Coral diseases are one of the most important causes of coral reef decline worldwide. Although they are well investigated in some areas of the Caribbean and the Great Barrier Reef, they have been poorly studied in the Indian Ocean, and particularly in the Maldives, where the presence of coral pathologies has been reported only recently. In order to fill this gap, we investigated the prevalence, local host range and distribution of five coral diseases, namely brown band disease, skeleton eroding band, black band disease (BBD), ulcerative white spot and white syndrome (WS), in the central part of the Maldivian Archipelago, focussing, in particular, on the dominant coral genus Acropora. We estimated an overall disease prevalence of approximately 1.51%. Malè North Atoll was the most affected area; however, coral diseases were present at all study sites. WS was the most widespread disease, whereas BBD affected the highest number of scleractinian genera. We found symptoms of disease on 18 genera belonging to eight families. Acropora was the most affected genus in terms of pathogen diversity. These results provide baseline disease levels for the Maldives and can therefore serve as gauge for monitoring future change.

In spite of growing evidence that climate change may dramatically affect networks of interacting species, whether—and to what extent—ecological interactions can mediate species' responses to disturbances is an open question. Here we show how a largely overseen association such as that between hydrozoans and scleractinian corals could be possibly associated with a reduction in coral susceptibility to ever-increasing predator and disease outbreaks. We examined 2455 scleractinian colonies (from both Maldivian and the Saudi Arabian coral reefs) searching for non-random patterns in the occurrence of hydrozoans on corals showing signs of different health conditions (i.e. bleaching, algal overgrowth, corallivory and different coral diseases). We show that, after accounting for geographical, ecological and co-evolutionary factors, signs of disease and corallivory are significantly lower in coral colonies hosting hydrozoans than in hydrozoan-free ones. This finding has important implications for our understanding of the ecology of coral reefs, and for their conservation in the current scenario of global change, because it suggests that symbiotic hydrozoans may play an active role in protecting their scleractinian hosts from stresses induced by warming water temperatures.