Larval electroreceptors indicate a larval electric system in mormyrids

The ontogeny of the electroreceptors of two species of mormyrids, Campylomormyrus cassaicus and Pollimyrus isidori, was studied. In young larvae (10 and 12 days old, respectively) ampullary organs, knollenorgans, promormyromasts and two types of tuberous organs (differing by their accessory cells) were found. These latter each possess a single sensory cell sitting on a platform of accessory cells. The platform is pierced by an unmyelinated nerve fibre. The promormyromast is composed of a single sensory cell surrounded by several accessory cells. The free apical area of the sensory cell and the accessory cells protrude into an intraepidermal cavity. The receptor cell is innervated by an unmyelinated nerve fibre ending in synaptic boutons. At the disappearance of the larval and appearance of the adult electric discharge the two types of tuberous organs degenerate, whereas the promormyromast differentiates into the typical mormyromast consisting of two types of sensory cells. The two types of tuberous organs and the promormyromast therefore are termed larval electroreceptors. Mormyrids are therefore the first group of electric fish possessing a complete larval electric system, comprising not only a larval electric organ, but also a larval electrosensory system.