Coral reefs rise up out of the sand like dazzling cities in a barren desert. Though constantly battered by surging waves and scouring sand, coral reefs continue growing upward - thanks to the millions of small animals that labor ceaselessly to build and rebuild them, in the process creating homes for thousands of other species.

Just who are the mysterious builders of these cities in the sea?

Reef Builders
Coral reefs are created by small, soft-bodied animals closely related to anemones. They consist of a soft cup of tissue surrounded by a ring of tentacles.

The soft bodies of living coral polyps consist of a ring of tentacles around a single opening that serves as both mouth and anus. © Nancy Sefton/Photo Researchers, Inc.

Each coral animal is called a polyp. Polyps can live individually, but most live in colonies of many thousands. The word "coral" can refer either to these animals or to the odd-shaped limestone skeletons they secrete to support their delicate bodies.

Some polyps form hard protective cups around themselves. These external skeletons form the building blocks of the reef. Some are no bigger than a match head, but fused together they can form massive structures.

Though the main reef-builders are corals, many worms and even algae also help build reefs. Other animals contribute shells and bones to the reef when they die. All are cemented by coralline algae, which secrete a crust of limestone that fuses the reef together. Oyster and mussel shells can also pile into huge shoals, but they are rarely cemented together as densely as coral reefs.

Reef Formations
Coral reefs come in many shapes and sizes. Though all coral reefs grow upward toward the sunlight, the growth pattern of a reef is shaped by the land nearest it.

Fringing reefs start at the coastline and gradually extend seaward as they grow.

Barrier reefs run parallel to the coastline but are separated from it by water. They start out as fringing reefs, but a gap forms when the land mass near them sinks over millions of years. The reef sinks too, but as it is constantly growing upward, the top remains near the surface.

Atoll reefs begin life as a fringing reef around a volcanic island. As the island slowly subsides below the surface, the coral anchored to it keeps growing upward, and eventually all that remains is a ring-shaped reef with a central lagoon where the volcano once was.