The effectiveness of Trapezia cymodoce in defending its host coral Pocillopora acuta against corallivorous Drupella

Population outbreaks of corallivorous Drupella gastropods have caused mass coral mortality, but there is insufficient information on their feeding behaviour to develop useful reef management strategies. This study examined the feeding rates of two Drupella species, D. rugosa (Born 1778) and D. margariticola (Broderip 1833), and investigated whether the presence of the coral guard crab, Trapezia cymodoce (Herbst 1801), could help to reduce corallivory on Pocillopora acuta (Lamarck 1816). Our mesocosm study showed that the feeding rate of D. rugosa (1.81 +/- 0.95 cm(2) coral tissue/day) was significantly higher than that of D. margariticola (0.51 +/- 0.75 cm(2) coral tissue/day). The presence of T. cymodoce reduced the feeding rate of D. rugosa by 22.9%; this was lower than that by other Trapezia congenerics. Trapezia cymodoce also did not display much aggressive behaviour towards D. rugosa. This study has highlighted that D. rugosa can cause considerable damage to corals and defence by T. cymodoce alone is unlikely to be effective against Drupella corallivory. Early detection of Drupella outbreaks and the subsequent development of mitigation measures remain of paramount importance in reducing the impact of corallivory on coral reefs.