5. Physical Description
Relatives of anemones and jellyfish, corals help to create reefs that house some of the greatest biodiversity on earth. Corals were first believed to be plant-animals but later were established to be solely marine invertebrate animals because they reacted to touch. Corals belong to the class Anthozoa of the Phylum Cnidaria. There are three subclasses: Hexacorallia, Octocorallia, and Ceriantharia. The subclass, Hexacorallia has the zoanthids, sea anemones, and stony corals. The Octocorallia has the seapens, gorgonia, blue coral, and the soft corals. The Ceriantharia are tube-dwelling corals and are solitary in nature. Most corals attach themselves to the substrate living in clumps and communities. The stony corals secrete a tough structure made of calcium carbonate around them. In soft corals, the hard outer structure is not present but instead, they possess sclerites for structural support. Tentacles that bring food to the mouth surrounds the polyp (body unit of the coral) of the coral. The tentacles also retract when not gathering food. Stony corals defend themselves by hiding inside their hard outer structures while soft corals release toxins that kill or paralyze their predators.
4. Diet and Coral Reef Formation
Since most corals are attached to the substrate, they feed by taking advantage of the water current that brings them an abundance of food. Small fish and tiny zooplanktons form the primary diet of corals. At the right moment, the nematocysts of the coral's tentacles are utilized to immobilize the prey. The tentacles then grab the prey, bringing it to the mouth of the coral. Later, the inedible parts and waste matter from the meal are released through the mouth. Some species of corals subsist on photosynthetic dinoflagellate algae that grow within its polyp structure. This symbiotic relationship feeds the algae while the coral gets energy. The photosynthetic process performed by the algae also helps to toughen the coral's outer calcium carbonate structure. Often, once the algae load on the coral polyp becomes stressful, the coral releases the algae. The loss of the brown colored algae thus results in coral bleaching. Corals are significant contributors to reef building activities. There are shallow-water reefs and deep-water reefs populated by coral builders. The majority of corals colonize shallow-water reefs with the help of energy obtained from photosynthetic algae harbored within them. Deep-water reef-building corals, however, do not harbor the symbionts. With the passage of time, most corals deteriorate and die-off. The succeeding generations of coral, debris accumulating between the dead corals, and other marine species contribute to the growth of reefs.
3. Habitat and Range
Corals in their primitive forms first appeared about 542 million years ago during the Cambrian period but became widespread only about 100 million years later. Most modern corals live in barrier reefs, fringing reefs, and atolls in warm waters of tropical and subtropical oceans. Coral reefs take millions of years to form. Today, coral reefs thrive in the Persian Gulf, Pacific Ocean, Indian Ocean, Caribbean Sea, and the Red Sea. The Great Barrier Reef, the world's biggest reef formation, located off the coast of Australia in the Coral Sea, stretches for a distance of 1,553 miles. The Red Sea Coral Reef is the second largest coral reef in the world extending for 1,180 miles, off the coasts of Israel, Egypt, and Djibouti. Third in line is the New Caledonia Barrier Reef at 932 miles, located off the coast of New Caledonia in the Pacific Ocean.
2. Threats and Conservation
Anthropological activities threaten the coral reefs around the world. Canal digging near inland bays and coves pose a threat to fringing reefs. Overfishing and blast fishing also are serious threats to the ecosystem that includes corals and fish population. Agriculture and pesticide runoffs also damage fringing reefs. Some corals are extra sensitive to water temperature changes and water salinity. Seaweed infestations that reach significant coral atolls also cause coral bleaching. Underwater springs that go directly to the ocean and transform the water salinity also threaten the sensitive reefs that cause coral and fish die-offs. Today, countries working with conservation groups have created many opportunities for coral reef protection. The creation of marine reserves, underwater parks, and heritage sites have all contributed to the saving and protection of coral reefs and its inhabitants. Additional efforts such as habitat protection and fishery management have also made coral conservation more effective. The use of corals as construction materials has been stopped. Jewelry and medicinal uses of certain types of corals have been reduced. Coral aquaculture has further enhanced coral restoration. This method entails the cultivation of coral fragments that are put back in the depleted reefs.
1. Reproduction and Life Cycle
Corals can multiply both asexually and sexually, but the latter is the predominant form of reproduction. Corals are either hermaphroditic or gonochoristic. Single sex colonies are formed by about 25% of stony corals. Reproduction is influenced by the cycle of the moon, length of daytime, and natural chemical signals. About 75% of the stony corals release their eggs and sperms into the water which after fertilization develops into tiny larvae called planulae. These larvae grow in the water until they are ready to settle on the substrate to form new colonies. Synchronous spawning is an interesting phenomenon exhibited by the corals. Multiple species of corals usually release their gametes at the same time. This phenomenon allows the formation of hybrids which might favor speciation. Another type of coral, the brooders inhabit waters with strong currents and waves. These corals release their sperms which sink in the waters to land on egg carriers loaded with eggs. Planula that develops after fertilization settles down for further growth. Division and budding are two forms of asexual reproduction in corals. The former phenomenon occurs when a smaller polyp buds out from the parent polyp. In the latter method, the adult coral splits into two equal sized polyps. The asexual reproduction methods endow regeneration abilities to the coral species, creating more individuals in a given population.