Quantification of sponge/coral interactions in a physically stressed reef community, NE Colombia

Coral reef sponges are considered to be important space competitors. Competitive interactions between sponges and corals often result in overgrowth of the coral. It is assumed that sponges are even more successful in environments sub-optimal for corals. In order to test the hypothesis that coral overgrowth by reef sponges increases when corals are under stress, the frequency of sponge/coral interactions was quantified along a gradient of physical stress. At 15 stations, encompassing 5 localities and 3 depths (5, 10 and 20 m) along the coast of Santa Marta (NE Colombia), the number and categories of interactions were scored in belt transects (10 x 1 m(2)). Four categories of interactions were distinguished. Physical factors such as sedimentation and visibility were measured. 21 coral species and 95 sponge species were encountered in a total of 3866 sponge/coral interactions. Only 2.5% (96 interactions) consisted of overgrowth of corals by sponges. The frequency of such overgrowth depended on the presence of particular sponge species, which appeared to be more aggressive towards corals in localities with high coral cover, relatively low sedimentation and high visibility. Thus, we reject the hypothesis that coral overgrowth by sponges occurs more frequently in localities under physical stress. Overgrowth was related to the presence of aggressive sponge species, rather than to characteristics of the corals. It is concluded that reef sponges differ notably in their competitive abilities. By influencing the sponge community composition on the reef, the physical environment may indirectly determine the extent of overgrowth of corals by sponges.