Award: OCE-1929979

Reef-building acroporid corals form the foundation of shallow tropical coral communities throughout the Caribbean. Yet, the once dominant staghorn coral (Acropora cervicornis) and the elkhorn coral (A. palmata) have decreased by more than 90% since the 1980s, primarily from disease. Their continuing decline jeopardizes the ability of coral reefs to provide numerous societal and ecological benefits, including economic revenue from seafood harvesting and tourism and shoreline protection from extreme wave events caused by storms and hurricanes. Despite their protection under the U.S. Endangered Species Act since 2006, threats to the survival of reef-building acroporid corals remain pervasive and include disease and warming ocean temperatures that may lead to further large-scale mortality. However, hybridization among these closely related species is increasing and may provide an avenue for adaptation to a changing environment. While hybrids were relatively rare in the past, they are now thriving in shallow habitats with extreme temperatures and solar irradiance and are expanding into the parental species habitats. Survey data on the prevalence of coral bleaching and disease in Caribbean acroporids suggested that hybrids might be more resistant to these stressors than one or both parental species. Hybridization may therefore have the potential to rescue the threatened parental species from extinction through the transfer of adapted genes via hybrids mating with both parental species, but extensive gene flow may alter the evolutionary trajectory of the parental species and drive one or both to extinction. This collaborative project with the Baums lab (award number 1537959) aimed to conduct genetic and ecological data to understand the mechanisms underlying increasing hybrid abundance and the ecological and evolutionary role hybridization has in the Caribbean acroporid system. Here, experimental data confirmed what previous surveys had suggested- Caribbean acroporid hybrids performing better than one and, in some cases, both parental species, thereby demonstrating hybrid vigor. Projected future oceanic conditions include increased oceanic temperatures and decreased pH. Exposing acroporid eggs and sperm to future oceanic conditions greatly decreased fertilization across all taxa. Yet across all treatments, hybrids had higher larval survivorship and settlement than at least one of the parental species. Acroporid hybrid adults exposed to increased temperatures showed little response in gene expression unlike the parental species. The microalgal symbionts in hybrids also showed no difference in abundance or chlorophyl concentration after heat exposure, suggesting hybrids may have enhanced survival to future temperature increases. This was confirmed in field surveys, where bleaching occurred less frequently in hybrids than the parental species. In disease transmission studies at three time points across the summer, hybrids demonstrated stronger disease resistance compared to the parental species, but disease resistance waned in hybrids during the warmest months. Resistance to thermal stress and disease is likely why some hybrid populations in the shallows have remained stable while adjacent parental species have declined. Additionally, hybrids, found to have higher growth rates than the parental species, can quickly attach and grow after a disturbance. The ability of hybrids to persist in shallow environments may also be contributed to its tightly clumped morphology, which is more like A. cervicornis at the colony scale but shows intermediate characteristics to the parental species at a microscopic scale. Taken together Caribbean acroporid hybrids are resistant to the primary stressors that have led to the threatened status of the parental species. Yet many attempts to collect spawn to create a second generation in the laboratory or confirm the existence of a second generation in the wild using genetics have failed. However, recruitment of the parental species is also rare. The use of a robust, persistent coral hybrid to replace shallow habitat left void by the loss of the parental species may offer an important alternative for shallow reef restoration, even if sexual reproduction among hybrids is unattainable. This NSF grant supported, trained, or provided research experience for 15 master?s and 17 undergraduate students, mostly women and several students from underrepresented groups. Fogarty incorporated products from this research into the lectures of four graduate and two undergraduate courses. Outreach products include a Caribbean Coral Restoration Webinar, 11 events with the public or school children, 1 interview on UNC-TV, 1 interview with local NPR affiliate?s news show, the creation of a coral spawning research FaceBook page with 4,100 members, and lab social media pages. Additionally, Fogarty serves on the Coral Restoration Consortium Larval Propagation working group and the South Atlantic Fishery Management Council. Fogarty presented at 10 scientific conferences or universities and her students gave 30 presentations at scientific conferences. This grant has supported many products at various stages of publication: 4 papers have been published, 4 are in review, 5 are in advanced preparation, and several more datasets are currently being analyzed. The knowledge gained from this research will help facilitate more strategic management of acroporid populations under current and emerging threats to their survival. Last Modified: 01/28/2021 Submitted by: Nicole D Fogarty