Science, Tech, Math › Animals & Nature 10 Facts About Corals Share Flipboard Email Print Photo © Raimundo Fernandez Diez / Getty Images. Animals & Nature Marine Life Marine Life Profiles Marine Habitat Profiles Sharks Key Terms Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Forestry Dinosaurs Evolution View More By Laura Klappenbach Ecology Expert M.S., Applied Ecology, Indiana University Bloomington B.S., Biology and Chemistry, University of Illinois at Urbana-Champaign Laura Klappenbach, M.S., is a science writer specializing in ecology, biology, and wildlife. our editorial process Laura Klappenbach Updated January 11, 2018 If you've ever visited an aquarium or gone snorkeling when on holiday, you're probably familiar with a wide variety of corals. You may even know that corals play a fundamental role in defining the structure of marine reefs, the most complex and diverse ecosystems in our planet's oceans. But what many don't realize is that these creatures, which resemble a cross between colorful rocks and various bits of seaweed, are in fact animals. And amazing animals at that. We've explored ten things we should all know about coral, what makes them animals and what makes them so unique. Corals Belong to the Phylum Cnidaria Other animals that belong to the Phylum Cnidaria include jellyfish, hydrae, and sea anemones. Cnidaria are invertebrates (they do not have a backbone) and all have specialized cells called nematocysts that help them capture prey and defend themselves. Cnidaria exhibit radial symmetry. Corals Belong to the Class Anthozoa (a Subgroup of the Phylum Cnidaria) Members of this group of animals have flower-like structures called polyps. They have a simple body plan in which food passes in and out of a gastrovascular cavity (stomach-like sac) through a single opening. Corals Typically Form Colonies Consisting of Many Individuals Coral colonies grow from a single founder individual that divides repeatedly. A coral colony consists of a base that attaches coral to a reef, an upper surface that is exposed to light and hundreds of polyps. The Term 'Coral' Refers to a Number of Different of Animals These include hard corals, sea fans, sea feathers, sea pens, sea pansies, organ pipe coral, black coral, soft corals, fan corals whip corals. Hard Corals Have a White Skeleton That Is Made of Limestone (Calcium Carbonate) Hard corals are reef builders and are responsible for the creation of the structure of a coral reef. Soft Corals Lack the Stiff Limestone Skeleton That Hard Corals Possess Instead, they have little limestone crystals (referred to as sclerites) embedded in their jelly-like tissues. Many Corals Have Zooxanthellae Within Their Tissues Zooxanthellae are algae that form a symbiotic relationship with the coral by producing organic compounds that the coral polyps use. This food source enables the corals to grow faster than they would without the zooxanthellae. Corals Inhabit a Wide Range of Habitats and Regions Some solitary hard coral species are found in temperate and even polar waters and occur as far as 6000 meters below the surface of the water. Corals Are Rare in the Fossil Record They first appeared in the Cambrian period, 570 million years ago. Reef-building corals appeared during the middle of the Triassic period between 251 and 220 million years ago. Sea Fan Corals Grow at Right Angles to the Current of the Water This enables them to efficiently filter plankton from the passing water.