Four oceans: Pacific, Atlantic, Indian, and Arctic

Pacific is largest and deepest64, 186,300square miles12,925 feet average depth

Fifth ocean delimited in 2000Southern Ocean north of Antarctica

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Corals are

colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built

from stony corals, which in turn consist of polypsthat cluster in groups. The polyps are like tiny sea anemones, to which they are closely related. But unlike sea anemones, coral polyps secrete hard

carbonate exoskeletons which support and protect their bodies. Reefs grow best in

warm, shallow, clear, sunny and agitated waters.

Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on Earth. They occupy less

than one tenth of one percent of the world's ocean surface, about half the area of France, yet they provide a

home for twenty-five percent of all marine species,[1]

including fish, molluscs, worms, crustaceans, echinoderms, sponges

, tunicates and other cnidarians.[2] Paradoxically, coral reefs flourish even though they are surrounded by ocean

waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but

deep water and cold water corals also exist on smaller scales in other areas.

Sunlight ( down to 650 feet)

Sea plants and many animals

Twilight (down to 3,300 feet)

Many different fish

Sunless ( down to 13,100 feet)

Animals feed on dead food from above.