Electric organ morphology and function in the lesser electric ray, Narcine brasiliensis

The lesser electric ray, Narcine brasiliensis, is a small, demersal ray capable of generating electricity through its main and accessory electric organs. Although closely related to the large piscivorous torpedo rays, it differs in size, habitat, and prey. Based on these differences, we hypothesized that the main electric organs are used for predator defense rather than feeding and that the accessory electric organs, specific to this species, are used for intraspecific communication. We found that the mass of the main and accessory electric organs were both significantly smaller in females than in males. Whereas the main electro-somatic index does not change with growth, the accessory electro-somatic index increases, providing support for the accessory electric organs' use in intraspecific communication. We characterized the discharge properties of the main electric organ throughout ontogeny by simulating a predation attempt on the ray. Rays always responded by generating electric organ discharges (EODs) and by flexing the tail dorsoventrally and laterally. The main EOD amplitude, measured directly at the source, increased logarithmically with disc width to a maximum measured amplitude of 56 V. Minimum amplitude was more variable, but followed a positive power relationship with disc width. Neonates produced trains comprised of significantly more EODs than the adults. Over the course of the first set of discharges, all age classes showed a decrease in fundamental frequency and an increase in train duration. In contrast to these defensive responses, the rays did not generate EODs while foraging or feeding on live prey.