Ultrastructural features and hormone-dependent sex differences of mormyrid electric organs

The electric organ of mormyrid fishes is composed of action potential-generating cells called electrocytes that together produce a species-typical electric organ discharge (EOD). The electrocytes of mormyrids are disc-shaped cells with distinct anterior and posterior faces, and a series of evaginations of one face that form a stalklike structure that is the site of innervation by spinal electromotoneurons (Bass: J. Comp. Neurol. 244:313-330, '86a). Here, we describe the major ultrastructural features of mormyrid electrocytes, which include surface invaginations along each face, myonuclei, myofilaments, and neuromuscularlike junctions formed by the axons of spinal electromotoneurons. The degree of surface invaginations along the anterior face is the most dramatic interspecific variable and is usually greater for species with the longer duration EODs. Among species with sexually dimorphic EODs, natural males, or females treated with gonadal steroid hormones, have longer-duration EODs and thicker electrocytes with more surface invaginations along the anterior face. The results are discussed in relation to the action potential-generating properties of the electrocyte's membranes.