The adult electric organ of the mormyrid Pollimyrus isidori consists of four longitudinal tubes, two dorsal and two ventral ones, entirely filling the space in the caudal peduncle, as in other mormyrids. Each tube comprises about 100 electrocytes possessing a large innervated stalk just behind their posterior faces. The numerous nerve terminals are deeply embedded into the surface of the stalk. The electrocytes begin to differentiate in 10-11 mm long fish (about 20 days old); they are initially arranged myotomically. The electromotoneurons are seen for the first time in 9.5 mm long fish (about 16 days old). In 12 mm long fish many putative nerve fibres are found surrounding the stalk, but it is only in 15-15.5 mm fish that nerve terminals establishing typical synapses (synaptic vesicles, synaptic cleft about 80 nm wide, and active zones) are first found. This coincides with the recording of the first electric discharges of the adult electric organ. At this stage the electrocytes are well developed, kidney-like and highly compressed, arranged in parallel in the caudal peduncle. Subsequently the number of nerve terminals increases, the terminals become more deeply invaginated into the membrane of the stalk, the myelin sheath terminating just at its surface. In 19 mm fish the first indication of penetrating stalks at the periphery of the electrocytes can be detected. With increasing length of the larvae the amplitude of the electric discharge also continuously rises. The electrocytes are acetylcholinesterase-positive, the activity is more pronounced over the stalk and the anterior face shows higher activity than the posterior face, mainly those at the periphery, where the penetrating projections are seen.

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The larval electric organ of Pollimyrus isidori consists of four longitudinal tubes, a dorsal and a ventral pair, which begin behind the skull, end at the beginning of the caudal peduncle and show myotomic segmentation. The elementary units are, apparently, transformed muscle fibres called electrocytes. They are shorter and thicker than muscle fibres, with long stalks and are found in the medial part of the deep lateral muscle. Electron microscopy reveals a clear difference between the anterior and posterior face of the electrocyte. Anteriorly, deep linear invaginations of the surface membrane together with many small vesicles of about 100 nm diameter can be seen. Posteriorly, many plasma membrane invaginations and vacuoles are found together with numerous cytoplasmic organelles--pleiomorphic nuclei, Golgi apparatus, oblong mitochondria and multivesicular bodies. The stalk originates at the posterior face and the nerve terminals are situated at the distal end of the stalk. In the electrocyte, myofibrils, similar to those found in muscle fibres, can be detected with clearly visible Z lines but with only a suggestion of H zones. Two bundles of myofibrils can be seen arranged orthogonally in the electrocyte. Strong acetylcholinesterase activity was found on the anterior face and on the innervated stalk. Under the given recording conditions the overall discharge amplitude of the larval electric organ reaches a maximum of about 100 mV peak to peak. The pulse duration is 1 millisecond and the main phase is head-positive.