Corals, and the zooxanthellae algae they contain, form the base of a coral reef ecosystem. Organisms like sea urchins feed on microorganisms that grow on the corals. Organisms including seahorses feed on plankton and fish that also inhabit the reef. Larger animals like sea turtles frequently enter the reef to forage. Seaweeds, which are competitors of corals, may grow along the reef bottom.