a subdivision of the preglomerular area, rather than an accessory optic or pretectal nucleus, and is related to electroreception. The preglomerulo-cerebellar connections in Gnathonemus are therefore interpreted as uniquely derived characters for mormyrids.

The receptive field of a sensory neuron is known as that region in sensory space where a stimulus will alter the response of the neuron. We determined the spatial dimensions and the shape of receptive fields of electrosensitive neurons in the medial zone of the electrosensory lateral line lobe of the African weakly electric fish, Gnathonemus petersii, by using single cell recordings. The medial zone receives input from sensory cells which encode the stimulus amplitude. We analysed the receptive fields of 71 neurons. The size and shape of the receptive fields were determined as a function of spike rate and first spike latency and showed differences for the two analysis methods used. Spatial diameters ranged from 2 to 36 mm (spike rate) and from 2.45 to 14.12 mm (first spike latency). Some of the receptive fields were simple consisting only of one uniform centre, whereas most receptive fields showed a complex and antagonistic centre-surround organisation. Several units had a very complex structure with multiple centres and surrounding-areas. While receptive field size did not correlate with peripheral receptor location, the complexity of the receptive fields increased from rostral to caudal along the fish's body.

Forlim CG, Pinto RD, Varona P, Rodríguez FB (2015) Delay-Dependent Response in Weakly Electric Fish under Closed-Loop Pulse Stimulation. PLoS ONE 10(10): e0141007. doi:10.1371/journal.pone.0141007, In this paper, we apply a real time activity-dependent protocol to study how freely swimming weakly electric fish produce and process the timing of their own electric signals. Specifically, we address this study in the elephant fish, Gnathonemus petersii, an animal that uses weak discharges to locate obstacles or food while navigating, as well as for electro-communication with conspecifics. To investigate how the inter pulse intervals vary in response to external stimuli, we compare the response to a simple closed-loop stimulation protocol and the signals generated without electrical stimulation. The activity-dependent stimulation protocol explores different stimulus delivery delays relative to the fish’s own electric discharges. We show that there is a critical time delay in this closed-loop interaction, as the largest changes in inter pulse intervals occur when the stimulation delay is below 100 ms. We also discuss the implications of these findings in the context of information processing in weakly electric fish.

Publisher Summary Many groups of fishes perceive naturally occurring electric stimuli. They possess ampullary electroreceptor organs that respond to low‐frequency electric fields. Two nocturnally active teleost groups produce weak electric signals (electric organ discharges, EOD), which they perceive with tuberous electroreceptor organs. In addition to “passive electrolocation,” these weakly electric fish perform “active electrolocation,” during which they detect alterations of their EOD caused by nearby objects. This enables them to perceive the complex impedance of objects and thus to identify animated objects. In addition, they can measure distance, size, shape, and other 3D object properties. When inspecting an object, the weakly electric fish Gnathonemus petersii employs two “electrical foveae" that are located on their moveable chin appendage and in the nasal region. The former is used for object inspection, while the latter is used for object detection during foraging. The brain of weakly electric fishes extracts information about objects by analyzing the input from the electroreceptor organs. The study of electroreceptive brain areas has revealed many general principals of neural processing of sensory and motor information.

Local field potentials evoked either by auditory or by mechanosensory (water displacement) lateral line stimuli were recorded in sensory subregions of the telencephalic nucleus dorsalis pars medialis (Dm) in the weakly electric fish Gnathonemus petersii. The neural tracer Neurobiotin was injected into these two physiologically defined subregions. A reciprocal connection between the two subregions of Dm, as well as cell bodies and terminals in other telencephalic regions, whose distribution was somewhat different for the two injection types, were found. The course of labeled fibers outside the telencephalon was similar after injections in both Dm regions. Fibers were seen running through the lateral forebrain bundle (lfb) to the ventral surface area of the brain within the diencephalic preglomerular region (PGv). Within a narrow streak along the ventral side of the brain densely arranged cell bodies were labeled. The locations of labeled cells within PGv were indistinguishable after tracer was injected into either acoustical or lateral line areas of Dm. Only after injection into the mechanosensory Dm region labeled cell bodies were found in the anterior preglomerular nucleus (PGa), in addition. When crystals of the fluorescent tracer DiI were inserted in the ventral part of PGv, a path of labeled fibers similar to that after telencephalic injections was found. Labeled terminals, but no cell bodies, were located both in the acoustical and in the mechanosensory regions of Dm as well as in several other telencephalic areas. Even though sensory regions in Dm that process acoustical and mechanical stimuli are segregated and unimodal, they both receive input from neurons of PGv. The specificity of the mechanosensory region of Dm might originate from the additional input from PGa and from other endbrain areas.

