Scleractinia

Stony Corals Temporal range: Triassic to recent
Brain Coral (Diploria labyrinthiformis)
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Anthozoa
Order:	Scleractinia Bourne, 1900
Families
Suborder Astrocoeiina   Acroporidae   Astrocoeniidae   Pocilloporidae Suborder Caryophylliina   Caryophylliidae Suborder Dendrophylliina   Dendrophylliidae Suborder Faviina   Rhizangiidae   Faviidae   Meandrinidae   Merulinidae   Mussidae   Oculinidae Suborder Oculina   Pectiniidae   Trachyphyllidae Suborder Fungiina   Agariciidae   Fungiidae   Poritidae   Siderastreidae   Thamnasteriidae

Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian. Much of the framework of coral reefs is formed by scleractinians.

There are two groups of Scleractinia:

• Colonial corals found in clear, 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. Some live in temperate, polar waters, or below the photic zone down to 6000 meters.

Anatomy

Stony corals may be solitary or compound. Most have very small polyps, ranging from 1 to 3 millimetres (0.04 to 0.1 in) in diameter, although some solitary species may be as large as 25 centimetres (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.

In a colonial Scleractinia, the repeated asexual division of the polyps causes the corallites to be interconnected, thus forming the colonies. There are also cases 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.

Skeleton

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.

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 that is termed serial and that produces a bilateral symmetry. Scleractinians are also distinguished from the Rugosa by their pattern of septal insertion. They secrete a stony exoskeleton in which the septa are inserted between the mesenteries in multiples of six.

In scleractinians, there are two main secondary structures:

• 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 scleractinians.

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 with septa built from thick structures, resembling little beams, called trabecules.
• Minitrabecular: Corals with septa built from thin trabecules.
• Fascilcular or non-trabecular: Corals with septa not built from trabecules, but from columns composed of bunches of aragonite fibres.

Ecology and life history

Hard coral Favites extends its polyps at night to feed

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 carbons produced by zooxanthellae are used as food by the polyps. The oxygen byproduct of photosynthesis and 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 feet - 60 meters), well-lit, warm water with moderate to brisk turbulence and abundant oxygen and prefer firm, non-muddy 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 that 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 there are a number of exceptions to this general rule.

Non-zooxanthellate corals are usually non-reef formers and can be found most abundantly beneath about 500 m 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 centimetres (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 centimetres (0.12 to 0.79 in) per year.

Life history

  A Stony Coral

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.

There are two main controls on the form of a scleractinian colony. One is the mode of budding and the other is the relative growth rate. There are two types of budding: 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.

Evolutionary history

Scleractinian coral from the Matmor Formation (Jurassic) of Makhtesh Gadol, Israel.

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.

Classification

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.

References

• Ezaki, Yoichi (1998). "Paleozoic Scleractinia: progenitors or extinct experiments?". Palaeobiology. 24 (2): 227–234. doi:10.2307/2401240. {{cite journal}}: Cite has empty unknown parameters: |month= and |coauthors= (help)

External links

• Tree of Life - zoantharia

Gallery

• 
  Tubastrea cup coral from East Timor
• 
  Closeup of Diploria strigosa (a Brain coral), Snapper Ledge reef, Florida Keys
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  Thin section of Holocene Scleractinian coral from Rice Bay, San Salvador Island, Bahamas. Scale bar 100 microns.