Hard Coral (Scleractinia)
They arise from colonies of an organism, polyps. Polyps have an external skeleton made of calcium carbonate that it can retract into in unfavourable conditions. They usually produce mucus or have tentacles to help them filter and collect food. Depending on their size, they feed on organisms from plankton to small fishes. They share a mutualistic relationship with algae (zooxanthella) where both parties benefit. It provides algae with shelter and nutrients and in turn receives products of algae’s photosynthesis process. Hence, it is usually situated somewhere that algae can receive some amount of light.