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 . Since photosynthesis requires sunlight, hard corals can only live in clear, shallow waters. Another way that corals eat is by catching tiny floating animals known as zooplankton .