Before answering that question, let us answer another question. What’s the difference between a plant and an animal? Without getting too scientific and some exceptions in mind, the main difference is that plants make their own food and animals have to get their food elsewhere. They depend on other plants or animals for their source of energy.
So, to answer our question, we need to know what a coral structure is made from and where it gets its food. A coral individual is actually made up of thousands of polyps. Each polyp is a circular mouth, surrounded by tentacles. Inside their tissues, in every polyp, they have millions of small plants: microscopic algae. These algae catch sunlight and just like any other plant, convert it into energy and nutrients, which the coral uses for food. They essentially have food factories living inside of themselves. This means a coral doesn’t make it’s own food, which makes it an animal!
During the day the algae photosynthesise and at night the corals even go on a hunt. They expand their polyps, the tentacles come out and everything that swims by is caught and swept into their mouths. Cool!
