Temporal range: 245.0–0Ma Triassic[verification needed] to recent
|Diploria labyrinthiformis, a species of Brain coral|
Bourne, 1900 
29, see text
Scleractinia, also called stony corals, are marine corals that generate a hard skeleton. They first appeared in the Middle Triassic and their relationship to the tabulate and rugose corals of the Paleozoic is currently unresolved. Much of the framework of modern coral reefs is formed by scleractinians. Stony corals numbers are expected to decline due to the effects of global warming.
There are two groups of Scleractinia:
- Compound corals live in colonies in clear, oligotrophic, shallow tropical waters; they are the world's primary reef-builders.
- Solitary corals are found in all regions of the oceans and do not build reefs. In addition to living in tropical waters some solitary corals live in temperate, polar waters, or below the photic zone down to 6,000 m (20,000 ft)
Stony corals may be solitary or compound. Most have very small polyps, ranging from 1 to 3 mm (0.039 to 0.118 in) in diameter, although some solitary species may be as large as 25 cm (9.8 in). The most common forms include conical and horn-shaped polyps. Colonies can reach considerable size, consisting of a large number of individual polyps.
The skeleton of an individual scleractinian polyp is known as a corallite. It is secreted by the epidermis of the lower part of the body, and initially forms a cup surrounding this part of the polyp. The interior of the cup contains radially aligned plates, or septa, projecting upwards from the base. Each of these plates is flanked by a pair of thin sheets of living tissue termed mesenteries.
The septa are secreted by the mesenteries, and are therefore added in the same order as the mesenteries are. As a result, septa of different ages are adjacent to one another, and the symmetry of the scleractinian skeleton is radial or biradial. This pattern of septal insertion is termed "cyclic" by paleontologists. By contrast, in some fossil corals, adjacent septa lie in order of increasing age, a pattern termed serial and produces a bilateral symmetry. Scleractinians are also distinguished from the Rugosa by their pattern of septal insertion[vague]. They secrete a stony exoskeleton in which the septa are inserted between the mesenteries in multiples of six.
The modern scleractinian skeleton, which lies external to the polyps that make it, is composed of calcium carbonate in the form of aragonite. However, a prehistoric scleractinian (Coelosimilia) had a non-aragonite calcium carbonate skeletal structure. The structure of both simple and compound scleractinians is light and porous, rather than solid as in the Rugosa.
In colonial Scleractinia, the repeated asexual division of the polyps causes the corallites to be interconnected, thus forming the colonies. Also, cases exist in which the adjacent colonies of the same species form a single colony by fusing. The living polyps are connected by horizontal sheets of tissue extending over the outer surface of the skeleton and completely covering it. These sheets are outgrowths of the main body of the polyp, and include extensions of the gastrovascular cavity, so that food and water can constantly circulate between all the different members of the colony.
In scleractinians, the two main secondary structures are:
- Stereome is an adherent layer of secondary tissue, which covers the septal surface. It consists of transverse bundles of aragonitic needles and protects the polyps. However, its function can be nullified by the thickening of the septa itself.
- Coenosteum is a perforated complex tissue that separates individual corallites in a compound scleractinian.
At the beginning of Scleractinia’s development, four groups with different microstructure can distinguished. These are:
- Pachytecal corals having very thick walls and rudimentary septa. This is the group which probably originated from Rugosa corals.
- Thick trabecular corals have septa built from thick structures, resembling little beams, called trabecules.
- Minitrabecular corals have septa built from thin trabecules.
- Fascilcular or nontrabecular corals have septa not built from trabecules, but from columns composed of bunches of aragonite fibres.
Ecology and life history
Scleractinians fall into one of two main categories:
- Zooxanthellate (hermatypic)
- Non-zooxanthellate (ahermatypic)
In hermatypic corals, the endodermal cells are replete with zooxanthellae symbiotic algae. These symbionts benefit the corals because nearly 95% of the organic compounds produced by zooxanthellae are used as food by the polyps. The oxygen byproduct of photosynthesis and the additional energy derived from sugars produced by zooxanthallae enable these corals to grow as much as three times faster as those without symbionts. These corals are restricted to shallow (less than 200-ft-deep, well-lit, warm water with moderate to brisk turbulence and abundant oxygen) and prefer firm, nonmuddy surfaces on which to settle.
Most stony corals feed on zooplankton, but those with larger polyps take correspondingly larger prey, including various invertebrates and even small fish. In addition to capturing prey with their tentacles, many stony corals also produce mucus films they can move over their bodies using cilia; these trap small organic particles and are then pulled into the mouth. In a few stony corals, this is the primary method of feeding, and the tentacles are reduced or absent.
Stony corals are generally nocturnal, with the polyps retreating into their skeletons during the day, but a number of exceptions to this general rule occur.
