WULFF, Janie L.; Florida State University: Trade-offs between resistance to competition and predation in mangrove and coral reef sponges

High regional diversity of tropical marine sponges typically results from high diversity within individual habitats and a low overlap in species composition among habitats, which is generally attributed to abiotic factors. Distinctive faunas of mangrove roots and shallow coral reefs contribute substantially to high sponge diversity in the Caribbean, but processes underlying the distinction between these faunas have not been clear. Determination of key factors has been complicated by the many abiotic factors that differ between these habitats. An unusual mangrove system in Belize, where typical reef sponges inhabit mangrove roots, permits comparisons between mangrove and reef faunas without complications of abiotic differences. Reciprocal transplant experiments, combined with caging and artificial substrata, demonstrated that competition and predation strongly influence membership in these sponge faunas. A positive relationship between growth rate and survival of mangrove species suggests control of this community by competition for space, mediated by growth rate. In contrast, growth and survival are not positively related for typical reef species, reflecting the more complex assortment of environmental factors, including predators, that control membership in reef communities. Growth rate was inversely related to defenses against spongivory in the species studied, suggesting a trade-off between resistance to competition and resistance to predation that might preclude success by individual species on both coral reefs and mangroves, enforcing the distinctness of the sponge faunas of these two habitats.