Halimeda opuntia and Diplosoma virens are common marine organisms in the Indo-Pacific. Halimeda grows to dense populations, and the ability of didemnids to overgrow coral has been well documented. In the light of their abundance in the Hikkaduwa Nature Park in Sri Lanka, this study investigated the effect of these two associates on the growth and survival of the commonly occurring staghorn coral, Acropora formosa. The results showed that both affected the growth rates of the host coral colonies significantly (P = 0.11 at 0.15 error level). The two associates also displayed a strong negative correlation between the spreading rate and the host growth rate (−0.71 for H. opuntia and −0.55 for D. virens). The results show that D. virens could have serious implications on coral survival with a population bloom.