The distribution of dopamine-containing cell bodies and fibers was studied with aid of specific antibodies against dopamine in the highly developed brain of the weakly electric fish Gnathonemus petersii. In the telencephalon, dopamine-containing cell bodies were observed in a small area, i.e., area ventralis pars dorsalis and supracommissuralis. In the diencephalon, moderate numbers of dispersed dopamine-immunoreactive cells were present in the preoptic region, while large numbers of dopamine-containing neurons occurred in the hypothalamic paraventricular organ and neighbouring regions. The paraventricular organ, located around small (anterior, intermediate, and posterior) recesses contained many dopamine-immunoreactive cerebrospinal fluid-(CSF)-contacting neurons. Dopamine-containing cells were also observed in a magnocellular hypothalamic cell group, in the nucleus of the lateral recess, and in the nucleus posterior tuberis. In the mesencephalon only a few dopamine-containing cells were observed in a dorsal tegmental (possibly pretectal) area, whereas in ventral mesencephalic regions dopamine-containing cells were lacking. More caudally, dopamine-containing cells were observed in the presumed locus coeruleus, in the caudal region of the reticular formation, and in the presumed area postrema. Dopamine-immunoreactive fiber density was very high in the medioventral hypothalamus and in the preoptic region, where a dense subependymal plexus was observed along the preoptic recess. Such a plexus was also present in the caudal rhombencephalon, where it probably arises from the area postrema. Moderate numbers of dopamine-immunoreactive fibers were present in medioventral parts of the brain along its total rostrocaudal extent as well as in several subnuclei of the torus semicircularis, in the tectum mesencephali, and in the medial part of the dorsal telencephalic area. Other parts of the dorsal telencephalic area, as well as the large cerebellum and the electrosensory lateral line lobe of Gnathonemus, did not contain detectable amounts of dopamine. In spite of the high differentiation of the brain of Gnathonemus, the distribution of catecholamines as visualized with dopamine immunohistochemistry appears to be basically similar to that described in other teleostean and actinopterygian fishes on the basis of formaldehyde-induced fluorescence or tyrosine hydroxylase immunohistochemistry.(ABSTRACT TRUNCATED AT 400 WORDS)

After injections of the posterior part of the lateral zone of the area dorsalis telencephalic (Dlp) with either horseradish peroxidase or the newly available carbocyanine dye DiI, efferent cells were labeled in the valvula cerebelli of the mormyrid fish, Gnathonemus petersii. This may be a unique connection for this group of electrosensory teleosts, since no other vertebrate has ever been reported before to have a direct cerebello-telencephalic projection.

The optic nerve and the retinal projections were studied in a mormyrid fish, Gnathonemus petersii, by using Fink-Heimer, HRP, cobalt labeling, and autoradiographic tracing techniques. The retinal fibers terminate bilaterally in the following places: suprachiasmatic nucleus, dorsolateral optic nucleus, optic nucleus of the posterior commissure, cortical nucleus, ventral pretectal area, optic tectum, and the accessory optic terminal field. The number of uncrossed fibers is relatively high in the suprachiasmatic nucleus, but negligibly small in the other retinal terminal fields. In the lateral geniculate nucleus and pretectal nucleus only crossed retinal fibers could be detected. The visual system of Gnathonemus is compared to that of other fishes, amphibians, and reptiles and the possible homologies are proposed. The comparison points to the conclusion that the visual system is less developed in Gnathonemus. This nocturnal species lives in turbid waters and has a special electric sense which may permit compensation for the reduced visual capacity.

