Morphological organization and ultrastructural evaluation of the oocyte–sperm bundle of the Southwestern Atlantic coral Mussismilia harttii

Most coral species of known reproductive traits are broadcast spawners, with about 65% being hermaphrodites that envelop their gametes together in oocyte–sperm bundles. While these bundles are crucial for the dispersion and successful cross-fertilization of gametes, research evaluating their structure in detail is rare in the literature. Here, we investigated the composition of the bundle and the arrangement of the gametes within, before and after dissociation, using histological and ultrastructural analyses. The endemic coral Mussismilia harttii was used, since it is one of the main reef builders in Brazil and is listed as an endangered species. Our results showed that there is no pattern to the distribution of gametes in the bundle, though there are regions of higher concentrations reaching 91 spermatozoa mm −2 . The dissociation of the bundle promotes important morphological changes to the oocytes, making them larger and with less dense and thicker microvilli on the surface. Spermatozoa are located within hexagonal-shaped structures, similar to the alveoli in a honeycomb. Given the morphological structural complexity observed, we believe that the release of spermatozoa occurs through the complete rupture of the walls of spermatic cysts and that such structure is a component of the spermatocytes. Once released into the gastrovascular cavity, the spermatocytes are enveloped in mucus, maintaining spermatozoa in the same organization. Our findings shed light on a poorly explored area that can assist in a better understanding of hermaphrodite broadcasting coral fertilization and provide support for future studies on reproductive biotechnologies for coral conservation.