Larval electroreceptor organs in the epidermis of larvae of Campylomormyrus cassaicus and Pollimyrus isidori (Mormyridae, Osteoglossomorpha).

Electroreceptor organs were studied in the epidermis of the head of 10 and 12 days old larvae of mormyrid fishes, Campylomormyrus cassaicus (Poll, 196 7) and Pollimyrus isidori (Valenciennes, 1846). In addition to the ampullar y organs and knollenorgans typically present in the epidermis of the adult fish, three other sensory organs were found. These were two different types of tuberous organs (types A and B) and promormyromasts; all of which have the typical characteristics of electroreceptor organs. Each organ has a sin gle sensory cell, The sensory cell of the promormyromast is surrounded by a number of accessory cells. The free apical area of the sensory cell and th e accessory cells protrude into an intraepidermal cavity. The sensory cell of the two types of tuberous organs is isolated in a cavity lined by epider mal cells which have long microvilli. The outer margin of the sensory cell also bears many microvilli and its base sits on a platform of accessory cel ls. An unmyelinated nerve crosses through the accessory cells and contacts the sensory cell in the promormyromast and the type B tuberous organ. Howev er, in the type A tuberous organ no synaptic buttons were found at the base of the sensory cell. When the larval electric organ disappears, both the t ypes A and B tuberous organs degenerate and the promormyromast differentiat es into the typical adult mormyromast. Mormyrids thus possess a complete la rval electric system comprising both a larval electric organ and several ty pes of larval electroreceptor organs which are specific to this stage.