Malapterurus

Abstract: The electric catfish, Malapterurus electricus, possesses electric organs that are innervated by a pair of identifiable electromotoneurons located withi the cervical spinal cord. The pattern of synaptic innervation of the electromotoneurons can be revealed by an antibody against the synaptic vesicle protein SV2. Both somata and proximal dendrites are densely innervated. Several transmitters contribute to this punctuate immunoreactivity. The previously dorsal root sensory fibers, reveals a weak punctuate immunoreactivity. The previouslydescribed electrical synapses of the electromotoneurons wete visualized by an antibody against a gap- junctional protein. In contrast to the electromotoneurons of other electric fish, the electric catfish electromotoneurons possess many inhibitory synapses. With antibodies against glycine and against the glycinee receptor, a dense immunoreactivity of the surface of the somata and proximal dendrites can be revealed. The glycine receptor-like immunoreactivity exhibits a patch-like distribution similar to that revealed by the anti-SV2 antibody. γ-Aminobutyric acid (GABA)- immunopositive terminals contribute to the inhibitory electromotoneuron innervations to a lesser degree. The chemical characteristics of the electromotoneuron innervations of Malapterurusresemble those of other spinal motoneurons rather than electromotoneurons of other electric fish. Thus our immunocytochemical study supports the view that the pattern of electromotoneuron innervations in Malapterurus reveals little specialization. The capacity for information processing required for the control of the electric organ discharge appears to be achieved by the increased integrational capacity of the newly evolved multiple dendrites and not by an additional parallel channel specific for the electromotor system. © 1994 Wiley-Liss, Inc.

Abstract: Morphological analysis of 28 Malapterurus specimens collected from the Nile and Omo‐ Turkana basins revealed two groups. The first was characterized by low vertebral counts (41‐44) and numerous gill rakers (14‐22) situated along the entire first ceratobranchial, the second group was characterized by high vertebral counts (47‐51) and a low number of gill rakers (6‐13) situated only on the upper portion of the first ceratobranchial. Through comparison with the type specimens of three currently recognized Malapterurus species and comparison with museum collections from the Nile, Omo‐Turkana, and Chad basins, these two groups were identified as M. electricus and M. minjiriya. A principal component analysis of 27 measurements allowed partial discrimination among species. Biogeographic aspects of finding M. minjiriya in the tributaries of the White Nile and in the middle reaches of the Omo River are discussed. ? 1999 The Fisheries Society of the British Isles

Abstract: Two giant electromotoneurons located within the cervical spinal cord form the centerpiece of the electromotor system in the electric catfish Malapterurus electricus. The cytoarchitectural organization suggests a high degree of input convergence onto the electromotoneurons. In order to obtain insights into the connectivities of the electromotor system, pre-neurons of the electromotoneurons within the brain stem and the spinal cord were labelled by application of FITC-dextran and horseradish peroxidase onto the surface of a single electromotoneuron. Our results show that the electromotoneurons receive their main inputs from the nucleus profundus mesencephali within the tegmentum and from large neurons of the medial reticular formation. Both nuclei possess an intimate connection to the optic tectum which mediates orientation responses. This pathway to the electromotoneurons could be instrumental in eliciting electric organ discharge during prey catching. The electric avoidance response in turn could be mediated by the Mauthner neurons which are also labelled. In addition to these neurons, cells of the nucleus fasciculi longitudinalis medialis, the descending octaval nucleus and the nucleus funicularis medialis were labelled. As compared to the corresponding neurons in ictalurid catfish, none of these neurons displays any alteration in its general morphology. It is concluded that the evolution of the electric organ from muscle tissue and the development of a central control system of the electromotor response in Malapterurus involved a minimum of alterations in central nervous system circuitry. In contrast to many other electric fishes the electromotor control is mainly accomplished at the level of the electromotoneurons.