Sexual Reproduction of Scleractinian Corals in Mesophotic Coral Ecosystems vs. Shallow Reefs

Corals utilize sex-derived diversity to adapt to environmental changes and to occupy new ecological niches. With major declines in coral reefs worldwide and calls for ecosystem-based management, understanding how environmental gradients affect coral reproductive performance over a species' range and within a demographically relevant timescale is critical. The study of coral reproduction is a mature field with the reproductive aspects of more than 450 species recorded. However, the vast majority of coral reproduction studies have been on shallow reefs, while knowledge of reproduction in mesophotic coral ecosystems (MCEs) is sparse. This knowledge gap hinders our ability to assess the resilience and functionality of MCEs and to understand ecosystem-scale connectivity. Environmental factors that influence coral reproduction, such as light, temperature, and disturbances, can vary dramatically with depth. Sexual reproduction has evolved, partly, to address environmental pressures. We, therefore, expect that environmental parameters can influence reproductive patterns and success. There is currently insufficient information to allow conclusions to be drawn regarding the effects of mesophotic depths on the phenology of coral reproduction, other than it might differ from shallow reefs. Nonetheless, it appears that reproductive performance decreases with depth, with most of the species studied so far in MCEs having exhibited either reduced fecundity or reduced oocyte size compared to shallower populations. Here, we summarize the current knowledge on mesophotic coral reproduction and propose several hypotheses regarding the changes in coral reproductive traits across depth and their implications for population connectivity and persistence. Additionally, we highlight crucial knowledge gaps and recommend future research.