Coral reefs occur in only about 1% of the world's oceans, but as the most diverse ecosystems, are home to around 25% of all marine life. Corals are animals, but they depend on a mutually beneficial association (called symbiosis) with microscopic plants called zooxanthellae. The hard structure of corals provides the zooxanthellae protection, while they synthesise food for the coral. This section showcases some of the common coral species and other animals that together make up a group called 'Cnidaria' (pronounced ni-dare-ee-yah). Cnidarians are characterised by their stinging cells (cnidocytes) that stun their prey, and this group includes sea anemones, sea pens, sea fans, and jellyfish. Cnidarians show two body forms, a solitary and immobile polyp form, and the swimming medusa form which often resembles an umbrella.
