Coral Persistence In The Anthropocene: Impacts To Reproduction, Implications For Restoration

Abstract

There have been numerous advances in our understanding of coral reproduction over the last ~40 years. Despite these achievements, it’s estimated that there are detailed records for the reproductive cycles and behavior of fewer than half of all coral species. Discoveries continue to be made as climate change and local stressors degrade reef habitat while also causing lasting damage to coral reproduction. The stark decline in coral cover around the world has prompted a boom in reef restoration activity. My research compares the annual gametogenic cycle and spawning behavior of the well-studied reef-building coral Montipora capitata to its lesser-known congeners M. flabellata and M. dilatata (collectively, Montipora spp.). Specifically, it examines their reproductive physiology years after consecutive bleaching events in 2014 and 2015 and explores the growth performance of fragments of these corals in an in situ coral tree nursery compared to their previous growth from a long-term ex situ mesocosm experiment. The results established that both species are broadcasting hermaphrodites that release egg-sperm bundles, but M. capitata gametes reach peak maturity simultaneously, similar to other acroporids, and spawn within a tightly coordinated window with the lunar cycle. In contrast, Montipora spp. have a protracted and asynchronous spawning season that doesn’t adhere to any apparent lunar cue and sporadically release small pulses of bundles per colony over several summer months. Additionally, there were notable differences in reproductive metrics, but most prominent were disparities in sperm motility and mitochondrial membrane potential. Montipora spp. displayed high levels of motility both years (2018 and 2019) while M. capitata sperm motility had not fully recovered to observed pre-bleaching levels. Finally, among fragments moved from an ex situ to an in situ coral nursery, individual genotype was the best predictor of success for growth in the reef nursery for M. capitata but not for Montipora spp. Thermal stress preconditioning did not convey growth or survival benefits during in situ warming for any species, and M. capitata was better suited to the coral tree method and location than Montipora spp. Questions remain regarding the mechanisms that explain the differences in reproductive characteristics between these species including habitat differences, historic cover and population densities, the potential role of ultraviolet radiation damage to coral germ cells and potential UVR mitigation strategies, and other factors to consider in restoration programs.