A coral polyp is a tubular saclike animal. Coral colonies vary in size. Some corals form only small colonies. Others may form colonies several feet (a few meters) high. Star coral (Montastrea annularis) colonies reach an average height of 10 to 13 ft. (3-4m).
The world's first coral reefs occurred about 500 million years ago, and the first close relatives of modern corals developed in southern Europe about 230 million years ago. By comparison, the Great Barrier Reef is relatively young at just 500,000 years old. The existing reef's structure is even younger; less than about 8,000 years old.Great Barrier Reef is relatively young Reefs grow horizon- tally once they reach sea level
Hard (stony) corals (Order = Scleractinia) have six (or a multiple of six) tentacles on their polyps. They have an internal limestone skeleton. Soft corals (Order = Alcyonacea) have eight tentacles on their polyps. They are soft or leathery in texture and have limestone sclerites instead of a solid sketelon). Soft corals are closely related to gorgonians (sea fans)
Most newly-settled corals contain zooxanthellae (pronounced zo-zan- thel-y), the single-celled algae that live inside the coral animal and help to supply it with food. Providing enough light for the zooxanthellae is one reason why most corals are found only in clear, shallow water. As the coral grows, the polyp divides repeatedly and produces more skeleton. The way the polyp grows and divides will determine the shape of the new coral colony. Growth is initially slow, so that after one year, the coral will still be quite small, perhaps 1 cm in diameter. After the first year, growth is more rapid, and a staghorn coral (Acropora) can increase in diameter at more than 15 cm per year.
TEMPORAL TRENDS IN THE FATALITY RATES OF SHARK ATTACKS ON DIVERS
Fatalities per attack Number of attacks Both, the frequency of attacks on divers and the proportion of fatal attacks on divers, show declining trends.
THE DIVING ACTIVITIES OF THE VICTIM (N=281) THE ACTIVITIES OF SHARK ATTACK VICTIMS, AS A FUNCTION OF DECADE (N=1459)
Sea surface conditions at the sites of shark attacks on divers
FATALITY RATE AS A FUNCTION OF ATTACK DEPTH DEPTH OF ATTACKS ON SCUBA DIVERS (N=55) DEPTH OF ATTACKS ON SNORKELERS AND FREE DIVERS (N=128)
http://www.zoo.co.uk/~z9015043/index.html Blacktip reef shark Blue shark
Coral reefs are underwater wave-resistant mounds that are virtual ecosystems all by them-selves. They are made up of corals, algae, mollusks, bryozoans, brachiopods, echinoderms, and sponges. Coral sands and solid limestone also play a large part in the buildup of a coral reef. Coral reefs grow upward by a rock like accumulation of calcium containing (calcareous) exoskeletons of past generations of coral animals.
Coral reefs are home to 25% of all marine species. However, the tiny colonial animals that build these intricate limestone masses are dying at alarming rates. If this trend continues, in 20 years the living corals on many of the world’s reefs will be dead and the ecosystems that depend on them severely damaged. The beauty and abundance of life in coral reef habitats, like this one in Florida, attract a variety of visitors, including sport divers and recreational fishermen. Such activities in U.S. waters alone generate billions of dollars a year for the tourist industry. http://geopubs.wr.usgs.gov/fact-sheet/fs025-02/
Extensive coral reefs are found in the waters of the United States and its territories. In the Atlantic Ocean, Gulf of Mexico, and the Caribbean Sea these include reefs off Florida, Texas, Puerto Rico, and the U.S. Virgin Islands. In the Pacific Ocean, they include those of the Hawaiian Islands, Wake Island, Johnston Atoll, the Northern Marianas, Saipan, Guam, Kingman Reef and Palmyra Atoll, Howland Island, Baker Island, Jarvis Island, and American Samoa. More than 60% of the Nation’s coral reefs are found in the extended Hawaiian Island chain.
Atoll Colonial “hard corals” form elaborate finger-shaped, branching, or mound- shaped structures and can create masses of limestone that stretch for tens or even hundreds of miles. Many coral reefs fringing coasts consist of nearshore inner reef flats that slope to deeper water fore reefs farther offshore. The reef crest, between the inner reef flat and outer fore reef, lies in extremely shallow water and may be exposed during the lowest tides. Waves commonly crash against or break on the reef crest. Fringing reefs help to protect harbors, beaches, and shorelines from erosion and wave damage by storms.
To illustrate cnidarian - zooxanthellae symbiosis, here are photos of zooxanthellae in the soft coral Xenia. Because the zooxanthellae are plants, they need sunlight to photosynthesise, so they need to find a part of their host animal which is not regularly shaded, or so far beneath the skin, that sunlight can't penetrate. In Xenia they live in the oral tentacles which during the day are fully extended, soaking up the sunlight.
Another excellent example of mutualistic symbiosis is presented by the clownfish that lives in the poisonous tentacles of tropical sea anemones. The latter animals are capable of stinging most fishes, but clownfish possesses a special mucous coating that somehow prevents the discharge of the anemone's stinging cells. Both partners apparently benefit from the relationship. The fish is protected from predators by the stinging anemone cells while the anemone is guarded against anemone-eating fishes by its highly territorial fish occupants.