Delineation of outbreak stages of the coral-killing sponge Terpios hoshinota

Outbreaks of the cyanobacteriosponge Terpios hoshinota are one of the many issues contributing to coral reef decline in the Indo-Pacific. This species occasionally undergoes local outbreaks, becoming an aggressive space competitor and overgrowing and killing zooxanthellate corals. Large outbreaks have been observed in Taiwan and the Ryukyu Islands in southern Japan. Many questions regarding the characteristics and chronology of T. hoshinota outbreaks remain unsolved. Therefore, the aim of this research was to identify outbreak stages at an outbreak site (Ogimi) on the west coast of northern Okinawa Island, Japan. We developed a novel approach to understand the chronology of outbreaks. Distinct outbreak stages were defined based on observations, in which Terpios: 1) starts overgrowing coral colonies, 2) covers a whole coral colony, 3) in the case of branching corals, causes the coral colony to collapse, and 4) the sponge is eventually overgrown by other organisms including turf algae and other sponges. Analyses were done based on photographic transects and the relative proportions of each stage were calculated. Results showed Ogimi is likely in final outbreak stages as overgrown Terpios had the highest share of all stages (57.9 % and 44.9 % at two outbreak sites), while smaller patches of Terpios, marking the starting point of the successional process, made up less than 1.5 % of all observations. We suggest collecting such stage information during Terpios monitoring as it can allow identification of outbreaks at early stages, and potentially identify global patterns.