The tropical location of the Great Barrier Reef is ideal for more than just the 2 million tourists who visit each year [source: Australian Government]. The sun and the shallow, clear, warm waters are perfect for coral growth. And where there's an abundance of healthy coral, you can bet there's a lot of other marine life as well. Reefs host more animal species than any other marine ecosystem [source: Microsoft Encarta].
Along with the 1,500 species of fish mentioned earlier, the reef also hosts more than 200 species of birds, 4,000 types of mollusks, 30 species of whales and dolphins, and six species of sea turtles [source: Australian Government, Microsoft Encarta] On just one chunk of coral, researchers found 1,441 sea worms from 103 different species, and 250 types of shrimp on a single reef [source: Chadwick].
The Great Barrier Reef is one of the few places where you might see a dwarf goby fish smaller than your fingernail, a whale shark bigger than your car and an overstuffed giant grouper on just one dive. The "non-fish" species are equally as stunning. Giant clams, whose rainbow-colored mantles are as unique as fingerprints, can reach up to 4 feet (1.2 meters) in length and weigh more than 440 pounds (200 kilograms) [source: National Geographic].
While you'll want to give all animals in the Great Barrier Reef some space, you may want to cut a wider berth around some, like the 20 species of sea snakes, some of which possess venom 20 times stronger than the most poisonous land snake [source: Zell]. Another critter you have to watch out for is the miniscule blue-ringed octopus. At a mere five to 20 centimeters, this poisonous mollusk has surprised greedy divers who try to sneak shells into their wetsuits only to be bitten by the venomous octopus lurking inside [source: Zell]. Coral, too, can pack a serious punch. Just barely brushing up against the stinging fire coral will leave you with a burning itch.
Another marine creature is deadly not to people but to the reef itself. The crown-of-thorns starfish, which feeds on coral, has devastated the reef on two separate occasions since the early 1960s. During an outbreak, thousands of these starfish descend on the reefs and feast on the fragile polyps, leaving a path of destruction in their wake. Algae often colonize the resulting coral graveyard, preventing the coral from swiftly recovering.
Reefs hit by the crown of thorns, one of the largest starfish in the ocean, can take more than 10 years to regenerate [source: Sammon]. Scientists are still trying to pinpoint what causes the sudden explosions of the sea stars, suggesting it could be anything from runoff to overfishing [source: Sammon].
Damage caused by starfish notwithstanding, the Great Barrier Reef has fared relatively well in comparison to other coral reefs; its reefs are ranked among the world's least threatened [source: Microsoft Encarta]. You'll find out how the reef has managed to stay out of danger on the next page.