Impacts of macroalgal competition and parrotfish predation on the growth of a common bioeroding sponge

Clionaids, an abundant group of bioeroding sponges, are important competitors of corals, but their interactions with other major taxa are poorly understood. This study examined the relative effect of predation and macroalgal competition on the growth of Cliona tenuis at Glover's Reef Atoll, Belize. A field experiment was designed to isolate the importance of each factor and its interactions, by means of exclusion cages to assess the effects of fish predators, and algal transplantation to assess the effects of macroalgal competition (Lobophora variegata), over 286 d. To obtain complementary data on the net outcome of predation and competition in situ, predation intensity and the competitive effects on C. tenuis of 5 different competitors were monitored in the field. Competition with macroalgae significantly reduced the size of the sponge by 38 +/- 11% (SE). Despite a high predation rate (1 bite 100 cm(-2) h(-1)), no effect of the interaction of predation with macroalgal competition was detected. The in situ observations found that L. variegata was the strongest competitor limiting the growth of C. tenuis. Overgrowth of corals by clionaids and macroalgae continues to be a matter of concern, but our results suggest that the antagonistic competition between sponges and algae might weaken their potential combined effect on corals.