Ontogeny Of The Sexually Dimorphic Sonic Muscle In Three Sciaenid Species

Male and female croaker (Micropogonias undulatus) have sonic muscle attached to the swimbladder, but in spot (Leiostomus xanthurus) and weakfish (Cynoscion regalis) these muscles are present only in males. In M. undulatus, muscle development began at 45 mm standard length (SL) (approx. 4-5 mo of age) in both sexes when the gonads were starting to develop, but before the onset of gametogenesis. Sonic muscles grew larger in males than in females. Sonic muscle appeared in L. xanthurus at 105 mm SL (approx. 6-7 mo of age) before testicular maturation. Sonic muscle was evident in C. regalis at 119 mm SL (approx. 12 mo of age), concurrent with testicular maturation. Sonic muscle development was synchronous with gonadal growth in all three species. Swimbladders grew larger in male M. undulatus than in females, but there were no sexual differences in the other two species. Swimbladders were small in L. xanthurus, intermediate in M. undulatus, and largest in C. regalis. Sonic muscles were also small in L. xanthurus, but large in the other species. We suggest that swimbladder size is determined by factors related to bouyancy and sound production.