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.