Development of the electrosensory nervous system in Eigenmannia (Gymnotiformes): I. The peripheral nervous system.

The nerves of the anterior lateral line system in embryonic and larval stages of the weakly electric gymnotiform fish Eigenmannia were visualized by injection of the fluorescent marker DiI into the primordium of the anterior (ALLN) and posterior (PLLN) lateral line nerves. Examination of developmental series reveals that the nerve fibers that innervate the electrosensory and mechanosensory components of the anterior lateral line system are present before the first mechanoreceptors and electroreceptors have differentiated. This suggests that nerve fibers might induce the formation of lateral line receptors. Whereas the innervation of the mechanoreceptive system is already established at an early stage, the afferent innervation of electroreceptors continues to arborize in the periphery, presumably by following pioneer axon pathways. The earliest recognizable stage of the anterior lateral line nerve ganglion (ALLNG) is evident 2 days after spawning. The ganglion shows two germinal cell masses that develop into the supraorbital-infraorbital and the hyomandibular placodes. The supraorbital-infraorbital placode forms the dorsal part of the ALLNG; the hyomandibular placode forms the ventral part of the ALLNG. Counts of ALLNG cells in embryonic, larval, and adult stages of Eigenmannia show that, at each stage examined, the number of ganglion cells is always significantly larger than the number of mechanoreceptors and electroreceptor units in the periphery. During development, the distribution of ALLNG cell diameters shifts from a unimodal distribution in juveniles to a bimodal distribution in adults, peaking at 8 microns and 18 microns. These results suggest that tuberous electroreceptive organs, which are innervated by the large ALLNG cells, may not be functional prior to day 18. Our results further suggest that the number of ALLNG cells correlates with the rate of induction of lateral line receptors in the periphery.