Biological consequences of an outbreak of growth anomalies on Isopora palifera at the Cocos (Keeling) Islands

Growth anomalies (GAs), a tumour-like disease affecting scleractinian corals, have been reported across the major reef systems of the Indo-Pacific and wider Atlantic regions, predominantly affecting Acropora and Porites species. In 2018, GAs were observed for the first time on Isopora palifera, an important observation from the isolated Cocos (Keeling) Islands in the East Indian Ocean, as the species is a key reef building coral at the atoll. In this study, the local distribution and abundance of GAs was quantified to determine if this occurrence could be classified as an outbreak, and the effects of this disease on I. palifera on reproductive potential and growth was described using histological and geochemical analysis. Growth anomalies were documented at 75% of sites and affected approximately one third of the I. palifera colonies examined. This disease compromises the biological and reproductive functioning of the host, as evidenced by a significant reduction in the density of oocytes, mesenteries, polyps, and zooxanthellae in infected tissues in comparison to healthy tissue. Furthermore, geochemical analysis indicates changes to key trace elements may be the result of bioerosion processes by infecting bacteria and the reprecipitation of calcite. The results of this study indicate the division of energy to the rapid skeletal development that characterises the disease, may have occurred at the detriment of the future reproductive potential of the population.