A short-term, localized outbreak of diatoms attached to live corals was observed along the coast of Sesoko Island, Okinawa, Japan in February, 2011. Diatoms are recognized as brown patches in the initial stage, becoming fluffy encrustations and resulting in complete or partial coral death. Attached diatoms, including Licmophora, Climacosphenia, Ardissonea and others, attached and overgrew exclusively Montipora corals, which are dominant corals in some parts of Sesoko reef. Heavily-covered colonies or branches died. The rate of affected corals reached 80% in the worst-affected area. Microscopic observation showed that most diatoms settled directly with polysaccharide stalks or pads onto the partly-bared skeleton of coral branches, although some settled on coral soft tissues. Although no similar phenomenon was reported from other areas of Japan, cold-water events might have important roles in coral weakening, as a consequence, enabling diatom attachment on corals, thus leading to coral death in this area.

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