Coral reefs constitute an important tropical environment that provides habitat for a rich variety of ocean life. A. Coral reefs are structures made by living creatures and can grow only in specific ocean conditions. B. Although atolls can sometimes grow to large proportions, barrier reefs are by far the largest coral structures. C. Mangrove swamps are as important as coral reefs as the habitat for numerous plant and animal species. D. Coral reefs are classified as fringing, barrier, or atoll, all of which are linked in a chain of progressive development. E. Although coral reefs are important to humans in several ways, they are currently threatened by dredging and mud from erosion. F. Fringing reefs lie close to the shore of continents and islands, forming a transition zone between the land mass and the sea.