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 density in the course of 20 minutes in a chamber with a filtering sponge to the change in prey density in the course of 20 minutes in a control chamber without a sponge. All six sponge species significantly removed cyanobacteria and heterotrophic bacteria compared to controls, none of the species removed picoeukaryotes, and all species except the two Aplysina species removed prochlorophytes. The two Aplysina species also cleared cyanobacteria more slowly than all other sponge species. Amphimedon compressa and Niphates erecta cleared heterotrophs more quickly than the other species and Amphimedon compressa also cleared cyanobacteria more quickly than all other sponge species. Finally, Aplysina fulva and Aplysina cauliformis removed heterotrophic bacteria more efficiently than other prey types, Iotrochota birotulata and Desmapsamma anchorata removed cyanobacteria more efficiently than other prey types, and Amphimedon compressa and Niphates erecta did not demonstrate selectivity for any of the prey types. This is the first study to quantify picoplankton abundances in the water directly available to sponges in reef, sea grass, and mangrove habitats and to also quantify the feeding ecology of six erect branching sponge species at multiple reef sites in the Caribbean. The differences documented in the picoplankton communities between sites indicate that differences in growth rates for individuals of the same species between sites may be controlled by bottom-up forces. Furthermore, if sponges partition picoplankton resources as this study suggests, then diverse sponge communities are essential to maintain water quality in these systems by effectively removing all types of picoplankton. Finally, sponges fulfill important ecological functions in these ecosystems, and it is possible that their filtering power could be harnessed in a species-specific manner for remediation efforts to improve water quality in these ecosystems.