The polysaccharides were studied in an electroreceptor organ, the tuberous organ of Gnathonemus petersii (Mormyridae). Histochemical methods (P.A.S., alcian blue, toluidine blue and iron colloidal reactions) allowed us to demonstrate the existence of glycogen in the sensory cytoplasm, and P.A.S. positive polysaccharides in the sensory cavity. The polysaccharides were shown to be amylase proof; they display an acidity due to the existence of sulphated radicals. The histochemical study was completed by a cytochemical analysis: a treatment with thiocarbohydrazide (TCH) according to the Thiery's method. This method allowed us to estimate the glycogen concentration, its localization, and relationship with cellular organites within the sensory cytoplasm, as well as to differentiate the highly glycogenous type II cells of the platform from the other accessory cells (Derbin and Szabo, 1968). After a treatment for 20 hours with TCH, silver stained grains were visible on the polysaccharide filaments of the sensory chamber, between the microvilli and the vacuoles of the epidermal cells lining to the sensory cavity. Silver grains coated the outer surface of the microvilli. Such polysaccharides were not identical to the filamentous polysaccharides of the cavity. In order to determine the cytochemical localization of the polysaccharide acid groups, sections were stained with iron salts. The colloidal iron constitutes a deposit opaque to electrons and located both on the filamentous polysaccharides of the sensory cavity and in the vacuoles of the epidermal cells, indicating that only these filamentous polysaccharides display acid radicals.

Phosphorylase activities were investigated by histochemical and ultrastructural procedures in the electroreceptive sensory cells of the tuberous organ of Gnathonemus petersii. Ater incubation in G1P, G1P activated by AMP (Takeuchi and Kuriaki medium) or in G1P activated by ATP+MgSO4 (Guha and Wegman medium) newly formed polysaccharides were analysed with the iodine and P.A.S. reactions under light microscopy and, under electron microscopy, with the periodic acid thiocarbohydrazide (TCH) silver proteinate (PATAg reaction, Thiery), The newly formed polysaccharides proved the presence of glycogen phosphorylase (2.4.1.1) activities and of their branching enzymes (2.4.1.18). When G1P was activated by ATP+MgSO4, they appeared as glycogen particles with the same constitution as native glycogen. After incubation in G1P and in G1P activated by AMP they appeared as glycogen and polyglucose filaments too. In the latter case they were high concentrated. The results show that the phosphorylases are principally present in this sensory cell in their inactive form.

The sequence of spike-like receptor potentials (RPs) of the tuberous organ of a Mormyrid fish (Gnathonemus petersii) has been studied in vivo. In stable conditions, the receptor is submitted only to the natural effector potentials of the fish, that is, the electric organ discharges (EODs). The intervals of time between RPs were analysed in histograms, considering only the cases with bimodal curves. Two modes of discharges were considered among the mean EOD frequencies: the stable and the burst modes. The burst EODs were shown to regulate the mean frequencies of RPs near a receptor equilibrium frequency. Two series of 10 experiments were performed. In the first series, in 8 experiments a regulation of the receptor mean frequency (RRMF) could be found. In the second, when the EODs of the fish were attenuated previously by a bath to which a curarizing agent had been added, only in 4 experiments a RRMF was found. Both modes of EODs are needed during RRMF, suggesting a counting mechanism during the stable mode of the effector. A parallelism between behavior, EODs and RP rates and RRMF is suggested.

