Nearctic Coral Reef and Cays - Code: Ne12G

Just below the surface of shallow tropical waters lie one of the most diverse habitats on the planet. Coral Reefs form only in a zone extending from approximately 30° N to 30° S of the equator. Tropical corals need warm water and significant amounts of sunlight and do not grow at depths of over 160 ft (50m). The optimum temperature for most coral reefs is extremely narrow 79–81 °F (26-27°C), and few reefs exist in waters below 64 °F (18 °C). Most reefs are less than 10,000 years old and formed as sea-levels rose following the end of the most recent glacial maximum. Depending on the underwater topography Coral Reefs can form as near-shore Fringe Reefs, Barrier Reefs which are separated by a deepwater channel, or isolated platform reefs.
Coral Reefs across the globe are formed from the skeletons and living biomass of tiny colonial cnidarians collectively called corals. A coral "group" denotes a cluster of numerous genetically identical polyps. Each individual polyp represents a sac-like organism, usually just a few millimeters in diameter and a few centimeters in height with tentacles enciricling a central mouth opening. These polyps secrete a calcium carbonate exoskeleton close to their base. Over numerous generations, the colony develops a skeleton with structural qualities unique to that species. Typically, stony corals form mounds, large branching structures, or plates all of which create unique microhabitats and structures for other animals. Mature coral groups can reach dimensions of several meters. The growth of individual colonies transpires through the asexual reproduction of polyps. In some cases, corals engage in sexual reproduction through spawning: polyps from the same species release gametes simultaneously during the night, frequently around the time of a full moon. Once fertilized, the eggs give rise to planulae, which is an early mobile stage of the coral polyp. Upon maturation, these planulae settle down and establish a new colony. Living corals grow on top of dead coral skeletons and provide important structural habitat and food for thousands of other species of soft coral, bivalves, sponges and anemone that are also important components of the reef.
As ocean waters transporting sediment cross a reef they may deposit sand in leeward areas. If enough sediment accumulates on the reef it may eventually form small sandy island called a cay. Cays are largely unvegetated but overtime larger cays may develop soil and plant life, especially when assisted by large deposits of guano from nesting seabirds. These small sandy islets are a good predator-free nesting area for tropical seabirds and share many wildlife characteristics with Nearctic Seacliffs and Offshore Islands.
The third habitat type included in here is the Hawaiian Atolls. Atolls are circular reefs that surround a central lagoon. Atolls may be entirely submerged reefs or consist of rings of cays. There are two competing theories about atoll formation – the first posits that atolls form from barrier reefs surrounding volcanic islands. As a volcanic island becomes extinct it will slowly subside below the oceans surface, leaving only the ring of barrier reefs behind. The other theory holds that atolls are derived from large plateau like reefs. Low ocean levels during the Pleistocene exposed these reefs which then collected rain water and eroded the central portion of the plateau. Whatever the origin, the result is the same.