We describe nesting and paternal care in two species of the weakly electric fish, Gymnotus: Gymnotus carapo from savanna swamps on the Caribbean island of Trinidad; and Gymnotus mamiraua from whitewater floodplains of the Amazon Basin. In both species, single adult males guard eggs and juveniles. Male G. carapo excavate depressions in the substrate or nest in the roots of aquatic macrophytes. Male G. mamiraua form nests exclusively in the root mass of floating meadows of macrophytes. Twelve nests of G. mamiraua were encountered, containing juveniles up to a maximum total length (TL) of 67 mm and estimated maximum age of 8–12 weeks. Two of these nests exhibit a bimodal size distribution, implying more than one spawning event. Juveniles remain close to the male during the day and disperse little more than 1 m away at night. Larval G. carapo and G. mamiraua (approximately 15– 20 mm TL) generate a characteristic Electric Organ Discharge (EOD) with a dominant positive phase followed by a very weak negative phase. The Peak Power Frequency of the larval EOD is in the range 0.015–0.04 kHz. It is possible that the EODs of larvae may be sensed by animals with ampullary electroreceptors, including the male as well as predatory catfishes. Larval G. carapo and G. mamiraua generate EODs at a low repetition rate compared to adults and with no increase at night. This low rate reduces the probability of coincident pulses and may mitigate sensory jamming between nestlings in crowded nests. The repetition rate, and the waveform and spectral features of the EODs from 40 mm TL specimens of G. mamiraua, resemble those of mature adults.

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