Billy Donahue, age 13, of Youngstown, Ohio, for his question:

What are reef builders?

We call them coral reefs because the small marine coral animals have a great deal to do with building them. These soft bodied polyps enjoy life in warm and tropical oceans and each one builds himself a home of minerals extracted from seawater. The little house of hard coral material has a door and the small animal inside pokes out a handful of tentacles, always ready to grab a passing morsel of food. He lives and finally dies in his own home. When he departs, no relative moves into the empty house. Instead, every young coral builds a brand new home of his own. It is attached to other coral homes, some still occupied and many others left by departed relatives.

In warm, shallow seas, generations of assorted corals build great masses of stony reefs. They share their territory with seaweeds, fishes, crustaceans and other marine dwellers. Stony seaweeds that stiffen their foliage with seawater minerals add to the rocky underwater formations. They are classified with the corals as reef builders. The remains of crustaceans and shellfish and even tiny diatoms also contribute to the building of reefs and atoll islands.