They are invertebrates or spineless animals, and are cousins of anemones and jellyfish. When thousands of these animals are grouped together, they are referred to as coral colonies . A polyp has a sack-like body and an opening or mouth encircled by numerous stinging tentacles called cnidae . The polyp uses calcium carbonate from the seawater to build itself a hard, cup-shaped skeleton. This limestone skeleton protects the soft, delicate body of the polyp.
Coral polyps are nocturnal and stay inside their skeletons during the day.
However, at night they can be seen as their tentacles come out to feed. Coral polyps eat in two different ways depending on their species. Hard coral polyps are nourished in a unique way by a tiny algae called zooxanthellae. Zooxanthellae live within the tissue of hard corals and supply them with up to 98 percent of their nutritional needs. The tiny algae use the energy from sunlight to convert carbon dioxide into oxygen and carbohydrates in a process known as photosynthesis .