Terpios hoshinota (Porifera): A black veil of death or an obnoxious neighbor!
Terpios hoshinota is a thin black encrusting sponge, and fierce space competitor. It kills scleractinian corals by overgrowing them. It can overtake several colonies and break out widely on the reefs. The causes of its outbreaks remain unknown. We investigated an outbreak of T. hoshinota at the island of Mauritius to determine the severity of its impacts on the coral community. Is it just an ephemeral event or a serious long-term threat? We set out to answer the following questions: What is the extent of its spread? How fast does it grow? Does it target specific coral species? How is it shaping the community it colonizes? We combined high-resolution measurement of T. hoshinotacolonies at small spatial scale with a large-scale community survey at our study site, to determine the extent of spread of the sponge and the species impacted. Quadrats were set up and monitored over 5-6 months to measure growth rate of the sponge, and to determine how fast the colonies were spreading. The photosynthetic capability of the sponge was also assessed.
Out of the 1256 m2 surveyed at the putative center of spread, there were 500 m2 of available hard substrate (hard corals and crustose algae) of which 11% had been overgrown by T. hoshinota. A colony of T. hoshinota could grow linearly by an average of 11.5 ± 3.6 (S.D.) mm month-1. The community survey showed that 70% of all corals were Acropora austera, and that 94% of coral surface killed was this single species. These results demonstrate that T. hoshinota has a marked preference to overgrow A. austera. Areas impacted with T. hoshinota had significantly more sponge cover after 5-6 months, while no new colonies of T. hoshinota appeared in adjacent (3-5 m away) sponge-free areas. The sponge seems to be as efficient as A. austera at performing photosynthesis, but it has a significantly higher electron transfer rate maximum. Although a time scale of 5-6 months may be too short to observe the appearance of new colonies, the extent of spread on the reef suggests that the sponge is fairly well established, and likely to colonize more area. A. austera seems to be its preferred target, but the sponge has been observed overgrowing at least six other coral species. Thus, T. hoshinota appears to be a black veil of death, and not just an obnoxious neighbor.