Community Ecology of Mesofauna Associated with the Caribbean Fire Sponge, Tedania ignis, and Context-Dependent Effects of Symbiosis

Sponges form symbioses with a wide array of mesofauna including polychaetes, crustaceans, brittle stars, and bivalves. These organisms use the sponge for food and shelter, but their effect on the sponge is largely unknown. The facultative association of mesofauna with their sponge hosts coupled with the ability to subdivide sponges to control for genotype and size make this a tractable system to study community ecology and the ecology of symbiosis. I posed three questions in my dissertation: (1) How does mesofaunal community composition of the Caribbean fire sponge, Tedania ignis, vary spatially and temporally? (2) How does a change in the regional species pool influence the invasibility and stability of mesofaunal communities? and (3) What is the effect of polychaete worm feeding on the sponge host? I used the Caribbean Fire Sponge to address these questions because it is the dominant sponge on mangrove roots in the Caribbean and it hosts abundant and diverse mesofaunal communities. To examine the spatiotemporal variation of mesofaunal communities, individuals of T. ignis were collected from a mangrove channel and seagrass meadow annually for three years and associated mesofauna were counted and identified. Mesofaunal communities were strongly influenced by habitat, which was particularly surprising because the environment that the mesofauna experience is mediated by their sponge host. This suggests that abiotic filters restrict dispersal from the regional species pool. Importantly, this study revealed that the regional species pool is dynamic – even when taxonomic composition was relatively stable through time there was turnover of species within a guild across years. In addition, local processes such as disturbance from a cold snap had large, but temporary effects on mesofaunal community composition. This study revealed that mesofaunal communities associated with sponges are influenced by both local and regional processes that occur at different spatial and temporal scales. To examine how a change in the regional species pool influenced the invasibility and stability of mesofaunal communities, I set up reciprocal transplants annually for three years between T. ignis from a seagrass meadow and T. ignis from a mangrove channel. Mesofaunal community composition differed significantly among transplants. Sponges that were transplanted back to the native habitat had mesofaunal communities that were distinct from each other and these differences may be explained by environmental filtering or differences between the mesofaunal regional species pool in the mangrove and seagrass site. Sponges that were transplanted between habitats had mesofaunal communities that resembled those of both the habitat source and the transplant site, but there was a stronger influence of the transplant site on the mesofaunal community composition. The change in the composition of mesofaunal communities moved between habitats suggests that a change in the regional species pool can precipitate turnover of species within a mesofaunal community. In addition, species that were gained (or lost) following the transplant tended to be species that occurred as epibionts, had weak ties to the sponge host, and only occurred in one habitat. Finally, to assess the effect of polychaete worm feeding on the sponge host, I tested two hypotheses: (1) mesofaunal symbionts are more likely to colonize their native host than a novel host and (2) polychaete feeding is costly to its sponge host. Field surveys and choice experiments revealed that different species of Branchiosyllis polychaete occupied different species of host sponges. Manipulative experiments revealed that polychaete feeding is costly to T. ignis. Sponges without polychaetes grew faster than sponges with polychaetes. A surprise was that polychaete density was influenced by brittle stars that live on the sponge and consume the polychaetes. The positive indirect effect of the brittle star on the sponge host has important implications for the sponge’s fitness and underscores the context-dependent nature of symbiosis.