Coral reefs grow along the coast of the Red Sea. They are famous for their beauty. Corals are really small animals that live in colonies. Over thousands of years, they build hard structures made of limestone. Tiny algae live inside their tissues. The algae give the coral food and color, and the coral gives the algae protection. Reefs shelter about one quarter of all sea life, although they cover only a small part of the ocean floor. One of the worst dangers is warmer water. It makes the coral push out the algae, so the coral loses its color and becomes weak. For this reason, many groups work to protect these reefs and to control diving around them.

Question: In the text, the word "shelter" means: