Filter feeding ecology of erect branching sponges in Caribbean coral reef and mangrove ecosystems
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. 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. This study quantified sponge filter feeding rates on coral reefs in Belize and Panama. Additionally, we quantified the feeding rates of genotype-controlled transplants of each sponge species in a nearby mangrove habitat. We focused on six of the most common erect branching sponge species representing three different orders (Verongida, Peocilosclerida, and Haplosclerida): Aplysina cauliformis, Aplysina fulva, Desmapsamma anchorata, Iotrochota birotulata, Niphates erecta, and Amphimedon compressa. Sponge filter 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. Water samples were collected at 10-minute intervals and picoplankton densities were quantified using flow cytometry. We analyzed picoplankton ingestion over time using repeated measures ANOVA.
The mangrove sites in Panama and Belize had higher densities of picoplankton cells in the water column than the reef sites in Panama, and the reef site in Belize had the lowest densities of all sites. All sponge species significantly removed cyanobacteria and heterotrophic bacteria from the water column in 20 minutes at all sites. Sponges ingested more carbon and nitrogen per minute from the picoplankton in Panama than in Belize. In Belize, sponges tested on the reef ingested more carbon and nitrogen per minute from cyanobacteria and less carbon and nitrogen per minute from heterotrophs than the sponges tested at the mangrove site. Additionally, ingestion rates were significantly higher in the first 10 minute time period than in the second, suggesting that these species adjusted their feeding rate in response to reduced resources. In Panama, there was no effect of site on ingestion rates, but the magnitude of the difference in picoplankton density between the reef and mangrove site was also smaller than in Belize. These results indicate that sponge filter feeding not only has a large effect on the water column, but that the water column has a large effect on sponge feeding rates and prey types.