But all corals are animals—minute, anemonelike invertebrates called polyps that colonize and build limestone skeletons around their delicate bodies. Hard corals number at least 300 species, and soft corals perhaps another 1,000 more.

Hard corals are the architects of the reef. They need shallow, clear, sunny waters to get the job done—and a big job at that. Reefs occupy a meager 1 percent of the Earth’s surface, yet house more than 25 percent of all marine species. When the polyps die, their skeletons supply the base for new polyps and their encasings to grow.

Microscopic algae called zooxanthellae (zoh-zan-THEL-ee) live in the tissues of hard corals. Like all plants, zooxanthellae convert sunlight into sugars, providing up to 98 percent of the corals’ food. They also give coral its feast of pulsating colors. In return, coral offers zooxanthellae a safe home and nutrients through its waste. This symbiotic relationship is the life force of the reef.

Shedd Aquarium is propagating corals, which was, until recently, difficult to achieve. Coral shapes—columnar, branching, or leaflike, for example—depend on such tightly controlled factors as temperature, light and turbulence. Many specimens are on exhibit in Wild Reef, including Heliopora coerulea, Acropana nana and Stylophora pistillata. But don’t let their scientific names keep you away. Come see their striking beauty and differences for yourself.