800 Introduction Fish of the gymnotiform family (Teleostei) are endowed with specialized electric organs (EO) whose activation generates weakly electric fields (Lissmann, 1958). Both the activation of the EO and the resultant electric field are referred to as the electric organ discharge (EOD). The electric field serves as a carrier of signals sensed by a mosaic of cutaneous electroreceptors (Bullock et al., 1961). These receptors are well tuned to this carrier, securing its optimal detection and a large signal-to-noise ratio (Bastian, 1986). Thus, the EOD is part of an active sense used for exploring the nearby environment (electrolocation) and for sending messages to con-specific neighbors (electrocommunication). Impedances different from water imprint signals in the self-generated field, in the same way that the voice of a radio presenter imprints signals on the emitted carrier of a broadcasting frequency (Bastian, 1986; Caputi and Budelli, 2006). This allows object detection, location and discrimination. The pattern of repetition of the EOD is itself an electrocommunication signal for conspecifics, in the same way that the sequence of spikes in a neuron codes a message (Black- Cleworth, 1970; Carlson, 2002). There are two main types of EOs, neurogenic and myogenic. While in neurogenic organs the discharge derives from the synchronic activation of parallel bundles of nerve fibers, in myogenic organs the EOD results from the activation of a population of specialized cells derived from embryonic myogenic tissue, the electrocytes (for reviews, see Bennett, 1971; Baas, 1986; Moller, 1995). The electrogenic system of electric fish with myogenic EO is organized in three main stages: (1) a command signal, generated by a medullary center that triggers each event in the series, (2) spinal and peripheral circuits that organize the spatio-temporal pattern of activation of the EO and (3) the differential excitability of the electrocytes (for reviews, see Dye and Meyer, 1986; Macadar, 1993; Hopkins, 1995; Caputi, 1999; Caputi et al., 2005). In Gymnotus, the multiphasic waveform of the EOD is generated by several neuronal and circuit mechanisms that are responsible for the precisely timed sequential activation of subcellular electrogenic units (the different faces of the electrocytes). Because these mechanisms have been well Some fish emit electric fields generated by the coordinated activation of electric organs. Such discharges are used for exploring the environment and for communication. This article deals with the development of the electric organ and its discharge in Gymnotus, a pulse genus in which brief discharges are separated by regular silent intervals. It is focused on the anatomo-functional study of fish sized between 10 and 300·mm from the species of Gymnotus, in which electrogenic mechanisms are best known. It was shown that: (1) electroreception and electromotor control is present from early larval stages; (2) there is a single electric organ from larval to adult stages; (3) pacemaker rhythmicity becomes similar to that of the adult when the body length becomes greater than 45·mm and (4) there is a consistent developmental profile of the electric organ discharge in which waveform components are added according to a programmed sequence. The analysis of these data allowed us to identify three main periods in post-natal development of electrogenesis: (1) before fish reach 55·mm in length, when maturation of neural structures is the main factor determining a characteristic sequence of changes observed in the discharge timing and waveform; (2) between 55 and 100·mm in length, when peripheral maturation of the effector cells and changes in post-effector mechanisms due to the fish’s growth determine minor changes in waveform and the increase in amplitude of the discharge and (3) beyond 100·mm in length, when homothetic growth of the fish body explains the continuous increase in electric power of the discharge. Key words: electric fish, electric organs, development, Gymnotus. Summary The Journal of Experimental Biology 210, 800-814 Published by The Company of Biologists 2007 doi:10.1242/jeb.000638 Post-natal development of the electromotor system in a pulse gymnotid electric fish Ana Carolina Pereira 1 , Alejo Rodríguez-Cattaneo 1 , María E. Castelló 1,2 and Angel A. Caputi 1, * 1 Departmento de Neurociencias Integrativas y Computacionales, Instituto de Investigaciones Biológicas Clemente Estable, Unidad Asociada de la Facultad de Ciencias and 2 Departmento de Histología y Embriología, Facultad de Medicina, Universidad de la República, Montevideo, Uruguay *Author for correspondence (e-mail: angel@iibce.edu.uy) Accepted 4 January 2007 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY