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Marine corals are colonial animals, except for exceptions, belonging to the phylum Cnidaria, most of the class Anthozoa although some are of the class Hydrozoa (like Millepora). The colonies, formed by hundreds or thousands of individuals called zooids, can reach large dimensions. In tropical and subtropical waters they form large reefs. The term "coral" has no taxonomic significance and it includes different types of organisms.
Although corals can catch plankton and small fish aided by the stinging cells of their tentacles, most corals obtain most of their nutrients from the photosynthetic unicellular algae, called zooxanthellae, which live within the coral tissue. These corals require sunlight and grow in clear, shallow water, usually at depths below 60 meters. Corals can be the main contributors to the physical structure of coral reefs that formed in tropical and subtropical waters, such as the huge barrier Reef in Australia and the Mesoamerican Caribbean Sea. Other corals, which do not have a symbiotic relationship with algae, can live in much deeper waters and in much lower temperatures, such as the species of the genus Lophelia that can survive to a depth of 3000 meters. Both in the world of diving and aquarium, corals are divided into soft and hard, as they have a skeleton or not. And the hard, in turn, are subdivided into corals of short hard polyps and long hard polyp. But this division is unscientific and generates not a few exceptions under a rigorous analysis of the various species. The scientific community refers to micro polyps when coral polyps are between 1 and 2 mm in diameter and refers to macro polyps for polyps between 10 and 40 mm in diameter. However, the vast majority of corals from all reefs in the world have polyps with a diameter between 2 and 10 mm, just between those categories.
The corals are divided into two subclasses depending on the number of tentacles or lines of symmetry: Hexacorallia and Octocorallia, and a series of orders depending on their exoskeletons, type of nematocysts and mitochondrial genetic analysis. The common classification of corals crosses the limits of the suborder/class.
The hermatypic corals are stony corals that build reefs. They secrete calcium carbonate to form a hard skeleton. They obtain a part of their energetic requirements of zooxanthellae, (symbiotic photosynthetic algae).
Ahermatypic corals do not build reefs because they do not generate a skeleton. They have eight tentacles and are also known as Octocorals, subclass Octocorallia. They include the corals of the order Alcyonacea, as well as some species in the order Antipatharia (black Coral, Cirrhipathes genera and Antipathes). Ahermatypic corals, such as the Gorgonians and Sessiliflorae, are also known as soft corals. Unlike stony corals, they are flexible, undulating in streams of water, and are often perforated, with a lace appearance.
Corals can be porous or non-porous. The former have porous skeletons which allow their polyps to connect among themselves throughout the skeleton. Non-porous hard corals have solid and hard skeletons. Huge concentrations at 450 meters deep, in the heart of the Atlantic, have been discovered. Virtually, nothing is known of the pelagic life of large turtles, and specifically of the huge and extremely rare turtle leatherback (Dermochelys coriacea). Also, there is no certainty as to the place in that huge basking shark spends most of the year. The everyday behaviour of sovereign marine fish of the sea is still often a mystery to the scholars of the world ocean.
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Polyps feed on a variety of small organisms, from microscopic demersal plankton to small fish. The tentacles of the polyp immobilize or kill their prey with their nematocysts. Then they contract to direct the prey to the stomach. Once the prey is digested, the stomach is re-opened, allowing the expulsion of waste and the beginning of the next hunting cycle. Polyps also collect organic molecules and dissolved organic molecules.
In terms of reproduction, there are species of sexual reproduction and asexual reproduction, and in many species where both forms are given. The sex cells are expelled to the sea all at once, following signs like the lunar phases or the tides. Fertilization is usually external, however, some species keep the egg inside (gastrovascular cavity) and that is where the eggs are fertilized. And the sets are so numerous that they come to dye the waters. Many eggs are eaten by fish and other marine species, but it is so much that although the survival rate varies between 18 and 25%, according to marine biology studies, the survivors guarantee the continuity of the species.
The eggs once on the outside, remain adrift drifted by the currents several days, later it forms a planula larva which, after wandering through the column of seawater, adheres to the substrate or rocks and begins its metamorphosis to become a polyp and a new coral.
Corals are mainly sexually reproduced. About 25% of the hermatypic corals (stone corals), form colonies composed of polyps of the same sex (unisexual), while the rest is hermaphrodite.
About 75% of all hermatypic corals spawn by diffusion, releasing gametes of eggs and sperm into the water to propagate their offspring. The gametes are fused during fertilization to form a microscopic larva, called planula, typically pink and elliptical in shape. A coral colony produces thousands of larvae per year to overcome obstacles that hinder the formation of a new colony.
Synchronous spawning is very typical of coral reefs, and often, even when several species are present, all corals spawn on the same night. This synchrony is essential to allow the male and female gametes to be found. Corals rely on environmental signals, which vary from species to species, to determine the appropriate time to spread the gametes. These signs include changes of temperature, lunar cycle, duration of the day and possibly chemical signs. The synchronous spawning can form hybrids and is possible that speciation of the coral may be involved. The immediate sign for the spawning is often the sunset. The event can be visually spectacular when millions of gametes concentrate in certain zones of the reefs.
Incubator species often they are ahermatypic (they are not constructors of the reef) and live zones with very much surge or current forts of water. The species incubators only liberate sperm without buoyancy, which they sink to the carriers of eggs that wait with the eggs not fertilized for weeks. Nevertheless, it is also possible that synchronous spawning happens with these species. After the fertilization, the corals liberate planulae, you list to establish itself in a suitable substratum.
The planulae exhibit positive phototaxis, swimming towards the light to reach the superficial waters where they derive and grow before descending in search of a hard surface a new colony to establish and to be begun. Also, they exhibit positive sonotaxy, moving towards the sounds that come from the reef, moving away from opened waters. Many stages of this process may be affected by high rates of failure, and though thousands of gametes are liberated by the colony, a few ones are those who manage to form a new colony. The period of the spawning to the accession in a new substrate lasts in general from two to three days, though it can be late up to two months. The larva turns into a polyp and finally, it turns into a colony of coral by means of gemmation and asexual growth.
Within a coral colony, genetically identical polyps are reproduced asexually, either through gemmation or by longitudinal or transverse division.
The gemmation is to separate a minor polyp from an adult. As the new polyp grows, the body parts form. The distance between the new polyp and the adult grows, and with it, the Cenosarc (the common body of the colony). The gemmation can be:
- Intratentacular — from their oral discs, producing polyps of the same size inside the tentacle ring.
- Extratentacular — from its base, producing a minor polyp.
The division can form two polyps, as large as the original.
- Longitudinal division — begins when a polyp widens and then divides its coelenteron, analogous to the longitudinal division of a trunk. The mouth is also divided and forms new tentacles. Then the two "new" polyps generate the other body parts and the exoskeleton.
- Transverse division — occurs when the polyps and the exoskeleton are divided transversely into two parts. This means that one part has the basal disk (the lower part) and the other has the oral disc (upper part), similar to cutting the end of a log. The new polyps have to generate the pieces that are missing individually.
The asexual reproduction has several benefits for these colonial sessile organisms:
- The cloning allows discharges rates of reproduction and a rapid exploitation of the habitat.
- The modular growth allows the increase of the biomass without a corresponding decrease in the relation surface - volume.
- The modular growth delays the aging, on having allowed that the clone can survive the loss of one or more modules.
- The new modules can replace the dead modules, reducing the mortality of the clones and preserving the territory occupied by the colony.
- The clones' diffusion to distant places reduces the mortality of clones caused by located threats.
Entire colonies can reproduce asexually forming two colonies with the same genotype.
- Fission happens in some corals, especially inside the family Fungiidae, in which the colony divides into two or more colonies during the first stages of development.
- Abandon takes place when just one polyp leaves the colony and anchors on a different bed to create a new colony.
- Fragmentation involves individual polyps removed from the colony during storms or other disturbances. The separated polyps can initiate new colonies.
The polyps of coral die with the time, but the calcareous structures are kept and can be colonized by other polyps of coral, which will continue creating calcic structures generation after generation. Along thousands or million years, big calcareous structures known as coral reefs are formed.
On occasions, the reefs are so big that they can manage to emerge from the surface. This way, when the coral grows on a volcanic island that later sinks, a coral structure in the shape of a ring is created with a central lagoon that receives the name of an atoll.
The reef of major length is the Great Barrier Reef, on Queensland's coast in Australia: it has more than 2,000 km² and is one of the biggest natural constructions in the world. The region of the world with more coral species and more biodiversity in its coral reefs is the Triangle of Coral, in Southeast Asia, which includes more than 500 species of corals (76 % of the coral known species) and at least 2,228 fish species.
The second biggest coral reef in the world is the Indo-American Reef (following the coast of Mexico, Belize, Guatemala and, finally, Honduras), is in the Caribbean sea, and spreads over more than 700 km from the Yucatan peninsula up to the Bay Islands on the north coast of Honduras. Even if it measures a third of what measures the Great Reef Barrier in Australia, the Indo-American Reef shelters a great diversity of organisms, including 60 types of corals and more than 500 fish species.
The coral reefs are decreasing all over the world. The principal threats determined for the coral ecosystems are the extraction of coral, the agricultural and urban run-off, the organic and inorganic pollution, the overexploitation of fisheries, the fishing with explosives, the coral diseases and the digging of channels to access to islands and bays. The widest threats include the increase of the temperature of the sea, the rise of the level of the sea, and the change of the pH due to ocean acidification, all of them associated with the emission of greenhouse gases. In 1998, 16 % of the total of coral reefs died as a consequence of the increase of the sea temperature.
Changes in water temperature of more than 1-2 degrees Celsius or changes in salinity, can eliminate the corals. Under such environmental pressures, the corals expel his zooxanthellae; without them, the tissues of the coral reveal the white of its skeleton, an event known as whitening coral. Global estimations indicate that approximately 10 % of the total of the coral reefs is dead. About 60 % of the coral reefs is at risk as a result of human activities. It is believed that the destruction of the coral reefs can reach 50 % in the year 2030. In response, most nations have established environmental laws in an attempt to protect this important aquatic ecosystem.
Between 40 % and 70 % of the common algae, transfer soluble-lipid metabolites causing discoloration and death between the corals, particularly when there is an overcrowding population of algae. The algae proliferate when they take sufficient nutrients as a result of organic pollution, and when overfishing dramatically reduces the shepherding for herbivores, as the dark brown fish.