A reef is a nautical term for a chain of rocks or coral, or ridges of sand, at or just beneath the surface of the water. Such formations can be a hazard to ships.
The best-known reefs are those of tropical waters, developed through biotic processes dominated by corals and calcareous algae. Others result from abiotic processes such as sand deposits and erosion of rock outcrops by waves.
Biotic reef types
There are a number of biotic reef types, including oyster reefs, but the most massive and widely distributed are tropicalcoral reefs. Although corals are major contributors to the framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault from ocean waves are calcarous algae, especially, although not entirely, species of red algae.
Reefs can be created artificially either by special construction or through deliberately sinking ships, but one can argue that these "reefs" are not real ones, as it is seldom the case that an artificial obstruction would be created that is a hazard to shipping.
Thus, "artificial reef" is a misnomer, though firmly established as the term used for man-made underwater habitat structures.
In addition, reef structures within a sequence of sedimentary rocks provide a discontinuity which may serve as a trap or conduit for fossil fuels or mineralizing fluids to form petroleum or ore deposits.
Coralreefs are either restricted or absent from along the west coast of the Americas, as well as the west coast of Africa.
Coralreefs are home to a variety of tropical or reeffishes, such as the colorful parrotfishes, angelfishes, damselfishes and butterflyfishes.
Reefs are also home to a large variety of other organisms, including sponges, Cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, spiny lobsters and crabs), mollusks (including cephalopods), echinoderms (including starfish, sea stars, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes.
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