An oyster reef is an ecological community made up of densely packed oysters (bivalve molluscs). It provides habitat for toadfish, skilletfish and blue crabs. Oysters suck water through gills and suspended tiny organisms and pollutants like nitrogen are trapped in the gills' fine lining. They eat the organisms and expel the rest as solid pellets of waste, which decompose and bubble up as nitrogen. A healthy oyster can filter up to five litres of water per hour, thus, cleansing it of algae and pollutants. Oyster reefs are mostly found with mangroves along the coast. Hence they also defend shorelines and reduce erosion.