Coral
The Coral Polyp lives inside a circular stone house called a corallite. The outer skin of the polyp has special cells that take calcium carbonate from the sea water and deposit it as a hard cup around the body of the polyp. This solid limestone cup cements the polyp to the reef. By day the poly curls up inside the limestone shelter. By night it stretches up out of the corallite and spreads it's tentacles to capture zooplankton food that comes it's way.

Even though the coral polyps hide in day and come out at night to feed in the darkness, sunlight is important to them. That is because they house many microscopic plant guests called zooxanthellae (zoh-uh-zan-THEL-ee).

The partnership between coral polyps and zooxanthellae is complex. Marine biologists are still trying to understand it. But most marine scientists agree that Coral polyps could not build a coral reef with out these plant companions. The zooxanthellae speed up the process by which coral polyps can remove calcium carbonate from the seawater.

Like all plants zooxanthellae use the energy from the sun to change water and carbon dioxide into oxygen, sugars and starches. This process is called photosynthesis. The zooxanthellae uses the waste from the polyp - carbon dioxide and nitrates - to help them grow. In return the plants supply food and oxygen to the polyp. The polyp also eats tiny animals called nematocysts it spears with it's stinging tentacles.