Spongivory on Caribbean reefs releases corals from competition with sponges

Competition for space is an important process on tropical coral reefs. Few studies have examined the role sponges play in community structure despite the fact that many sponges are competitively superior to reef-building cora ls in space acquisition. Surveys conducted throughout the Florida Keys indi cated that Chondrilla nucula was involved in about 30% of all coral-sponge interactions; this sponge has also been observed in 40-50% of coral-sponge interactions on other Caribbean reefs. C. nucula is also the top prey item of the Hawksbill turtle, and among the preferred prey of several spongivoro us fish. I examined how predation influenced sponge competitive abilities ( particularly those of C. nucula), and whether this type of indirect effect had important consequences for community dynamics in the Florida Keys. Excl usion of sponge predators (primarily angelfish) resulted in increased spong e overgrowth, with a subsequent greater loss of coral cover, compared to un caged pairwise interactions. When caged, the corals Dichocoenia stokesii an d Siderastrea sideraea lost significantly greater surface area and number o f polyps to the sponge C. nucula compared to uncaged interactions. For cage d interactions involving the sponge Ectyoplasia fever, there was a trend fo r greater loss of S. sideraea surface area and polyps compared to uncaged i nteractions. Predation had a greater affect on C. nucula than on any of the other sponges examined. Predator exclusion experiments performed with natu rally occurring coral-sponge interactions demonstrated a significant decrea se in total coral cover compared to uncaged controls. It is proposed that i ndirect effects arising from spongivory (especially consumption of C. nucul a) may have large community consequences. Species diversity on Caribbean re efs may be maintained, at least in part, by spongivores.