Sexual dimorphisms in the vocal control system of a teleost fish: ultrastructure of neuromuscular junctions.

In the sound-generating fish, Porichthys notatus, large, nest-guarding 'Type I' males use their swimbladder 'drumming' muscles to produce acoustic communication signals. Females and another group of smaller sexually mature males ('Type II') have not been observed to produce sounds. Electron microscopy was used to compare the morphology of the neuromuscular junctions in vocalizing Type I males to those of Type II males and females. Significant differences were seen in synaptic vesicle density, terminal size, degree of terminal invagination below the muscle fiber surface, number of Schwann cell processes along the non-synaptic boundary of boutons, and the number of boutons per innervation site.