QUANTIFICATION OF SPONGE CORAL INTERACTIONS IN A PHYSICALLY STRESSED REEF COMMUNITY, NE COLOMBIA/

Coral reef sponges are considered to be important space competitors. C ompetitive interactions between sponges and corals often result in ove rgrowth of the coral. It is assumed that sponges are even more success ful in environments sub-optimal for corals. In order to test the hypot hesis that coral overgrowth by reef sponges increases when corals are under stress, the frequency of sponge/coral interactions was quantifie d 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 Mart a (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 interacti ons) consisted of overgrowth of corals by sponges. The frequency of su ch overgrowth depended on the presence of particular sponge species, w hich appeared to be more aggressive towards corals in localities with high coral cover, relatively low sedimentation and high visibility. Th us, we reject the hypothesis that coral overgrowth by sponges occurs m ore frequently in localities under physical stress. Overgrowth was rel ated to the presence of aggressive sponge species, rather than to char acteristics of the corals. It is concluded that reef sponges differ no tably in their competitive abilities. By influencing the sponge commun ity composition on the reef, the physical environment may indirectly d etermine the extent of overgrowth of corals by sponges.