This small coral structure looks like a beautiful underwater castle. It is a hard coral which is home to millions of teeny tiny individual polyps in a castle constructed out of calcium carbonate. Other likely residents within this castle are single-celled algae. The algae use energy from the sun to make sugars and fats which they share with the coral allowing it to grow faster. The coral animals also make waste which feeds the algae. Together they make a very large and happy family of teeny residents in a beautiful castle. They even have a blue Christmas tree worm as a pet in the yard!