Before learning how reefs form, it’s helpful to define a reef. Acoral reef is made up of animals called stony corals. The stony corals are made up of tiny, soft colonial organisms called polyps. Polyps look a lot like a sea anemone, as they are related to these animals. They are invertebrates in the Cnidaria phylum. As the polyps live, reproduce, and die, they leave their skeletons behind. A coral reef is built up by layers of these skeletons covered by living polyps. The polyps reproduce either through fragmentation (when a piece breaks off and new polyps form) or sexual reproduction through spawning. A reef ecosystems may be made up of many species of corals. Healthy reefs are typically colorful, highly biodiverse areas made up of a mishmash of corals and the species that inhabit them, such as fish, sea turtles, and invertebrates such as sponges, shrimp, lobsters, crabs, and seahorses. Soft corals, like sea fans, may be found within a coral reef ecosystem, but do not build reefs themselves.  The corals on a reef are further cemented together by organisms like coralline algae, and physical processes like waves washing sand into spaces in the reef.