A.N.Severtzov Institute of Ecology and Evolution, Leninskii pr. 33, Moscow 117071, Russia

Abstract

Hypothetical causes and mechanisms of natural selection that led to polyphyletic development of electric organs (EO) in number of Elasmobranchii and Teleostei species are considered. From the known EO functions, the primary function in evolution might be only the signal one (an analogy and, perhaps, originally an "electric component" of visually perceived movements associated with aggression, etc.). This follows from the only function of EO in contrast to the others - location or electric stressor - can be accomplished in a shot range, by the slow components of common muscle impulsion, detected by low-frequency ampulated electric receptors, the only ones detected in some of non-electric fishes. EO could also be a product of selection for the increase in the amplitude and, as a result, the long-range of prolonged (juvenile or those emerging in case of partial fusion of excitatory postsynaptic potentials in tetanus) muscle discharges as a signal, addressed to such electric receptors of other conspecific individuals or in those (not having electric receptors Uranoscopidae) - of the "false signal" simulating EO discharges of potential predators or competitors (the rays Rajidae) associated with territorial aggressive behaviour ("electric mimicry"). Further selection for longer range of reception may lead to the replacement of primary signal EO function by the secondary - of scaring off predators and immobilization of prey. Selection for species-specificity and reliable reception of electric signals under cohabitation of numerous species of weakly electric fish in the same biological community promoted the development in the ancestors of Mormyridae and Gymnotidae of initially a single ability of generating and recognition increasingly short and frequent species-specific discharges and also to measure stepwise the frequency of the own signal in the appearance of noises in it. The same conditions resulted in the development of tubercular highly frequent electric receptors due to electric communication ("passive location") and finally the advent of the mechanisms of spatial "active" electric location ("electrovisory"). Thus active electric location is the "highest" EO function, the product their longest and most complicated evolution, associated with repeated and complicated changes of function, rather than the initial function as was thought by some authors.