The corpus cerebelli of mormyrid fishes is strongly developed and differentiated into four lobes: C1−C4. Although all of these lobes contain the characteristic cerebellar layers: granular, ganglionic and molecular, they show distinct architectonic differences. A previous study revealed that the ganglionic layer of C1, in addition to Purkinje elements contains conspicuous giant cells. In the present paper the results of a further analysis of C1 are reported. This analysis is based on serially sectioned brains of Gnathonemus petersii, stained according to Nissl, Bodian and Haggquist. Semi-thin sections were stained with p-phenylenediamine. Routine EM techniques were used to visualize synaptic relations. Mossy fibres and granule, Golgi, Purkinje and stellate cells are located characteristically throughout C1. It appeared that the giant cells of a previous study represent the largest elements of a population which has been termed now the eurydendroid cells. The average size of the latter is somewhat larger than that of the Purkinje cells, but both groups of cells show a considerable overlap in the size of their somata. Purkinje cells and eurydendroid cells are present throughout the ganglionic layer and both have a flattened, sagittally oriented, dendritic tree that extends into the molecular layer. Yet, the eurydendroid cells (EC) display the following characteristics which distinguish them from Purkinje cells (PC): (1) In EC the Nissl substance is dispersed diffusely throughout the soma, whereas in PC it tends to be concentrated around the nucleus, (2) The soma/nucleus ratio for EC is distinctly larger than for PC, (3) The dendritic trees of EC extend over a larger stretch of the molecular layer than those of PC, hence the term EC, (4) The dendrites of EC are more widely spaced and oriented less strictly parallel to each other than those of PC, (5) The dendrites of EC are somewhat irregular in outline and not covered with spines, in contrast to those of PC, (6) The axons of EC are oriented radially and join bundles of coarse fibres which leave the cerebellum whereas the axons of PC extend and ramify within the ganglionic layer, (7) The somata of EC, contrary to those of PC, are enveloped by a dense axonal plexus which forms numerous synaptic terminals on them. The numerical ratio of EC: PC was 1:5.5. The circuitry in C1 and the possible functional roles of its constituent neurons are discussed. It is pointed out that in this lobe the axons of PC impinge on EC and that the latter constitute its output system.

The cutaneous ampullary organs (type A) of the specific lateral line system in Gnathonemus petersii (mormyrids) have been investigated with ultrastructural methods. The stratified epithelium of the ampulla contains sensory cells which bear microvilli on their luminal surface and many sensorineural junctions on their basal cytoplasm. All nerve terminals for a single organ are issued from a single lateral nerve. The epidermal basal membrane only is in contact with the accessory cells. The three types of accessory cells show excretory functions. They fill up the ampulla and the intraepidermic canal with morphologically different mucopolysaccharides. The accessory cells of the type III appear to be similar to “goblet” cells of the intestinal epithelium.

The rhombencephalic posterior lateral line lobe nucleus (nLLL) and its connections were investigated in the mormyrid fish Gnathonemus petersii, at light and electron microscopical levels using HRP tracing. The nLLL, constituted on each side of about 1500 large, round shaped, adendritic cells, is located in the intermediate cell and fibre layer exclusively, in the ventrolateral zone of the posterior lateral line lobe. The cells show a complex synaptology: boutons with chemical synapses cover the largest part of the soma and the long initial segment of the axons. In addition each nLLL cell bears generally two club endings which form gap junctions with the postsynaptic membrane. On the unmyelinated portion of the club ending, a particular synaptic complex (= serial synaptic connections) was observed; large endings, bearing boutons with chemical synapses, contact the club ending with gap junctions. HRP injection into either lateral line nerve showed that the club endings represent the peripheral input of the nLLL. This input is exclusively ipsilateral; the anterior and posterior lateral line nerve projection does not seem to overlay at this level. Retrograde labelling of nLLL cells after HRP injection into the anterior mesencephalic exterolateral nucleus (nELa) confirms the electrophysiological results according to which the nLLL projects directly and bilaterally to the nELa. This rhombo-mesencephalic connection is established through club endings of the nLLL axons which form gap junctions with the large cells of the nELa. About two thirds of the nLLL axons form a crossed and one third an uncrossed bundle within the lateral lemniscal pathway. Anterograde transport in the axon collaterals shows that some of the nLLL cells project simultaneously to the ipsi- and to the contralateral nELa permitting this system a high degree of electrosensory information processing.

A morphological investigation was done on the epidermal and dermal melanophores of two mormyrid fish, Marcusenius isidori and M. longianalis. The epidermal meleanophores were located in the outer and middle layers of the epidermis as well as in the inner layer. The dermal melanophores occurred just beneath the basal lamina and in the deeper part of the dermis. The dermal melanophores of M. longianalis had sometimes elongated melanosomes as well as ovoid-shaped ones. The premelanosomes of the epidermal melanophores had a doughnut-like inner structure in addition to a striped inner structure. The melanophores of Marcusenius isidori and M. longianalis possess the basic characteristics common to other vertebrates, and they resemble closely those of other mormyrid fish, Gnathonemus petersii, and other fish.

