Phase III – The Future of Coral Reefs in the WIO region: Influence of climate change on the sexual reproduction and recruitment of reef-building corals

In the context of climate change, coral reproduction and recruitment are key processes that can ensure the renewal and recovery of reefs. Yet the effects of climate change and global warming on these processes are poorly known and may vary with latitude. This project aimed to compare the sexual reproduction and recruitment of two reef-building corals along a latitudinal gradient to ascertain how fecundity, larval survival and recruitment rate varied for similar species depending on latitude and environmental conditions. The trends in these parameters were correlated with seawater temperature to ascertain how climate change may influence the adaptive capacity of coral communities in the coming decades. Study objectives were to (i) compare the reproductive features of coral (gametogenesis, fecundity, seasonality, recruitment rate) between a tropical and a sub-tropical reef, (ii) compare the recruitment rate and diversity between a tropical and a sub-tropical reef (ii) ascertain the influence of water temperature on coral gametogenesis and larval development. The study was conducted in two countries of the WIO region, namely, Reunion and South Africa. The beneficiaries of the project include management and conservation organizations, policy makers, the scientific community, and local communities.

To achieve the objectives, a series of activities were undertaken. Representative species Acropora austera (Acroporidae) and Platygyra daedalea (Faviidae) that occur in both South Africa and Reunion were selected for this study. Gamete development and measurements of fecundity were collected in the selected coral species.  Spawning monitoring was conducted by both in situ observations and ex situ aquarium monitoring. Coral recruitment on reefs was investigated using replicate artificial substrata (recruitment tiles). The influence of water temperature was tested on coral reproduction (timing of spawning and gametogenesis) and the larval and juvenile development in the aquarium.   

Gamete development was annual in A. austera off Reunion and South Africa and it was initiated at the beginning of summer in September and ended in March with the spawning of gamete in both studied sites. Like in A. austera, one cycle of gametogenesis per year was observed in P. daedalea off South Africa and Reunion; however, it started in October in South Africa while it was slightly earlier (September) in Reunion. Coral recruitment rate was significantly higher in South Africa than in Reunion over the two-year study. Recruitment was highly seasonal in the two countries, with an overall abundance of recruits approximately four times higher in summer than in winter. South African coral recruits were characterised by a higher number of recruits from the Family Acroporidae, while Reunion was characterised by a high number of Poritidae. The mean abundance of coral recruits was not significantly higher in the No-Take Areas of Reunion compared to the exploited areas of the reefs. Diversity in recruitment was not consistent between the no take areas with a high number of Pocilloporid recruits in Saline while St Leu was dominated by Pocilloporids. The increase in gamete size in A. austera was significantly correlated with the increase in seawater temperature. Development of the coral embryo was faster in the warmest treatment, and the duration of larval development was 3, 4 and 6 days in the 28°C, 26°C and 24°C respectively.

The coral reproductive features showed little variation between Reunion and South Africa in the two studied species. Despite being a slow-growing coral, Platygyra daedalea invested a high amount of energy into reproduction and produced up to 60 oocytes per polyp. Its fecundity, in addition to its stress-tolerant capacity, may enhance its chance of survival in the face of climate change. The recruitment rate was high in South Africa and close to the value observed on the Great Barrier Reef. In Reunion, the average recruitment rate ranged between values observed on other tropical fringing reefs. The South African reefs seemed, therefore, to be suitable for coral settlement despite being at the limit of the coral environmental threshold. Apart from Pocilloporids, Acroporids were the most abundant in South Africa while it was Poritids in Reunion. The recruit composition on tile seemed to reflect the taxonomic composition of adult assemblage. No clear pattern in coral recruitment rate or diversity was visible between the No-Take Areas and exploited reefs of Reunion. The recruitment rate was the lowest of all study sites in the northern NTA site (SAT), where degradation and signs of eutrophication have been reported since the 1980s. Other sites outside the NTAs showed high recruitment rates; therefore, there may be a need to re-evaluate the position of NTA within the MPA. Gametogenesis in the two studied species was strongly correlated with a change in seawater temperature in Reunion and South Africa. Temperature variation may, therefore, affect the synchronicity and timing of reproduction in coral and have major consequences on fertilization and reproductive success. In addition, increased seawater temperature strongly affected the coral development in the aquarium by reducing the larval pre-competency period. A shortened benthic phase may, therefore, favour localised recruitment and lead to greater reef isolation. This may have serious consequences for reef recovery following damage.

This project will continue through the research program developed by the Observatory of the Universe Sciences. Its contribution towards management involved the long-term survey of coral recruitment in Reunion and the training of the eco-guards for the tile collection and analysis.