Crown-of-thorns starfish predation and physical injuries promote brown band disease on corals

Brown band (BrB) disease manifests on corals as a ciliate-dominated lesion that typically progresses rapidly causing extensive mortality, but it is unclear whether the dominant ciliate Porpostoma guamense is a primary or an opportunistic pathogen, the latter taking advantage of compromised coral tissue or depressed host resistance. In this study, manipulative aquarium-based experiments were used to investigate the role of P. guamense as a pathogen when inoculated onto fragments of the coral Acropora hyacinthus that were either healthy, preyed on by Acanthaster planci (crown-of-thorns starfish; COTS), or experimentally injured. Following ciliate inoculation, BrB lesions developed on all of COTS-predated fragments (n = 9 fragments) and progressed up to 4.6 ± 0.3 cm d^?1, resulting in ~70 % of coral tissue loss after 4 d. Similarly, BrB lesions developed rapidly on experimentally injured corals and ~38 % of coral tissue area was lost 60 h after inoculation. In contrast, no BrB lesions were observed on healthy corals following experimental inoculations. A choice experiment demonstrated that ciliates are strongly attracted to physically injured corals, with over 55 % of inoculated ciliates migrating to injured corals and forming distinct lesions, whereas ciliates did not migrate to healthy corals. Our results indicate that ciliates characteristic of BrB disease are opportunistic pathogens that rapidly migrate to and colonise compromised coral tissue, leading to rapid coral mortality, particularly following predation or injury. Predicted increases in tropical storms, cyclones, and COTS outbreaks are likely to increase the incidence of coral injury in the near future, promoting BrB disease and further contributing to declines in coral cover.