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111-553: About 35, see text . Scleractinia , also called stony corals or hard corals , are marine animals in the phylum Cnidaria that build themselves a hard skeleton . The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mouth is fringed with tentacles. Although some species are solitary, most are colonial . The founding polyp settles and starts to secrete calcium carbonate to protect its soft body. Solitary corals can be as much as 25 cm (10 in) across but in colonial species
222-601: A monophyletic group. The earliest scleractinians were not reef builders, but were small, phaceloid or solitary individuals. Scleractinian corals were probably at their greatest diversity in the Jurassic and all but disappeared in the mass extinction event at the end of the Cretaceous, about 18 out of 67 genera surviving. Recently discovered Paleozoic corals with aragonitic skeletons and cyclic septal insertion – two features that characterize Scleractinia – have strengthened
333-419: A symbiotic union of a fungus and photosynthetic algae or cyanobacteria , reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia , dust-like particles consisting of fungal hyphae wrapped around photobiont cells. Clonal Fragmentation in multicellular or colonial organisms is a form of asexual reproduction or cloning where an organism
444-556: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in
555-407: A character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make
666-497: A chemical cue accumulates and induces the transition to sexual reproduction. Many protists and fungi alternate between sexual and asexual reproduction. A few species of amphibians, reptiles, and birds have a similar ability. The slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In
777-402: 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 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
888-414: A fertilization event. These haploid individuals produce gametes through mitosis . Meiosis and gamete formation therefore occur in separate multicellular generations or "phases" of the life cycle, referred to as alternation of generations . Since sexual reproduction is often more narrowly defined as the fusion of gametes ( fertilization ), spore formation in plant sporophytes and algae might be considered
999-418: A form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy . However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle. Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into
1110-402: A group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,
1221-602: A group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below. Since
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#17327728433021332-564: A male gamete. Examples are parthenogenesis and apomixis . Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. It has been documented in over 2,000 species. Parthenogenesis occurs in the wild in many invertebrates (e.g. water fleas, rotifers , aphids, stick insects , some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish). It has also been documented in domestic birds and in genetically altered lab mice. Plants can engage in parthenogenesis as well through
1443-415: A mate becomes difficult. For example, female zebra sharks will reproduce asexually if they are unable to find a mate in their ocean habitats. Parthenogenesis was previously believed to rarely occur in vertebrates, and only be possible in very small animals. However, it has been discovered in many more species in recent years. Today, the largest species that has been documented reproducing parthenogenically
1554-429: A new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algae Polysiphonia , and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants and many algae, are produced by meiosis . Fragmentation
1665-402: A phenomenon known as "egg parasitism." This method of reproduction has been found in several species of the clam genus Corbicula , many plants like, Cupressus dupreziana , Lomatia tasmanica , Pando and recently in the fish Squalius alburnoides . Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
1776-400: A phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by
1887-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae
1998-706: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to
2109-538: A process called apomixis . However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction. Parthenogenetic organisms can be split into two main categories: facultative and obligate. In facultative parthenogenesis, females can reproduce both sexually and asexually. Because of the many advantages of sexual reproduction, most facultative parthenotes only reproduce asexually when forced to. This typically occurs in instances when finding
2220-402: A process called thelytoky . The freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities
2331-639: A rate up to three times faster than similar species without symbionts. These corals typically grow in shallow, 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 extend their tentacles to feed on zooplankton , but those with larger polyps take correspondingly larger prey, including various invertebrates and even small fish. In addition to capturing prey in this way, many stony corals also produce mucus films they can move over their bodies using cilia ; these trap small organic particles which are then pulled towards and into
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#17327728433022442-687: A result of teaming up with symbiotic algae . Nine of the sub-orders were in existence by the end of the Triassic and three more had appeared by the Jurassic (200 million years ago), with a further suborder appearing in the Middle Cretaceous (100 million years ago). Some may have developed from a common ancestor, either an anemone-like coral without a skeleton, or a rugose coral . A rugose coral seems an unlikely common ancestor because these corals had calcite rather than aragonite skeletons, and
