Seagrass beds develop on sandy and muddy coastal floors with algae and animals forming a community that relates to the coral reefs. Since they retain and accumulate sediment, the seagrass beds modify the topography of the floor until they rise almost to the surface. In this phase, they can be invaded by mangroves, enabling thus the advance of coastland over the sea. The seagrass is eaten by turtles and manatees while fish and urchins only take partial advantage of it.