Dynamics behind standoff interactions in three reef sponge species and the coral Montastraea cavernosa

Benthic organisms compete for space, and standoff interactions, i.e, interactions with no clearly observable outcome such as win or loss, are very common in marine hard substratum communities. Standoffs were more common than overgrowths among all sponge-coral interactions observed in a coral reef community off Santa Marta, NE Colombia. The dynamics of these standoffs and the role played by each of the interacting organisms were examined in a series of standard observations of sponge coral interactions involving the sponge species Niphates erecta, Rhaphidophlus venosus, Scopalina ruetzleri and the coral Montastraea cavernosa. Most sponge-coral standoffs in this study displayed rapid changes in number of polyps along the coral border, sponge area and sponge-coral distance. The outcome of sponge-coral standoffs depended on the ecological stategy (e.g., growth) of the sponge species. Scopalina ruetzleri and N. erecta did not show any signs of direct competition with Monlastraea cavernosa. R. venosus, however, was often observed to take over vacant space formed by dead coral polyps (54 % of interactions). The impact of coral damage (lesions) on the interaction process between R. venosus and M. cavernosa was shown by a significant increase in coral polyp death in damaged compared with undamaged M. cavernosa colonies (22.7 % versus 6.9 %). This study demonstrates that (1) sponge-coral standoffs are actually quite dynamic because of the continually changing distance between the organisms, (2) the dynamic nature of sponge-coral standoffs depends on growth-related strategies (growth form, growth rate) of The competing sponge species, (3) the actual frequency of coral overgrowth by (thinly encrusting) sponge species can only be determined by means of intermittent observations; space gained at the cost of coral tissue cannot be seen by single observations, (4) coral damage (lesions) on reefs may enhance deterioration of corals by increasing their susceptibility to sponge overgrowth.