2553-408: A ring of tentacles . The base of the polyp secretes the stony material from which the coral skeleton is formed. The body wall of the polyp consists of mesoglea sandwiched between two layers of epidermis. The mouth is at the centre of the oral disc and leads into a tubular pharynx which descends for some distance into the body before opening into the gastrovascular cavity that fills the interior of
2664-401: A set of characters shared by all its living representatives. This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on
2775-525: A sperm cell is used to initiate reproduction. However, the sperm's genes never get incorporated into the egg cell. The best known example of this is the Amazon molly . Because they are obligate parthenotes, there are no males in their species so they depend on males from a closely related species (the Sailfin molly ) for sperm. Apomixis in plants is the formation of a new sporophyte without fertilization. It
2886-401: A subjective decision about which groups of organisms should be considered as phyla. The approach is useful because it makes it easy to classify extinct organisms as " stem groups " to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display
2997-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with
3108-400: A year. This switch is triggered by environmental changes in the fall and causes females to develop eggs instead of embryos. This dynamic reproductive cycle allows them to produce specialized offspring with polyphenism , a type of polymorphism where different phenotypes have evolved to carry out specific tasks. The cape bee Apis mellifera subsp. capensis can reproduce asexually through
3219-474: Is a paraphyletic taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by
3330-548: Is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms. Animals that reproduce asexually include planarians , many annelid worms including polychaetes and some oligochaetes , turbellarians and sea stars . Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in mosses and liverworts . Most lichens , which are
3441-484: Is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and
Scleractinia - Misplaced Pages Continue
3552-559: Is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis . Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe ( Bryophyllum daigremontianum ) and many produce new plants from rhizomes or stolon (for example in strawberry ). Some plants reproduce by forming bulbs or tubers , for example tulip bulbs and Dahlia tubers. In these examples, all
3663-471: Is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera . In the nine-banded armadillos , this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence
3774-402: Is common. There are at least 10 million identical human twins and triplets in the world today. Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through
3885-405: Is defined in various ways by different biologists (see Current definitions of Plantae ). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta , to form the clade Viridiplantae . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida ,
3996-418: Is derived entirely from pollen . Androgenesis occurs when a zygote is produced with only paternal nuclear genes . During standard sexual reproduction , one female and one male parent each produce haploid gametes (such as a sperm or egg cell, each containing only a single set of chromosomes ), which recombine to create offspring with genetic material from both parents. However, in androgenesis, there
4107-481: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish
4218-414: Is important in ferns and in flowering plants, but is very rare in other seed plants. In flowering plants, the term "apomixis" is now most often used for agamospermy , the formation of seeds without fertilization, but was once used to include vegetative reproduction . An example of an apomictic plant would be the triploid European dandelion . Apomixis mainly occurs in two forms: In gametophytic apomixis,
4329-766: Is no recombination of maternal and paternal chromosomes, and only the paternal chromosomes are passed down to the offspring (the inverse of this is gynogenesis , where only the maternal chromosomes are inherited, which is more common than androgenesis). The offspring produced in androgenesis will still have maternally inherited mitochondria , as is the case with most sexually reproducing species. Androgenesis occurs in nature in many invertebrates (for example, clams, stick insects, some ants, bees, flies and parasitic wasps ) and vertebrates (mainly amphibians and fish ). The androgenesis has also been seen in genetically modified laboratory mice. One of two things can occur to produce offspring with exclusively paternal genetic material:
4440-449: Is particularly challenging. Many species were described before the advent of scuba diving , with little realisation by the authors that coral species could have varying morphologies in different habitats. Collectors were mostly limited to observing corals on reef flats, and were unable to observe the changes in morphology that occurred in more turbid, deeper-water conditions. More than 2,000 nominal species were described in this era, and by
4551-667: Is relatively rare among multicellular organisms , particularly animals . It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Almost all asexual modes of reproduction maintain meiosis either in
Scleractinia - Misplaced Pages Continue
4662-441: Is split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. In echinoderms , this method of reproduction is usually known as fissiparity . Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different. Agamogenesis is any form of reproduction that does not involve
4773-525: Is the Komodo dragon at 10 feet long and over 300 pounds. Heterogony is a form of facultative parthenogenesis where females alternate between sexual and asexual reproduction at regular intervals (see Alternation between sexual and asexual reproduction ). Aphids are one group of organism that engages in this type of reproduction. They use asexual reproduction to reproduce quickly and create winged offspring that can colonize new plants and reproduce sexually in
4884-649: The Bacteriological Code Currently there are 2 phyla that have been validly published according to the Bacteriological Code Other phyla that have been proposed, but not validly named, include: Asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes . The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit
