Formation OF CORAL REEFS
Corals are constructed of many smaller organisms called polyps, which belong to the phylum Cnidaria and the class Anthozoa. A polyp has a sac-like body and an opening that serves as a mouth, surrounded by stringing tentacles called cnidae. The polyp builds a hard limestone skeleton around its vital and delicate body. Polyps are nocturnal for the most part, being that they stay inside their skeletons during the day. At night, however, polyps extend their tentacles in order to feed on zooplankton floating by. These polyps secrete a calcium carbonate exoskeleton (also called calyx or correlate) for protection. Hard, or true corals, build limestone skeletons under the living tissue. Soft corals instead secrete limestone crystal structures called sclerites, which are contained within a jelly-like layer beneath the polyps.
Image of coral polyps
HISTORY: Before the continents took the form hey have today (put in a year), there was a body of water called Tethys Sea, bordered in the north by Northern Europe and Asia and in the south by Southern Europe. Due to continental drift, the bordering land closed up on Tethys Sea, moving all of it's marine life toward the present day Indo-Pacific area. Due to warm water, and numerous islands, a healthy and abundant population of corals was fostered here (the greatest marine diversity is still found in the Indo-Pacific). As Africa moved north, the Atlantic corals and the life they supported was blocked from cross-fertilization with the creatures of the Indo-Pacific. Thus, these two regions developed separately, and today there are no species in common in the two regions.
There are three stages of development of coral reefs, discovered by Darwin in 1842, each consisting of increasingly independent structures:
FRINGING REEF: As the first stage, fringing corals form as a band along a coastline or around an island, gradually extending seaward. The best conditions of a substrate for developing corals are hardness, gentle slope, and clean water.
BARRIER REEF: Here the band of reef becomes separated from the substrate by a shallow lagoon. Barrier reefs form as the rate at which the land sinks and the reef rises become the same.
ATOLLS: As the last stage, land is completely submerged, leaving only a ring of reef around a central lagoon.
The lagoons attract marine turtles and dolphins, who use them as recreational and breeding areas where they are safe from predators (i.e. sharks).
The crest is the seaward side of the coral, impacted by tides and passing life, while the windward or lee side is more sheltered, and thus has a richer species diversity.