No abstract

No abstract

Sponges belonging to the genus Cliona are common inhabitants of many coral reefs, and as bioeroders, they play an important role in the carbonate cycle of the reef. Several Cliona species maintain intracellular populations of dinoflagellate zooxanthellae (i.e., Symbiodinium spp.), which also form symbioses with a variety of other invertebrates and protists (e.g., corals, molluscs, foraminifera). Unlike the case of coral symbioses, however, almost nothing is known of the metabolic interaction between sponges and their zooxanthella symbionts. To assess this interaction, we performed a tracer experiment to follow C and N in the system, performed a reciprocal transplant experiment, and measured the stable carbon isotope ratio of Cliona spp. with and without zooxanthellae to study the influence of environment on the interaction. We found strong evidence of a transfer of C from zooxanthellae to their sponge hosts but no evidence of a transfer of N from sponge to zooxanthellae. We also saw significant influences of the environment on the metabolism of the sponges. Finally, we observed significant differences in carbon metabolism of sponge species with and without symbionts. These data strongly support hypotheses of metabolic integration between zooxanthellae and their sponge host and extend our understanding of basic aspects of benthic-pelagic coupling in shallow-water marine environments.

Competition for space is an important process on tropical coral reefs. Few studies have examined the role sponges play in community structure despite the fact that many sponges are competitively superior to reef-building corals in space acquisition. Surveys conducted throughout the Florida Keys indicated that Chondrilla nucula was involved in about 30% of all coral-sponge interactions; this sponge has also been observed in 40-50% of coral-sponge interactions on other Caribbean reefs. C. nucula is also the top prey item of the Hawksbill turtle, and among the preferred prey of several spongivorous fish. I examined how predation influenced sponge competitive abilities (particularly those of C. nucula), and whether this type of indirect effect had important consequences for community dynamics in the Florida Keys. Exclusion of sponge predators (primarily angelfish) resulted in increased sponge overgrowth, with a subsequent greater loss of coral cover, compared to uncaged pairwise interactions. When caged, the corals Dichocoenia stokesii and Siderastrea sideraea lost significantly greater surface area and number of polyps to the sponge C. nucula compared to uncaged interactions. For caged interactions involving the sponge Ectyoplasia ferox, there was a trend for greater loss of  S. sideraea surface area and polyps compared to uncaged interactions. Predation had a greater affect on C. nucula than on any of the other sponges examined. Predator exclusion experiments performed with naturally occurring coral-sponge interactions demonstrated a significant decrease in total coral cover compared to uncaged controls. It is proposed that indirect effects arising from spongivory (especially consumption of  C. nucula) may have large community consequences. Species diversity on Caribbean reefs may be maintained, at least in part, by spongivores.

The ecological and evolutionary forces that shape interactions between marine sponges and their symbiotic microbiota remain poorly understood. Considerable work has been done to characterize the sponge microbiome, and this research has shown that there are two distinct types of sponges: those with high microbial abundances (HMA) and those with low microbial abundances (LMA). There is no clear evolutionary driver for this distinction, and we have only a limited understanding of how HMA or LMA status affects host phenotypes (e.g. feeding behavior, capacity for nutrient cycling). We had two primary goals with the present study. First, we used a variety of microscopic techniques to compare aspects of host biology (e.g. choanocyte chamber morphology and density) in the context of symbiont status. Secondly, we used molecular approaches to uncover components of ecological structure of bacterial communities in HMA versus LMA sponges (e.g. species richness, evenness). We found that choanocyte chamber density is greater in LMA sponges than in HMA sponges. We also found distinct patterns of organization for bacterial communities in HMA and LMA sponges, although environmental factors, to a lesser degree, also influence community structure in the Floridian sponges we examined. These results suggest that the large bacterial communities found in HMA sponges may allow the host to decrease their heterotrophy versus that of LMA sponges with minimal bacterial communities.