Non-zooxanthellate corals are usually not reef formers and can be found most abundantly beneath about 500 m (1,600 ft) of water. They thrive at much colder temperatures and can live in total darkness, deriving their energy from the capture of plankton and suspended organic particles. The growth rates of most species of non-zooxanthellate corals are significantly slower than those of their counterparts, and the typical structure for these corals is less calciferous and more susceptible to mechanical damage than that of zooxanthellate corals. The rate at which a stony coral colony lays down calcium carbonate depends on the species, but some of the branching species can increase in height or length by around 10 cm (3.9 in) a year (about the same rate at which human hair grows). Other corals, like the dome and plate species, are more bulky and may only grow 0.3 to 2 cm (0.12 to 0.79 in) per year.
Stony corals can reproduce both sexually and asexually. Many species have separate sexes, but others are hermaphroditic. Sexual reproduction results in the birth of a free-swimming planula larva that eventually settles to form a polyp. In colonial species, this initial polyp then repeatedly divides asexually, to give rise to the entire colony.
The two main controls on the form of a Scleractinian colony are the mode of budding and the relative growth rate. The two types of budding are intratentacular and extratentacular. In an intratentacular budding, polyps are divided by simple fission across the stomodaeum, and each bud retains part of the original stomodaeum and regenerates the rest. Extratentacular budding takes place outside the tentacular ring of the parent. These daughter buds do not share any part in the functions within the parent scleractinians as do the products of intratentacular budding.
The overwhelming majority of Scleractinia taxa are hermaphroditic in their adult colonies. In these species the usual pattern is synchronized release of eggs and sperm into the water during brief spawning events. Fertilization occurs subsequently. Immediately after spawning, the eggs are delayed in their capability for fertilization until after the release of polar bodies. This delay, and possibly some degree of self-incompatibility, likely increase the chance of cross-fertilization. A study of four species of Scleractinia found that cross-fertilization was actually the dominant mating pattern, although three of the species were also capable of self-fertilization to varying extents.
There are two main hypotheses about the origin of Scleractinia. The closest scleractinian analog in the Paleozoic is the Rugosa, which suggests direct, possibly polyphyletic, descent, with different scleractinian suborders having originated in different rugosan families. The second hypothesis suggests the similarities of scleractinians to rugosans are due to a common non-skeletalized ancestor in the early Paleozoic. Recently discovered Paleozoic corals with aragonitic skeletons and cyclic septal insertion - two features that characterize Scleractinia - have strengthened the hypothesis for an independent origin of the Scleractinia.
The evolutionary relationships among stony corals were first examined in the 19th and early 20th centuries. Early classification schemes used anatomical features of the polyps to propose evolutionary relationships. The two most advanced 19th century classifications both used complex skeletal characters; Milne Edwards and Haime’s 1857 classification was based on macroscopic skeletal characters, while Ogilvie’s 1897 scheme was developed using observations of skeletal microstructures, with particular attention to the structure and pattern of the septal trabeculae.
Vaughan and Wells in 1943, and Wells in 1956, used the patterns of the septal trabeculae to divide the group into five suborders. In addition, they considered polypoid features such as the growth of the tentacles. They also distinguished families by wall type and type of budding.
Alloiteau’s 1952 classification built off of these earlier studies but included more microstructural observations and did not involve the anatomical characters of the polyp. Alloiteau recognized eight suborders. Bryan and Hill, in 1942, stressed the importance of microstructural observations by proposing that stony corals begin skeletal growth by configuring calcification centers, which are genetically derived. Therefore, diverse patterns of calcification centers are vital to classification. Alloiteau later showed that established morphological classifications were unbalanced and that the comparison of micro and macrostructural characters uncovered many convergences (convergent evolution) between fossils and recent taxa.
The rise of molecular techniques at the end of the 20th century prompted new evolutionary hypotheses that were different from ones founded on skeletal data. Results of molecular studies explained a variety of aspects of the evolutionary biology of the Scleractinia, including connections between and within extant taxa and supplied support for hypotheses about extant corals that are founded on the fossil record.
Through Romano and Palumbi’s 1996 analysis of mitochondrial RNA, it was found that molecular data supported the assembling of species into the existing families (biology), but not into the traditional suborders. For example, some genera affiliated with different suborders were now located on the same branch of a phylogenetic tree. In addition, there is no distinguishing morphological character that separates clades, only molecular differences.
Veron et al. analyzed ribosomal RNA in 1996 to obtain similar results to Romano and Palumbi, again concluding that the traditional families were plausible but that the suborders were incorrect. Veron et al. also established that stony corals are monophyletic, including all the descendants of a common ancestor, but that they are divided into two groups, the robust and complex clades. He suggested that both morphological and molecular systems be used in future classification schemes.
- Montlivaltiidae †
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Tubastrea cup coral from East Timor