In oceans around the world, clear and well-circulating waters encourage corals to anchor and grow. Despite their plant or stone like appearance, corals are actually animals. They come in many colorful forms and textures.

 

Starting as tiny cylindrical polyps just a couple of millimeters in diameter, they grow about half an inch a year and can become several feet in diameter. Corals form reefs as successive generations build on the skeletal remains of older generations. Reefs can be hundreds of feet wide and stretch for miles.

 

The greatest collection of reefs is located off the northeast coast of Australia, the Great Barrier Reef. This collection of reefs encompasses 135,000 square miles.

 

Thousands of species of reef fish，mollusks， sea stars and other creatures inhabit coral reefs. The reef system's spiny grottos offer refuge and feeding grounds for a variety of aquatic life.

 

In spring, this Caribbean reef plays host to the fanatic mating of fish. A male yellow head garfish is left alone to tender fertilized eggs he carries in his mouth. He spits them out from time to time in order to aerate them over the 5-day gestation period.

 

In this aquatic community, unusual relationships develop. A cleaning station thrives around this coral head, often patronized by the tiger grouper. Tiny gobies fearlessly enter the tiger's mouth to feed on dead tissue and parasites. But just as the reef fosters life, it can harbor danger. Lurking on the fringes of this Caribbean reef, the barracuda is a fierce predator equipped with razor-sharp teeth. Although they do hunt in groups, sometimes adults branch off and prowl for food on their own. This small ballyhoo fish is a mere snack for this hungry barracuda.

 

Coral reefs have been evolving for about 500 million years, but these days they are under threat. Global warming, pollution and overfishing have contributed to their decline. Earnest efforts are under way to protect the world's reefs and restore them. Artificial reefs created from sunken ships and other man-made objects have shown some short-term promise. But man's impact on the environment continues to make the future of coral reefs uncertain.