The Morphology of the Urogenital Region of the Bluehead Wrasse, Thalassoma bifasciatum: Female Egg Release via Epidermal Rupture

Females of the diandric, protogynous species Thalassoma bifasciatum release a single all-or-nothing explosive burst of eggs in a once-a-day spawning event. The mechanism controlling egg release is unknown but must provide proper spatiotemporal distribution of eggs, i.e., for successful fertilization, one that conforms sufficiently to the corresponding externally released sperm cloud geometry. This is particularly important in pair spawns, where one female releases eggs approximately simultaneously with the sperm release of a single male in a rapid, highly synchronized spawning rush. In order to determine how egg release is controlled, we examined the surface and internal morphology of the urogenital region of mature females using in vivo light microscopy, histology, and scanning electron microscopy. In anesthetized females, manual pressure to the abdomen distended the epidermis between the anus and urinary papilla until it ruptured and eggs burst outward en masse . Females captured before or after natural spawning, and with or without manual pressure applied under anesthetic, consistently exhibited a path for egg release from the oviduct(s) that was a caudoventrally descending irregular sinus within a loose connective tissue mass, which opened with the exterior where the epidermis ruptured; there was no distal duct, surface gonopore, or sphincter. This pathway was immediately rostral to, and separate from, the urinary papilla, its urinary duct, and urinary pore, which had no communication with the oviduct(s). We suggest that the egg load expands the sinus and distends the epidermis until it ruptures due to increased internal pressure, rapidly releasing hundreds of eggs. These results are consistent with a burst-release mechanism that occurs during the pelagic spawning event, perhaps optimizing spatiotemporal mixing of gametes.