Phase III – Effects of global warming on coral disease outbreaks in the Western Indian Ocean

Coral reefs are among the most productive, diverse, and complex ecosystems in the world (Odum and Odum 1955) and support almost one-third of the world’s marine fish species (Newton et al. 2007). They provide many goods and services, such as recreational activities and coastal protection (Moberg and Folke 1999). The goal of this project is to provide data on the prevalence and variability at local, regional, and temporal scales, factors involved in the occurrence and spread, and the effect on host populations of coral diseases with an emphasis on cause and effect on Reunion, South African, and Mayotte reef ecosystems. Additionally, this study will focus on the identification and characterization of the agents of the pathogens using field observation combined with techniques from microbiology. The following objectives were studied: 1) Investigating the impact of diseases on coral communities by quantifying their prevalence; 2) Determining the spatial distribution and temporal variations of coral disease outbreaks; and Identifying the potential pressures that induce and/or facilitate the introduction and spread of disease. 3) Describing and comparing bacterial communities associated with both healthy and diseased coral colonies; and 4) Identifying the pathogenic agents associated/responsible for the selected disease signs/syndromes. Results from this will contribute to identifying management strategies that can help decision-making in coral reef management. This study was undertaken on three WIO reef ecosystems in South Africa, Reunion, and Mayotte. Major beneficiaries include the scientific community and decision-makers.

To achieve the above objectives the following activities were undertaken: 1) Identification and description of coral diseases, 2) studies on coral disease prevalence, host susceptibility and spatiotemporal variability, 3) Microscopic characterisation of coral diseases including description of microscopic morphology and spatial structure of the microorganisms of main coral disease lesions in the Wio region.

The study results are summerised below: 1) Six coral disease were found present manifesting discoloration, tissue loss, growth anomalies or color changes generally resembling diseases already identified in the indo-Pacific regions; 2) Prevalence of all disease syndromes varied significantly among the three regions where the lowest total mean prevalence was recorded in Mayotte and the highest in Reunion. In South Africa, the proportion of infected coral colonies was low but quite variable between the two surveys. Combining all disease signs and syndromes, there was no significant evidence for seasonality between winter and summer in both Reunion and Mayotte. In South Africa, the disease prevalence was significantly higher in the first summer compared to the winter. Finally, Coral disease prevalence was significantly different between reef zones in both Reunion and South Africa with more diseases in shallow than deep reefs. 3) In Reunion, Acropora, Goniopora, Hydnophora, and Porites seemed to be the most susceptible to disease. White syndromes were the most common disease affecting branching colonies of Acropora spp., specifically in the shallowest zone of the reef. Coral colonies of Goniopora sp., hydnophora sp, and Porites spp. Seemed to exhibit the highest susceptibilities to BBD. Massive colonies of Porites spp. were the most vulnerable genus to diseases, exhibiting multiple infections. In South Africa, the most susceptible coral genera seemed to be Astreopora, Hydnophora, Pocillopora and Porites. Coral colonies of Pocillopora spp. Seemed to exhibit a high susceptibility to WS, whereas massive Porites lutea and lobata seemed to be more vulnerable to PWPS and PLS. In Mayotte, Acropora and Porites seemed to be the most susceptible to disease. Coral colonies of Astreopora sp. seemed to be highly susceptible to GA and WS. Acropora sp. seemed to be particularly vulnerable to WS, whereas Porites spp. Exhibited a high susceptibility to diseases. 4) BB and PWPS seemed to vary between seasons and the two geomorphological reef zones in Reunion. WS on Acropora, Astreopora, and Pocillopora taxa were significantly seasonal in South African reefs. A Significant higher prevalence of WS on both Acropora and Pocillopora taxa was detected in shallow reef zones. In Mayotte, the prevalence of GA was observed on Astreopora, as well as both WS and Nec on Astreopora. Diploastrea and Fungia were higher during the cooler and drier season. Porites black band disease was characterized by a clear dense population of filamentous cyanobacteria genetically identified as Oscillatoria sp. Serial sections of infected tissues showed that this cyanobacterium has a direct mechanical effect by perforating coral tissues. Black band disease observed mainly on both Goniopora sp. and Hydnophora sp. was also characterized by a clear visible mat of cyanobacteria that appeared to generate tissue lysis and/or necrosis. 5) Microscopic analysis revealed another black syndrome characterized by a dense population of sessile folliculinid ciliates similar to those implicated in the skeleton eroding band associated with black cyanobacteria. The protist organisms seemed to be embedded in the host skeleton, forming a compact black front that may be confused with black band disease (BBD) in the field. However, no evidence of direct effects on tissues was observed in histological sections. 

The study has contributed towards management by providing a state of knowledge on coral and their prevalence in different ups. Knowledge on vulnerability, susceptibility and resistance by different taxa to various diseases is crucial for coral reef management.