Acropora muricata mortality associated with extensive growth of Caulerpa racemosa in Magoodhoo island, Republic of Maldives

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 mats formed a continuous dense meadow (Fig 1a) that occupied an area of 95 x 120 m (~11,000 m2) previously dominated by the branching coral Acropora muricata. Partial and total mortality (Fig 1 b-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 are known to promote green algal blooms (Lapointe and Bedford 2009). Considering the current state of regression of many reefs in the Maldives (Lasagna 2010) we report an unusual phenomenon which could possibly become more common.