There are two types of corals, hard and soft. Hard corals have an internal, rock-like, chalky skeleton that remains when they die. Huge colonies of hard corals form coral reefs. Soft corals do not have a hard skeleton.

Habitat and Distribution: Corals are found in seas around the world. Large colonies are found in some warm, shallow waters, where colonies of millions of coral polyps form vast coral reefs. Reefs are slow-growing; they only grow about an inch each year.

Life Cycle: A coral polyp begins its life as a tiny, free-swimming larva; the larva is only the size of the head of a pin. It settles on a hard support and will never move again. It reproduces by budding (in which an identical polyp sprouts out of the polyp's side) and by sexual reproduction (in which polyps release eggs and sperm, which mix in the water).

Anatomy: The soft body of a coral polyp is about the size of a pencil eraser. It has radial symmetry; some corals have 8 tentacles, others have multiples of 6. The polyp of hard corals makes a hard, protective shell out of calcium carbonate. When the polyp dies, the chalky skeleton remains, and another polyp will grow on top of the old one. Coral colonies grow in many shapes and come in many colors.

Many corals have symbiotic algae that live inside them. (Symbionts are two organisms that help one another.) These algae (called Zooxanthellae) help form the coral's stony exoskeleton.

Diet: Corals are carnivores that eat zooplankton (like copepods and tiny marine larvae). They catch food using tentacles that surround the mouth; the tentacles have poisonous stingers (called nematocysts).

Predators: Corals are eaten by the crown-of-thorns (a large starfish that eats coral polyps), parrotfish, and other animals.

Classification: Kingdom Animalia (animals), Phylum Coelenterata (corals, jellyfish, sea anemones, hydroids), Class Anthozoa meaning "flower-like animals" (corals and sea anemones).