Morphology of gonoducts and male genital papilla, in the bluehead wrasse: implications and correlates on the control of gamete release

In the coral reef fish Thalassoma bifasciatum, males vary the number of sperm they release in successive spawnings with individual females in accordance with female body and clutch size. The morphology and histological structure of the male genital papilla, sperm duct, oviduct and surrounding musculature were examined in an effort to elucidate the mechanism permitting control of the number of gametes released during mating. In males, urinary and genital ducts pass separately through a common urogenital papilla and are associated with a striated sphincter muscle and a pair of thin, smooth ligament muscles arising from the first proximal anal fin radial and passing laterally around the sperm duct and oviduct. Within the papilla, the sperm duct resembles a narrow funnel whose inner walls contain longitudinal folds or septa protruding into the lumen of the duct. Dorsal to the papilla, the sperm duct enlarges and is divided into numerous, open chambers by irregular, longitudinal trabeculae. The wall of the duct and the trabeculae contain flat epithelium, smooth muscle and loose connective tissue. In females, the oviduct contains no trabeculae and is not divided into chambers. The ligament muscles are more thoroughly embedded in the sphincter muscle of the rectum than in males. Some ways in which these structures might control gamete release are suggested.