Coral reefs, composed of small animals called coral polyps, are an integral part of marine life. Only the upper layer of the coral reef is made up of living corals. While more than 600 species of coral exist, there are two main types of coral: soft and hard (stony) coral. Stony corals get their structure by taking calcium from the saltwater, which helps to create their hard outer shell. The stony coral only extends its polyps, while the soft coral's whole body swells. Polyps emerge when the coral is feeding. Some eat in the daytime, though others are nocturnal. Many coral reefs depend upon the sunlight to live, because most coral live on internal algae. Other animals live on this algae as well, and also rely on the coral's polyps, to survive. This makes most coral reefs vulnerable. For example, starfish eat the algae and polyps from the coral. A starfish can eat their own mass in coral in a single day. Other fish make the coral reef their home, either for themselves or for their offspring.