Sexual reproduction in octocorals

For octocorals, sexual reproductive processes are fundamental to maintaining populations and influencing macroevolutionary processes. While ecological data on octocorals have lagged behind their scleractinian counterparts, the proliferation of reproductive studies in recent years now enables comparisons between these important anthozoan taxa. Here we review the systematic and biogeographic patterns of reproductive biology within Octocorallia from 182 species across 25 families and 79 genera. As in scleractinians, sexuality in octocorals appears to be highly conserved. However, gonochorism (89%) in octocorals predominates, and hermaphroditism is relatively rare, in stark contrast to scleractinians. Mode of reproduction is relatively plastic and evenly split between broadcast spawning (49%) and the 2 forms of brooding (internal 40% and external 11%). External surface brooding which appears to be absent in scleractinians may represent an intermediate strategy to broadcast spawning and internal brooding and may be enabled by chemical defenses. Octocorals tend to have large oocytes, but size bears no statistically significant relationship to sexuality, mode of reproduction, or polyp fecundity. Oocyte size is significantly associated with subclade suggesting evolutionary conservatism, and zooxanthellate species have significantly larger oocytes than azooxanthellate species. Based on biogeographic patterns, reef scleractinians appear to disperse longer distances over ecological timescales compared to reef octocorals. However, differences in reproductive characteristics between the 2 taxa do not offer an obvious explanation for these different biogeographic patterns.