Filter feeding ecology of erect branching sponges on Caribbean coral reefs

Sponges are unique filter feeding organisms with complex canal and flagellated chamber aquiferous systems. These systems allow them to specialize in clearing the smallest plankton size class (picoplankton) from the water. Sponges serve many important ecological functions, but they may be best known for efficiently filtering picoplankton, such as phytoplankton bloom species, from the water column. This ecological function increases water clarity and allows more light to penetrate to photosynthesizing corals and sea grasses. As abundant and efficient members of the benthic filter-feeding community, it is important to understand how and to what extent sponges can maintain water quality on healthy coral reefs and restore water quality on declining coral reefs. The first goal of this research was to quantify and compare the abundance of picoplankton resources, in terms of number of cells and carbon and nitrogen content, available to sponge filter feeders in Caribbean coral reef, mangrove, and sea grass habitats. The four primary picoplankton prey types (picoeukaryotes, cyanobacteria, prochlorophytes, and heterotrophic bacteria) were quantified using flow cytometry. Data were also collected on light intensity and dissolved nutrient concentrations because some sponge species harbor phototrophic and heterotrophic bacterial symbionts that can utilize these resources. We studied sites in all three habitats near Carrie Bow Cay, Belize, two reef sites and one mangrove site in Bocas del Toro, Panama, and one reef site in Curacao. These sites were chosen specifically to explain observed patterns in long-term sponge growth rate data that suggest resource limitation. Overall, heterotroph cell densities and carbon and nitrogen were greater at the mangrove sites than at the reef sites, but autotroph cell densities and carbon and nitrogen at the reefs sites were greater than or equal to those at the mangrove sites. Furthermore, there were greater heterotroph and autotroph cell densities and carbon and nitrogen at the reef sites in Bocas del Toro than at the reef sites in Belize and Curacao, and the autotroph cell densities and carbon and nitrogen were also greater at the mangrove site in Bocas del Toro than the mangrove site in Belize. The second goal of this research was to quantify and compare the effect of sponge grazing on the four picoplankton prey types, the clearance rate of each prey type for each species, and selectivity (i.e. clearance rate standardized by relative abundance of prey types in the ambient water). These variables were quantified and compared for six of the most common erect branching sponge species representing three different orders at the reef sites in Belize and Panama: Aplysina cauliformis (Order Verongida), Aplysina fulva (Order Verongida), Desmapsamma anchorata (Order Poecilosclerida), Iotrochota birotulata (Order Poecilosclerida), Niphates erecta (Order Haplosclerida), and Amphimedon compressa (Order Haplosclerida). Sponge feeding was quantified using chamber experiments that compared the change in prey…