THE EFFECTS OF SEASONAL HYPERTROPHY AND ATROPHY ON FIBER MORPHOLOGY, METABOLIC SUBSTRATE CONCENTRATION AND SOUND CHARACTERISTICS OF THE WEAKFISH SONIC MUSCLE

Male weakfish Cynoscion regalis possess highly specialized, bilateral, striated sonic muscles used in sound production associated with court ship, Androgen-driven hypertrophy of the muscles during the late sprin g spawning period results in a tripling of sonic muscle mass followed by post-spawning atrophy, This study examined the morphological and bi ochemical changes underlying seasonal changes in sonic muscle mass and the functional effects of these on contraction as measured by sound p roduction, Sonic muscle fiber cross-sectional area (CSA) increased sig nificantly during the period of hypertrophy and then decreased by near ly 60%, Both the CSA of the contractile cylinder and that of the perip heral sarcoplasm decreased significantly by late summer, with the peri pheral ring of sarcoplasm virtually disappearing, Muscle protein conte nt followed a similar trend, suggesting a major loss of structural ele ments during atrophy, Muscle glycogen and lipid content decreased prec ipitously in early June during the period of maximal sound production, Sound pressure level increased and sound pulse duration decreased wit h increasing sonic muscle mass, indicating that sonic muscle fibers co ntract with greater force and shorter duration during the spawning sea son, Neither the pulse repetition rate nor the number of pulses varied seasonally or with muscle mass, suggesting that the effects of steroi ds on the acoustic variables are more pronounced peripherally than in the central nervous system, Seasonal sonic muscle hypertrophy, therefo re, functions as a secondary sexual characteristic that maximizes voca lization amplitude during the spawning period.