Variation in the mode of receptor cell addition in the electrosensory system of gymnotiform fish

Age-related changes in ampullary and tuberous receptor organ morphology were studied in six species of gymnotiform weakly electric fish. Cheek skin was silver-stained, whole-mounted, and viewed under Nomarski differential interference contrast optics. The ampullary receptor units of all species show an increasing number of receptor organs per afferent fiber with fish size, presumably the result of addition of newly formed receptor organs. Ampullary units composing over a dozen organs were observed in large specimens of a few species. Receptor cells were also added in the tuberous receptor system of all species, but in different ways. As previously reported for Sternopygus, small specimens of Eigenmannia had only a single tuberous receptor organ per afferent. Fish of increasing size retained a population of afferents that innervated only a single receptor organ and, in addition, had a population of afferents that innervated a cluster of receptor organs. The mean number of receptor organs per cluster increased in fish of increasing size. In addition, the mean number of sensory receptor cells per organ increased. New organs presumably derive from older ones, which divide under the stimulus of continued addition of new receptor cells. Apteronotus, Adontosternarchus, and Hypopomus all added more receptor cells to their tuberous organs. In these species, every afferent innervated only a single tuberous organ and there was no indication of division of receptor organs. Gymnorhamphichthys and Gymnotus were intermediate in that they added new receptor cells to each receptor organ, and, in larger fish, these were segregated into discrete patches within a single receptor organ. It is likely that the addition of new receptor cells aids in increasing sensitivity of both ampullary and tuberous receptors as fish grow.