4995-565: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to
5106-550: The Convention on International Trade in Endangered Species (CITES) meaning that their international trade (including in parts and derivatives) is regulated. The World Register of Marine Species lists the following families as being included in the order Scleractinia. Some species have not been placeable ( Incertae sedis ): Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; pl. : phyla )
5217-666: The Meselson effect that have allowed them to survive better in periods of dehydration. Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy. These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks. This repair mechanism was studied in two Bdelloidea species, Adineta vaga , and Philodina roseola . and appears to involve mitotic recombination between homologous DNA regions within each species. Molecular evidence strongly suggests that several species of
5328-450: The effects of global warming and ocean acidification . Scleractinian corals may be solitary or colonial . Colonies can reach considerable size, consisting of a large number of individual polyps. Stony corals are members of the class Anthozoa and like other members of the group, do not have a medusa stage in their life cycle. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc surrounded by
5439-461: The endodermal cells are usually replete with symbiotic unicellular dinoflagellates known as zooxanthellae . There are sometimes as many as five million cells of these per 1 square centimetre (0.16 sq in) of coral tissue. Up to 50% of organic compounds produced by symbionts are used as food by polyps. The oxygen byproduct of photosynthesis and the additional energy derived from sugars produced by zooxanthellae enable these corals to grow at
5550-491: The hammerhead shark and the blacktip shark . In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers. The New Mexico whiptail is another example. Some reptiles use the ZW sex-determination system , which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Until 2010, it
5661-486: The pyrgomatids , but a recent study recorded evidence of living pyrgomatids in stylasterids , casting doubt on this idea. Stony corals have a great range of reproductive strategies and can reproduce both sexually and asexually. Many species have separate sexes, the whole colony being either male or female, but others are hermaphroditic , with individual polyps having both male and female gonads. Some species brood their eggs but in most species, sexual reproduction results in
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#17327728433025772-431: The rotifer Brachionus calyciflorus asexual reproduction (obligate parthenogenesis ) can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring. Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum . Asexual reproduction is found in nearly half of the animal phyla. Parthenogenesis occurs in
5883-457: The stick insect genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect. Similar findings suggest that the mite species Oppiella nova may have reproduced entirely asexually for millions of years. In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes. A complete lack of sexual reproduction
5994-478: The 19th and early 20th centuries. The two most advanced 19th century classifications both used complex skeletal characters; The 1857 classification of the French zoologists Henri Milne-Edwards and Jules Haime 's was based on macroscopic skeletal characters, while Francis Grant Ogilvie 's 1897 scheme was developed using observations of skeletal microstructures, with particular attention to the structure and pattern of
6105-497: The Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina , Kickxellomycotina , Mucoromycotina , and Zoopagomycotina . Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi. Protista
6216-458: The above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine
6327-420: The adjacent colonies of the same species form a single colony by fusing. Most colonial species have very small polyps, ranging from 1 to 3 mm (0.04 to 0.12 in) in diameter, although some solitary species may be as large as 25 cm (10 in). 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
6438-523: The age of the coral. Solitary corals do not bud. They gradually increase in size as they deposit more calcium carbonate and produce new whorls of septa. A large Ctenactis echinata for example normally has a single mouth, may be about 25 cm (10 in) long and have more than a thousand septa. Stony corals occur in all the world's oceans. There are two main ecological groups. Hermatypic corals are mostly colonial corals which tend to live in clear, oligotrophic, shallow tropical waters; they are
6549-477: The amount of water movement and other factors. Many shallow-water corals contain symbiont unicellular organisms known as zooxanthellae within their tissues. These give their colour to the coral which thus may vary in hue depending on what species of symbiont it contains. Stony corals are closely related to sea anemones , and like them are armed with stinging cells known as cnidocytes . Corals reproduce both sexually and asexually. Most species release gametes into
6660-502: The assembling of species into the existing families , 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. The Australian zoologist John Veron and his co-workers analyzed ribosomal RNA in 1996 to obtain similar results to Romano and Palumbi, again concluding that
6771-451: The body and tentacles. Unlike other cnidarians however, the cavity is subdivided by a number of radiating partitions, thin sheets of living tissue, known as mesenteries . The gonads are also located within the cavity walls. The polyp is retractable into the corallite , the stony cup in which it sits, being pulled back by sheet-like retractor muscles. The polyps are connected by horizontal sheets of tissue known as coenosarc extending over
