Waveform generation in the weakly electric fish Gymnotus coropinae (Hoedeman): the electric organ and the electric organ discharge

SUMMARY This article deals with the electric organ and its discharge in Gymnotus coropinae , a representative species of one of the three main clades of the genus. Three regions with bilateral symmetry are described: (1) subopercular (medial and lateral columns of complex shaped electrocytes); (2) abdominal (medial and lateral columns of cuboidal and fusiform electrocytes); and (3) main [four columns, one dorso-lateral (containing fusiform electrocytes) and three medial (containing cuboidal electrocytes)]. Subopercular electrocytes are all caudally innervated whereas two of the medial subopercular ones are also rostrally innervated. Fusiform electrocytes are medially innervated at the abdominal portion, and at their rostral and caudal poles at the main portion. Cuboidal electrocytes are always caudally innervated. The subopercular portion generates a slow head-negative wave (V 1r ) followed by a head-positive spike (V 3r ). The abdominal and main portions generate a fast tetra-phasic complex (V 2345ct ). Since subopercular components prevail in the near field and the rest in the far field, time coincidence of V 3r with V 2 leads to different waveforms depending on the position of the receiver. This confirms the splitting hypothesis of communication and exploration channels based on the different timing, frequency band and reach of the regional waveforms. The following hypothesis is compatible with the observed anatomo-functional organization: V 1r corresponds to the rostral activation of medial subopercular electrocytes and V 3r to the caudal activation of all subopercular electrocytes; V 2 , and part of V 3ct , corresponds to the successive activation of the rostral and caudal poles of dorso-lateral fusiform electrocytes; and V 345ct is initiated in the caudal face of cuboidal electrocytes by synaptic activation (V 3ct ) and it is completed (V 45ct ) by the successive activation of rostral and caudal faces by the action currents evoked in the opposite face.