Growth facilitation by the octocoral Gorgonia ventalina explains spatial difference in the population size structure of the common demosponge Ircinia felix

摘要

In this study, the demography of the common demosponge Ircinia felix was examined at Tamarindo, a coral reef located in the island municipality of Culebra, Puerto Rico. A preliminary study comparing the size structure of two subpopulations within the reef, Tamarindo Norte (TN) and Tamarindo Sur (TS), indicated that sponges at TN are significantly larger than sponges at TS. This result served as a baseline for the present comparative study in which we aimed to determine whether the spatial differences in population size structure can be explained either by a difference in rates of survival, growth, or recruitment, or a combination of these. To accomplish our goal, we followed the growth, survival and recruitment of I. felix at the two localities for one year. Growth was the only demographic parameter that differed significantly between localities. Because the most obvious distinction between the study sites was the absence of the octocoral Gorgonia ventalina at TS, we hypothesized that the faster overall growth rate of sponges at TN was related to the presence of the octocoral. To test this hypothesis, we compared growth rates between sponges associated with the octocoral and those individuals not associated. We found that sponges growing near G. ventalina grew significantly faster than non-associated sponges. This result suggests that the octocoral facilitates the growth of I. felix and therefore may account, at least in part, for the spatial differences in population size structure.