Caulerpa racemosa, a common and opportunistic species widely distributed in tropical and warm-temperate regions, is known to form monospecific stands outside its native range (Verlaque et al. 2003). In October 2011, we observed an alteration in benthic community due to a widespread overgrowth of C. racemosa around the inhabited island of Magoodhoo (3°04′N; 72°57′E, Republic of Maldives). The algal mats formed a continuous dense meadow (Fig. 1a) that occupied an area of 95 × 120 m (~11,000 m2) previously dominated by the branching coral Acropora muricata. Partial mortality and total mortality (Fig. 1b, c) were recorded on 45 and 30% of A. muricata colonies, respectively. The total area of influence of C. racemosa was, however, much larger (~25,000 m2) including smaller coral patches near to the meadow, where mortality in contact with the algae was also observed on colonies of Isopora palifera, Lobophyllia corymbosa, Pavona varians, Pocillopora damicornis, and Porites solida. Although species of the genus Caulerpa are not usually abundant on oligotrophic coral reefs, nutrient enrichment from natural and/or anthropogenic sources is known to promote green algal blooms (Lapointe and Bedford 2009). Considering the current state of regression of many reefs in the Maldives (Lasagna et al. 2010), we report an unusual phenomenon that could possibly become more common.

Fig. 1
figure 1

a Extensive meadows of C. racemosa overgrowing colonies of the dominant coral A. muricata. bc Partial and total mortality of A. muricata following algal overgrowth