CONTINUOUS ADULT DEVELOPMENT OF MULTIPLE INNERVATION IN TOADFISH SONIC MUSCLE

The sonic muscle of the oyster toadfish Opsanus tau produces unfused c ontractions at over 200 Hz for mating call production, requiring extre me muscle fiber synchronization. This multiply innervated muscle is se xually dimorphic and grows for life by fiber proliferation and hypertr ophy. Previous descriptions of its multiple innervation did not consid er fish size or sex. We examined neuromuscular junction (NMJ) developm ent in adult fish of both sexes between 123 and 343 mm in total length (24.7-790 g in mass). The NMJ was a tubelike trough that varied in le ngth from 8 to 178 mu m. Troughs were usually straight, although some consisted of consecutive ovals and some were branched. Median length o f NMJs increased linearly with fish length (r(2) = .40; p = .002) from 58 to 75 mu m. Modal lengths were mostly between 50 and 60 mu m and d id not increase ontogenetically, indicating that the median increase w as 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 pm and did not vary with fish size (r (2) = .06; p = .285). Considering muscle fiber elongation, the data in dicate an increase from 60 to 140 NMJs per fiber during fish growth. T here were no sexual differences in NMJ length or spacing. In view of t he slow conduction velocity of sonic muscle fibers, the addition of ne w NMJs and the relatively constant distance between them supports rapi d and synchronized contraction necessary for sound production in both sexes. (C) 1998 John Wiley & Sons, Inc.