Coral: The Rock That Grows

Written by Dr. Sherwood B. Idso

Located off the northeastern coast of Australia is a rocky formation known as the Great Barrier Reef. Measuring almost a hundred miles in width and attaining a thickness of well over 100 yards, it stretches for over 1,200 miles. What exactly is it? And how did it get there?

As with many other magnificent works of nature, reefs have very humble beginnings; their genesis is found in the life and death of billions upon billions of tiny coral polyps.

Coral polyps are simple animals that can best be described as stomach-mouths ringed by a number of tentacles. Anchored to a single spot throughout their entire life, they were once thought to be plants. However, they are true animals that capture and digest even smaller animals.

As they grow, corals add to the size of their rocklike skeletons that are produced from minerals dissolved in the seawater. And as one generation of individuals dies and leaves its hardened skeletons behind, a new generation builds upon them, expanding the size of the colony. In time, great masses of skeletal materials are accumulated, forming the basic framework of the reef.

Once formed, a coral reef is much more than just the fossilized remains of coral polyps. It is a living ecosystem (community of living things) that probably supports a larger number of animal and plant species than any other ecosystem on earth. Indeed, hundreds of species similar to the corals live among the reef’s topmost layers, binding them together. And innumerable fish and invertebrates, such as crabs and shrimp, live among the many nooks and crannies of the reef. Where the reef encloses a sandy lagoon, there will also be a community of bottom-dwelling organisms such as sea urchins and sea cucumbers. Last of all, there is a host of burrowing animals that churn up the sands and sediments.

How are all of these creatures sustained? Where does their food come from? To find out, we must go back to the primary reason for the reef itself—the coral polyp.

Hidden within the cells that line their stomachs, coral polyps harbor large numbers of one-celled algae (sea plants) known as zooxanthellae. These algae produce food by photosynthesis (new combinations of chemical compounds formed with the help of sunlight). What’s more, the raw materials they use are the waste products of the corals. And the corals, in turn, make use of the products produced by the algae.

As this symbiotic (cooperative) relationship enhances the development of a coral skeletal framework, other creatures are at work producing the total ecosystem. Many types of algae, fungi, sponges, sea worms, crustaceans, and mollusks bore into this hard material. The holes are soon filled, however, by secretions of the various organisms living there, including some bacteria.

With all of this biological activity, it is easy to understand that there is intense competition for food and living space in the reef ecosystem. It is eat or be eaten. And coral polyps are one of the delicacies enjoyed by a large number of fishes, sea urchins, starfish, and other animals. In fact, some fish will actually eat all of the polyps in an area so that they can start algae “farms” on the hard coral skeleton.

The end result of all of these interrelated physical, chemical, and biological processes is the “living rock” of the coral reef, one of the true wonders of the world. In subsequent articles we will look more closely at many of the different animals that live there and at some of the life-and-death dramas that are played out every day. Scientists are particularly interested in the medicinal uses of many substances produced by reef animals, and we will learn of their great importance to man.

Photographed by Dr. Sherwood B. Idso

1. At the bottom lives a colony of coral polyps.

2. The law of the reef in action: eat or be eaten.

a. A crab devours a small fish.

b. A sea star about to do likewise.

c. A hermit crab munching on a starfish.

d. A sea anemone waiting for an unsuspecting hermit crab looking for a new home.

3. The protective coloration (or lack of color, in this case) shown by this ghost shrimp helps it avoid capture.

4. This nudibranch or “snail without a shell” can show off its bright red color, since it is distasteful to most reef creatures.

5. This coral-banded shrimp shows off its dramatic colors too. It is sought out by fish, not to be eaten, but because it removes parasites from their skin.