The spatial ecology of a coral reef sponge, aplysina fistularis

Sessile communities provide an ideal opportunity to understand how population interactions are structured in space, and in turn, how this structure influences population dynamics. The yellow tube sponge, Aplysina fistularis is common on Caribbean coral reefs and is involved in four main types of ecological interactions: symbiosis, predation, disease, and competition. We used spatial and multivariate analysis to describe the population structure of A. fistularis on the Belizean barrier reef, and consequently, how the observed spatial structure influenced sponge morphology and the frequency of population interactions. We found that sponges were non-randomly distributed across the reef, with both local density and tendency towards a clustered spatial distribution increasing with depth. Sponge morphology also varied with depth; deeper sponges were larger and had fewer tubes then their shallower conspecifics. Local density and spatial autocorrelational patterns were not significant predictors of population interactions. The frequency of symbiosis, characterized by the presence of the fish Elacatinus lori, increased with depth, sponge size, and number of sponge tubes. The incidence of predation increased with depth, number of tubes, and the interaction between these two variables. The chance of disease decreased with increasing sponge size. Lastly, the frequency of spatial competition decreased with depth. These results highlight the unexpected finding that in this system, symbiosis, predation, disease, and competition, are density independent, specifically they are not predicted by local density or spatial autocorrelational patterns. Overall, this study provides an essential framework that will greatly enhance our knowledge of sponge ecology on coral reefs