The projection regions of electroreceptor and mechanical lateral line afferents in electric fish of the mormyridae family are described. Electroreceptor afferents from the posterior dorsal skin run in the dorsal branch of the posterior lateral line nerve. Electroreceptor afferents from ventral skin and mechanical lateral line afferents and efferents run in the ventral branch of the nerve. Horseradish peroxidase (HRP) injections into each branch resulted in filling of its central terminals with the marker enzyme. The method yields a Golgi-like staining of afferent terminals, allowing some aspects of their morphology to be described. Comparison of results from dorsal and ventral branch injections shows the separate medullary regions to which electroreceptor and mechanical afferents project, and also demonstrates four separate somatotopic maps within the electroreceptor region. Mechanical afferents end predominantly ipsilaterally in nucleus anterior and eminentia granularis as has been suggested by others. Ipsilateral endings in nucleus octavius are also seen. Electroreceptor afferents end exclusively in the cortex and nucleus of posterior lateral line lobe (PLLL). Within the cortex there are three distinct maps of the skin surface which are separated from each other by discontinuities in the cellular layers. Somatotopic mapping is also present in the nucleus of PLLL though it is less precise than in the cortical zones. Large club endings of the cells of this nucleus are filled with HRP. Labeled cells are seen within a small midline nucleus located at the level of the eighth nerve just above the medial longitudinal fasciculus. These are probably the cell bodies of lateral line efferents.

The connections of a cerebellar structure, the eminentia granularis ( e.gr. ) in the mormyrid fish Gnathonemus petersii were demonstrated by the HRP method. Injections of horseradish peroxydase in this area and in several cranial nerves enabled us to distinguish two regions in this extensive granular structure: the pars anterior ( p.a. ) which receives root fibres from the lateral line and vestibular nerves, and the pars posterior (p.p.) towards which converge afferent fibres from the spinal cord, the medulla oblongata and the mesencephalon. In the light of these results the former is considered as a primary centre for specific sensory impulses and will be called the eminentia granularis, sensu strictiori ; in contrast the latter, which may be considered as the auricula , represents an integration centre for secondary impulses arriving from the common (cutaneous, proprioceptive) sensory system.

The sensory basis of group cohesion in the weak-electric fish Gnathonemus petersii was investigated in a circular tank with groups of four fish each, interacting through a wide-meshed plastic screen with intact or operated conspecifics, or with other stimulus objects. We confined these stimuli to one or two peripheral holding compartments. The response measures were obtained from the free swimming fish and included (1) the time the fish spent together as a group, (2) the time they spent in front of the holding compartments, (3) the circular distribution of the fish's positions, and (4) the mean nearest neighbour distances. Under empty compartment conditions, four different groups were tested, consisting of either (1) intact, electrically active fish, or (2) electrically ‘silent’ fish (with their electric organ surgically rendered inoperative), or (3) blind, or (4) ‘silent’ and blind animals. The loss of either sensory modality, vision or feedback from electric organ discharge, led to changes of comparable size, decreasing the time spent as a group and increasing the mean nearest neighbour distance. In fish lacking both modalities, group cohesion was further impaired. With stimuli present in one or both holding compartments, the strength of social attraction depended on the nature of the stimulus: the more intact stimulus conspecifics were present, the more densely did the fish group in front of the stimulus compartment. ‘Wired-in’ electric organ discharges (simulating waveform and intensity) and electrically ‘silent’ fish were equally attractive, but only half as attractive as intact fish. Blind free swimming fish aggregated with intact and also with ‘silent’ conspecifics. Under dim light conditions, group cohesion was predominantly, though not exclusively, affected by electrosensory feedback from the electric organ discharge and visual input. Mechanical and olfactory cues may also be involved.

