Continuous adult development of multiple innervation in toadfish sonic muscle.

The sonic muscle of the oyster toadfish Opsanus tau produces unfused contractions at over 200 Hz for mating call production, requiring extreme muscle fiber synchronization. This multiply innervated muscle is sexually dimorphic and grows for life by fiber proliferation and hypertrophy. Previous descriptions of its multiple innervation did not consider fish size or sex. We examined neuromuscular junction (NMJ) development in adult fish of both sexes between 123 and 343 mm in total length (24.7790 g in mass). The NMJ was a tubelike trough that varied in length from 8 to 178 microm. Troughs were usually straight, although some consisted of consecutive ovals and some were branched. Median length of NMJs increased linearly with fish length (r2=.40; p=.002) from 58 to 75 microm. Modal lengths were mostly between 50 and 60 microm and did not increase ontogenetically, indicating that the median increase was caused by a greater number of large junctions in older fish. Median interval between NMJs (measured from the beginning of one junction to the next) ranged from 92 to 116 microm and did not vary with fish size (r2=.06; p=.285). Considering muscle fiber elongation, the data indicate an increase from 60 to 140 NMJs per fiber during fish growth. There were no sexual differences in NMJ length or spacing. In view of the slow conduction velocity of sonic muscle fibers, the addition of new NMJs and the relatively constant distance between them supports rapid and synchronized contraction necessary for sound production in both sexes.