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Mormyrid fish use active electrolocation to detect and analyze objects. The electrosensory lateral line lobe in the brain receives input from electroreceptors and an efference copy of the command to discharge the electric organ. In curarized fish, we recorded extracellularly from neurons of the electrosensory lateral line lobe while stimulating in the periphery with either a local point stimulus or with a more natural whole-body stimulus. Two classes of neurons were found: (I) three types of E-cells, which were excited by a point stimulus; and (2) two types of I-cells, which were inhibited by point stimulus and responded with excitation to the electric organ corollary discharge. While all neurons responded to a point stimulus, only one out of two types of I-units and two of the three types of E-units changed their firing behavior to a whole-body stimulus or when an object was present. In most units, the responses to whole-body stimuli and to point stimuli differed substantially. Many electrosensory lateral line lobe units showed neural plasticity after prolonged sensory stimulation. However, plastic effects during whole body stimulation were often unlike those occurring during point stimuli, suggesting that under natural conditions electrosensory lateral line lobe network effects play an important role in shaping neural plasticity. [ABSTRACT FROM AUTHOR]

East African and south African churchills (Petrocephalus, Mormyridae) were synonymised in 1959 to become members of a single species of subcontinental, southern African distribution, Petrocephalus catostoma (Günther, 1866). By comparison with the type material for P. catostoma from the Ruvuma River and P. stuhlmanni from the Ruvu River, both of East African origin, we confirm the South African form of churchill to represent a new species, P. wesselsi, ranging from the northern Limpopo and Incomati systems south to the Pongola River (Natal) as its southern limit. We also compared churchills from the Sabie River (25° S, South Africa, Incomati system) with churchills from the Upper Zambezi River (17° S, Namibia), using electric organ discharges (EODs) and morphology. The duration of an EOD pulse of the South African form (N = 39; 943.2±S.E. 18.82 µs) is, on average, more than twice that of the Upper Zambezi form (N = 37; 436.6±15.1 µs), and the amplitude of the second head-positive phase (P2 phase relative to P1 = 1) significantly weaker (0.133 ± 0.0005 vs. 0.472 ± 0.002 for Upper Zambezi males, 0.363 ± 0.03 for Upper Zambezi females). In contrast to the Upper Zambezi form, the EOD of the South African form exhibits no difference between the sexes. Fish from the two origins differ significantly in 11 out of 14 anatomical characters studied, confirming molecular genetic differentiation on the species level. [ABSTRACT FROM AUTHOR]