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#17327728433026882-404: The budding of new polyps. There are two types of budding, intratentacular and extratentacular. In intratentacular budding, a new polyp develops on the oral disc, inside the ring of tentacles. This can form individual, separate polyps or a row of partially separated polyps sharing an elongate oral disc with a series of mouths. Tentacles grow around the margin of this elongated oral disc and not around
6993-413: 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 calcified and more susceptible to mechanical damage than that of zooxanthellate corals. Scleratinians were previously believed to be obligatory hosts of another group of barnacles,
7104-446: The cellular level occurs in many protists , e.g. sporozoans and algae . The nucleus of the parent cell divides several times by mitosis , producing several nuclei. The cytoplasm then separates, creating multiple daughter cells . In apicomplexans , multiple fission, or schizogony appears either as merogony , sporogony or gametogony . Merogony results in merozoites , which are multiple daughter cells, that originate within
7215-487: The embryo arises from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis. In nucellar embryony , the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as in the Saharan Cypress Cupressus dupreziana , where the genetic material of the embryo
7326-588: 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. The 1996 analysis of mitochondrial RNA undertaken by American zoologists Sandra Romano and Stephen Palumbi found that molecular data supported
7437-438: The fall to lay eggs for the next season. However, some aphid species are obligate parthenotes. In obligate parthenogenesis, females only reproduce asexually. One example of this is the desert grassland whiptail lizard , a hybrid of two other species. Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations. Gynogenesis is a form of obligate parthenogenesis where
7548-507: The first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders , many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia
7659-673: The form of "polyp bail-out", which may allow polyps to survive even though the parent colony dies. It involves the growth of the coenosarc to seal off the polyps, detachment of the polyps and their settlement on the seabed to initiate new colonies. In other species, small balls of tissue detach themselves from the coenosarc, differentiate into polyps and start secreting calcium carbonate to form new colonies, and in Pocillopora damicornis , unfertilised eggs can develop into viable larvae. The overwhelming majority of scleractinian taxa are hermaphroditic in their adult colonies. In temperate regions,
7770-442: The form of crystals of aragonite , however, a prehistoric scleractinian ( Coelosimilia ) had a non-aragonite skeletal structure which was composed of calcite . The structure of both simple and compound scleractinians is light and porous, rather than solid as is the case in the prehistoric order Rugosa . Scleractinians are also distinguished from rugosans by their pattern of septal insertion. In colonial corals, growth results from
7881-408: The full set of genes of their single parent and thus the newly created individual is genetically and physically similar to the parent or an exact clone of the parent. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria . Many eukaryotic organisms including plants , animals , and fungi can also reproduce asexually. In vertebrates ,
7992-583: The fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as
8103-519: The hypothesis for an independent origin of the Scleractinia. Whether the early scleractinian corals were zooxanthellate is an open question. The phenomenon seems to have evolved independently on numerous occasions during the Tertiary, and the genera Astrangia , Madracis , Cladocora and Oculina , all in different families, each have both zooxanthellate and non-zooxanthellate members. The fact that zooxanthellate coral make up only about half of
8214-428: The individual mouths. This is surrounded by a single corallite wall, as is the case in the meandroid corallites of brain corals. Extratentacular budding always results in separate polyps, each with its own corallite wall. In the case of bushy corals such as Acropora , lateral budding from axial polyps form the basis of the trunk and branches. The rate at which a stony coral colony lays down calcium carbonate depends on
8325-527: The individuals are clones, and the clonal population may cover a large area. Many multicellular organisms produce spores during their biological life cycle in a process called sporogenesis . Exceptions are animals and some protists, which undergo meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals called gametophytes , without
8436-512: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
8547-404: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
8658-412: The maternal nuclear genome can be eliminated from the zygote, or the female can produce an egg with no nucleus , resulting in an embryo developing with only the genome of the male gamete. Other type of androgenesis is the male apomixis or paternal apomixis is a reproductive process in which a plant develops from a sperm cell (male gamete) without the participation of a female cell (ovum). In this process,
8769-415: The maternal nuclear genome. Obligate androgenesis is the process in which males are capable of producing both eggs and sperm, however, the eggs have no genetic contribution and the offspring come only from the sperm, which allows these individuals to self-fertilize and produce clonal offspring without the need for females. They are also capable of interbreeding with sexual and other androgenetic lineages in
8880-597: The maternal nuclear genome. Some species can alternate between sexual and asexual strategies, an ability known as heterogamy , depending on many conditions. Alternation is observed in several rotifer species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects. One example of this is aphids which can engage in heterogony. In this system, females are born pregnant and produce only female offspring. This cycle allows them to reproduce very quickly. However, most species reproduce sexually once
