Postembryonic changes in the peripheral electrosensory system of a weakly electric fish: addition of receptor organs with age.

The organization of the peripheral electrosensory system of the cheek was studied in an age-graded series of Sternopygus dariensis in Nissl-stained sections and silver-stained whole mounts of skin. As in other gymnotoids, both ampullary and tuberous electroreceptors are present. Small fish have only one ampullary organ or tuberous organ per axon, and the number of receptor organs per axon increases with age in both ampullary and tuberous systems. Large fish may have up to ten tuberous organs per axon, although the distribution of tuberous organs per axon is bimodal with one peak occurring at a single receptor organ per axon and the other peak shifting upward in relation to the age of the fish. The ampullary system adds receptor organs at a faster rate and a large fish may have 20 ampullary organs per axon. With increasing size, the number of sensory receptor cells in each organ remains constant for both types of electroreceptors. Evidence is presented for addition of new electroreceptor units by de novo production in small fish and increases in the number of organs in existing electroreceptor units by division of previously formed organs in medium-sized and large fish. As the surface area of the skin increases with growth, the density of electroreceptor units decreases and, although new receptor organs are still being added to existing receptor units, no generation of new receptor units occurs in medium-sized to large fish.