The northern reef of Yongxing Island, the largest reef island of the Xisha Islands in the South China Sea, was in good condition with significant cover of scleractinian corals until 2002. Surveys in 2008 and 2010, however, found that coral coverage had declined rapidly and severely, implying that catastrophic coral mortality occurred during the past 8 years. A blackish mat was observed covering live and dead corals in both 2008 and 2010 that was identified as an encrusting sponge, Terpios hoshinota, by special surface morphology and spicule structure. In addition, spicule residues were found on the surface of long-dead corals, indicating a previous invasion of T. hosinota. T. hoshinota is referred to as the ''black disease'' because it rapidly overgrows and kills corals. Our evidence indicates that outbreaks of black disease are at least partially responsible for the massive coral mortality at the northern reef of Yongxing Island over the past 8 years, although human activities and heat-related coral bleaching cannot be discounted as minor causes for this coral decline.

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