Several groups of invertebrates construct their own quarters: Ants, for example, create hills, and bees build hives; however, no other group of animals builds a more dramatic home for itself than the corals. A coral reef, which resembles a pile of rocks, is actually an invertebrate superstructure made up of a living colony and the skeletons of their dead ancestors. The colony is always growing, but at a painfully slow rate of an inch or two (2.54 to 5 cm) each year. Reefs that are miles long were formed over thousands of years.
Chief inhabitants of the reef are the corals themselves. These simple invertebrates live alongside sponges, worms, anemones, hydroids, and thousands of other types of animals. Coral polyps wave their stinging tentacles in the water all through the night to grab any potential food item that comes their way. In addition, they stock their gastrovascular cavity with millions of zooxanthellae, tiny dinoflagellates that photosynthesize in the daytime.
A coral reef may support as many sponges, in both population size and diversity, as it does coral animals. Sponges, the simplest kinds of animals on Earth, are efficient filter feeders that continuously gather food. They grow in the company of colorful soft corals that resemble brightly colored plants, giant feathers, or ornate fans.
Close cousins to the corals are the hydroids, colonial cnidarians. Some of these sessile colonies form flexible, branching stalks, and others build limestone skeletons similar to those of the true corals. Nearby, growing singly or in clumps, are the anemones, cnidarians that lack any type of body covering. Anemones hold out their delicate but poisonous tentacles during the day to gather food. Some live in symbiotic relationships with shrimp, small fish, or crabs.
Scattered throughout the reef are the worms. Flatworms can be identified by their thin, transparent bodies and highly branched digestive systems. These simple animals feed by secreting digestive enzymes on their food, then sucking it up. The more advanced worms, the polychaetes, either crawl around the reef in search of food or catch food particles in their halo of radioles. Many are brightly colored predators that carry dangerous toxins.
The color and beauty of the coral reef make it easy to forget that each day is a life-and-death struggle for the simple animals that live there. Every invertebrate is a potential meal for another animal and survives simply because it has evolved some method of protection from predators.