Scanning electron microscopic studies were conducted on the sensory epithelia of the auditory portions of the ears in teleost species representing wide taxonomic diversity. A number of the features of the ears investigated resem- bled features found in other teleost species, although some major exceptions to earlier patterns were found, particularly in the saccular sensory epithelium. The saccular maculae of all but one species contained basically similar ciliary bundles on the sensory hair cells while there was some significant variation on the lagenar maculae. Hair cell orientation patterns on the sacculus contained four orientation groups in all of the species, other than the mormyrid, Gnathonemus, which only had two groups. Lagenar maculae had two orientation groups, and the orientation patterns were similar to one another. The most divergent form of lagenar macula was found in Gnathonemus. These data, combined with data from earlier investigations, provide a broad overview of the surface features of the ear in teleost fishes. Most significantly, it now appears that there are at least five different saccular hair cell orientation patterns among teleost fishes, and all of these patterns are found spread through many major teleost taxa. While there is some similarity in ear structures among some groups of closely related species, such as the Elopomorpha and the Gadiformes, it is becoming more apparent that there is extensive convergence in a number of features of the teleost ear that most likely reflect similar selective pressures during the evolution of the ear. The nature of these selective pressures, however, are not well understood.

In the presence of a novel stimulus such as an aluminum or a plastic rod mormyrid fish (Marcusenius cyprinoides and Gnathonemus petersii) exhibit characteristic motor probing acts (PMAs): 'chin probing', 'radial' and 'lateral va-et-vient', 'lateral probing', 'tangential probing', and 'stationary probing'. During the display of these PMAs the fish maintain characteristic probing distances from the object which were 4.5 cm for the metal and 2.5-3.7 cm for the plastic stimulus. G. petersii with their electric organ rendered inoperative ('silent fish') no longer exhibited 'radial' and 'lateral va-et-vient'. Regardless of the nature of the stimulus the probing distances were shorter in 'silent' fish and ranged from 1.8 to 2.6 cm. During the display of PMAs intact fish changed their variable electric organ discharge rate to a unique and stable, regularized rate, with the interdischarge interval maintained at 28-30 ms. The fish's electric and non-electric (motor) probing behavior in the presence of novel objects (and following their removal during phantom PMAs) is discussed in light of theories on exploratory behavior.

Des substances muqueuses ont ete examinees par des methodes histochimiques au niveau des organes recepteurs cutanes de type ampullaire (organe de type A, Szabo, 1965) faisant partie du systeme de la ligne laterale des poissons electriques a faible decharge Gnathonemus petersii (Mormyrides), Gymnotus carapo (Gymnotides) et Gymnarchus niloticus (Gymnarchides).

The electric organ discharges (EODs) of pairs of weakly electric fish, Gnathonemus petersii, were simultaneously recorded to study the significance of the EODs as communication signals. In a 400-litre tank a larger fish (12 to 15 cm) was passively moved within a shelter tube toward a smaller specimen (6 to 9 cm), either in steps or a continuous move. The movement was stopped at that distance when at least one fish significantly lowered or ceased its EOD activity. From this ‘threshold interfish distance’ the spatial range of a ‘communication field’ was found to extend about 30 cm from the fish. At threshold distances an EOD frequency increase caused a temporary EOD activity cessation in the second fish. The spontaneous irregular EOD pattern of the fish displaying the increased EOD rate changed into a regular one with almost equal time intervals between fish pulses.

This article deals with the gross and microscopic anatomy of the tongue and tongue region of the lower jaw of Gnathonemus petersii (Mormyriformes, Teleostei). The osteology coincides largely with that of Mormyrus caschive; the cranial muscles differ from those of most other fish through the absence of the m. intermandibularis posterior. The innervation of the tongue appears to be by the ramus palatinus superior n. facialis (VII). Microscopically the overall picture of the tongue is that of a deflated tube which consists at the outer portion of a compact continuous fibrous connective tissue surrounding an inner part which contains mainly loosely arranged collagen and reticular or elastic fibers with a few cells and little recognizable matrix. A comparison is made between the tongue structure of G. petersii and those of three other bony fish, Phoxinus phoxinus (Cyprinoidei), Plecoglossus altivelis (Salmonoidei) and Polypterus (Braehyopterygii).