The pink-blue spot syndrome in Acropora eurystoma (Eilat, Red Sea): A possible marker of stress?

The appearance of pink-blue spots (termed here as pink-blue spot syndrome - PBSS) in the branching coral Acropora eurystoma from the Gulf of Eilat, Red Sea, is described. We monitored 18 transects (10×1 m2 each) in front of the H. Steinitz Marine Laboratory (Eilat), at 3, 6 and 9 m depth, during March and August in 2001 and 2002. Transect measurements revealed high frequencies of colonies with PBSS (up to 100% of colonies) between 3 and 9 m depth. Ten PBSS-affected colonies of A. eurystoma were labelled and monitored for the development of spots. From March to August 2001, the number of spots per colony increased and remained constantly high at both sampling dates in 2002. Spot size ranged between 7 and 149 mm2. Spots were primarily recorded in areas where coral tissues contacted foreign biological matter, either around regenerative wounds or when surrounded by encrusting organisms, in fast-growing areas and in allogeneic interactions. A preliminary biochemical examination suggested that the pink-blue pigment in A. eurystoma is part of a family of compounds (pocilloporin) responsible for the pink-blue colours in pocilloporid and acroporid corals. Pink-blue colour could be experimentally induced in A. eurystoma by tissue-to-tissue contacts between distressed and non-distressed allogeneic branches. PBSS was also induced in healthy coral tissue by contact with inert objects, e.g., by bandaging a branch with plastic strips. Any specific pink-blue colour spots faded within 1-3 months from onset. These results suggest that PBSS in A. eurystoma may not be considered a regular coral disease, but rather a locally induced syndrome caused by restricted environmental and/or biological stress conditions.