Scleractinia
Encyclopedia
Scleractinia, also called stony coral
s, are exclusively marine animals; they are very similar to sea anemone
s 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:
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.
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:
At the beginning of Scleractinia’s development four groups with different microstructure can distinguished. These are:
Scleractinians fall into one of two main categories:
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 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 cm (0.118110236220472 to 0.78740157480315 in) per year.
. 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.
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.
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.
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, are exclusively marine animals; they are very similar to sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s but generate a hard skeleton. They first appeared in the Middle Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
and replaced tabulate
Tabulate coral
The tabulate corals, forming the order Tabulata, are an extinct form of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to a honeycomb. Adjacent cells are joined by small pores...
and rugose corals that went extinct at the end of the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "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 reefReefIn nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
-builders. - Solitary corals are found in all regions of the oceans and do not build reefs. Some live in temperateTemperateIn geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
, polarPolar regionEarth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...
waters, or below the photic zonePhotic zoneThe photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...
down to 6000 meters.
Anatomy
Stony corals may be solitary or compound. Most have very small polyps, ranging from 1 to 3 mm (0.0393700787401575 to 0.118110236220472 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
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
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 carbonateCalcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
in the form of aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
. However, a prehistoric scleractinian (Coelosimilia
Coelosimilia
Coelosimilia is a genus of extinct scleractinian coral from the Late Cretaceous period. The specimens were found in rocks around 70 million years old dating from the Late Cretaceous of the Mesozoic Era. Coelosimilia is similar to modern-day scleractinians, except for the composition of its...
) 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
Rugosa
Disambiguation:The Rugosa Rose is also sometimes just called "Rugosa". For the moon in Star Wars: The Clone Wars, see .The Rugosa, also called the Tetracoralla, are an extinct order of coral that were abundant in Middle Ordovician to Late Permian seas.Solitary rugosans are often referred to...
.
The skeleton of an individual scleractinian polyp
Polyp
A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...
is known as a corallite. It is secreted by the epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...
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
Septum
In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...
, projecting upwards from the base. Each of these plates is flanked by a pair of thin sheets of living tissue termed mesenteries
Mesentery
In anatomy, the mesentery is the double layer of peritoneum that suspends the jejunum and ileum from the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity.-Mesentery :The...
.
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
Symmetry (biology)
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry .In nature and biology,...
. 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
- Zooxanthellate (hermatypic)
- Non-zooxanthellate (ahermatypic)
In hermatypic corals, the endodermal cells are replete with zooxanthellae symbiotic algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
. 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 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
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
, 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
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
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 cm (0.118110236220472 to 0.78740157480315 in) per year.
Life history
Stony corals can reproduce both sexually and asexually. Many species have separate sexes, but others are hermaphroditicHermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
. Sexual reproduction results in the birth of a free-swimming planula
Planula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
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
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
is the Rugosa
Rugosa
Disambiguation:The Rugosa Rose is also sometimes just called "Rugosa". For the moon in Star Wars: The Clone Wars, see .The Rugosa, also called the Tetracoralla, are an extinct order of coral that were abundant in Middle Ordovician to Late Permian seas.Solitary rugosans are often referred to...
, 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
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
.
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
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
) 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
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, it was found that molecular data supported the assembling of species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
into the existing families (biology), but not into the traditional suborders. For example, some genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
affiliated with different suborders were now located on the same branch of a phylogenetic tree
Phylogenetic tree
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
. 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.