Court and spark: electric signals in the courtship and mating of gymnotoid fish

By mimicking tropical rainy season conditions in aquaria, we stimulated two species of gymnotoid electric fish, Eigenmannia virescens and Apteronotus leptorhynchus, to spawn in captivity. Their courtship activity, breeding behaviour and electric social communication were monitored in several groups over 2 years. Groups of both species established dominance hierarchies correlated with electric organ discharge frequency, aggressiveness and size. Spawning was preceded by several nights of courtship during which the male modulated its electric organ discharge to produce ‘chirps’. Continual bouts of chirping lasted for hours on evenings prior to spawning. These electrical signals play a significant role in courtship and spawning, as gravid E. virescens females could be stimulated to spawn by playing back into the tank a tape recording of male courtship chirps. In both species the chirp invovves a slight increase in frequency followed by a cessation of the dominant frequency. This suggests a common mode of signal production in these two different genera of fish. Chirps are short and abrupt during aggressive encounters, but assume a softer and more raspy quality during courtship.