Meeting Abstract
Recent evidence suggests that top-down factors (predation) primarily determine the diversity and abundance of sponges in reef environments. In the Caribbean, one-third of the dominant sponge species host high densities of photosynthetic cyanobacteria, with some sponges deriving significant nutritional benefit from this symbiosis. To test whether these symbioses play a role in structuring sponge communities, we examined the interactive effects of predation and irradiance on the growth of 6 sponge species. Sponges were held in a two-factor field experiment that manipulated irradiance and predation in situ for 6 weeks. Sponges hosting high densities of photosymbionts had significantly lower growth and cyanobacterial abundance under reduced irradiance irrespective of the presence of predators, indicating that shading had a greater impact on holobiont biomass than predation. In contrast, sponges lacking cyanobacteria displayed significantly reduced growth in uncaged treatments, supporting the contention that top-down forces influence growth of some sponge species. Neopetrosia rosariensis, which hosts high densities cyanobacterial symbionts, demonstrated higher growth rates under shaded conditions when predators were excluded than in uncaged treatments, suggesting that an interactive effect of irradiance and predation can influence growth. These results suggest that sponge communities in the Caribbean are likely structured by multiple factors varying in importance among species, and highlight the necessity of examining multiple ecological variables in combination when examining the forces that structure communities.
