Embryonic, larval and adult development of the sonic neuromuscular system in the oyster toadfish

Unlike the case in birds and mammals where addition of neurons and muscle fibers is almost exclusively an embryonic process, fish form new sensory receptors, central neurons and muscle fibers postnatally. We have investigated the development of the sonic neuromuscular system, consisting of a spinal sonic motor nucleus (SMN) and a pair of sonic muscles intrinsic to the swimbladder, in the oyster toadfish Opsanus tau. The SMN differentiates late in embryonic development, and neurons increase in number and size through the larval period and for 7-8 years of adult life. Neuron number increases from a median of 35 to 322 cells, from the time of differentiation through the larval period, and to almost 3,000 cells in large fish. Immature neurons appear to originate from precursor cells in the base of the ventricular zone of the central canal and are added throughout the SMN. No evidence of cell death was seen. The number and diameter of sonic muscle fibers increase for life (about 16- and 3-fold, respectively); fiber number ranges from almost 31,000 in a 1-year-old juvenile to almost 500,000 in large animals. Fiber splitting provides a possible mode of fiber addition. Multiyear addition of neurons and muscle fibers indicates that embryonic processes continue beyond the termination of larval life.