8991-428: The more common apomixis, where development occurs without fertilization, but with genetic material only from the mother. There are also clonal species that reproduce through vegetative reproduction like Lomatia tasmanica and Pando , where the genetic material is exclusively male. Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
9102-485: The most common form of asexual reproduction is parthenogenesis , which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited. Some monitor lizards , including Komodo dragons , can reproduce asexually. While all prokaryotes reproduce without the formation and fusion of gametes, mechanisms for lateral gene transfer such as conjugation , transformation and transduction can be likened to sexual reproduction in
9213-703: The mouth. In a few stony corals, this is the primary method of feeding, and the tentacles are reduced or absent, an example being Acropora acuminata . Caribbean stony corals are generally nocturnal, with the polyps retracting into their skeletons during the day, thus maximising the exposure of the zooxanthellae to the light, but in the Indo-Pacific region, many species feed by day and night. Non-zooxanthellate corals are usually not reef-formers; they 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
9324-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within
9435-643: The order is unusual, as symbiosis is almost always an all-or-nothing phenomenon. This symbiotic equilibrium suggests that there must be evolutionary processes simultaneously maintaining and limiting symbiotic relationships. This is likely because despite the energetic benefits it provides, photosymbiosis appears to be an evolutionary disadvantage during mass extinctions. Traits that generally enable corals to survive mass extinction include deep-water or large habitat range, non-symbiotic, solitary or small colonies, and bleaching resistance, all of which tend to characterize azooxanthellate (non-symbiotic) corals. Endosymbionts, on
9546-539: The other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes . This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size. A definition of
9657-521: The other hand, which rely on specialized conditions and access to light to survive, are especially vulnerable to prolonged darkness, temperature change, and eutrophication, all of which have been hallmarks of past mass extinctions. This makes zooxanthellate coral especially vulnerable to unstable conditions. Therefore, it is possible that coral and zooxanthellate coevolved loosely, with the relationship dissolving when advantages decreased, then reforming when conditions stabilized. The taxonomy of Scleractinia
9768-427: The outer surface of the skeleton and completely covering it. These sheets are continuous with the body wall of the polyps, and include extensions of the gastrovascular cavity, so that food and water can circulate between all the different members of the colony. In colonial species, the repeated asexual division of the polyps causes the corallites to be interconnected, thus forming the colonies. Also, cases exist in which
9879-557: The parent organism. Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii . It involves an unusual process in which two ( endodyogeny ) or more ( endopolygeny ) daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation. Also, budding (external or internal) occurs in some worms like Taenia or Echinococcus ; these worms produce cysts and then produce (invaginated or evaginated) protoscolex with budding . Vegetative propagation
9990-399: The polyps are usually only a few millimetres in diameter. These polyps reproduce asexually by budding , but remain attached to each other, forming a multi-polyp colony of clones with a common skeleton, which may be up to several metres in diameter or height according to species. The shape and appearance of each coral colony depends not only on the species, but also on its location, depth,
10101-462: The production of a free-swimming planula larva that eventually settles on the seabed to undergo metamorphosis into a polyp. In colonial species, this initial polyp then repeatedly divides asexually, to give rise to the entire colony. The most common means of asexual reproduction in colonial stony corals is by fragmentation. Pieces of branching corals may get detached during storms, by strong water movement or by mechanical means, and fragments fall to
10212-475: The relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On
10323-409: The risk of self-fertilization. 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 increases 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
10434-640: The rules of nomenclature, the name given to the first described species has precedence over the rest, even when that description is poor, and the environment and even sometimes the country of the type specimen is unknown. Even the concept of "the species" is suspect, with regard to corals which have large geographical ranges with a number of sub-populations; their geographic boundaries merge with those of other species; their morphological boundaries merge with those of other species; and there are no definite distinctions between species and subspecies. The evolutionary relationships among stony corals were first examined in
10545-446: The same cell membrane, sporogony results in sporozoites , and gametogony results in micro gametes . Some cells divide by budding (for example baker's yeast ), resulting in a "mother" and a "daughter" cell that is initially smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra , which reproduces by budding. The buds grow into fully matured individuals which eventually break away from
10656-469: 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 secrete a stony exoskeleton in which the septa are inserted between the mesenteries in multiples of six. All modern scleractinian skeletons are composed of calcium carbonate in
10767-467: The sea bed. In suitable conditions, these are capable of adhering to the substrate and starting new colonies. Even such massive corals as Montastraea annularis have been shown to be capable of forming new colonies after fragmentation. This process is used in the reef aquarium hobby to increase stock without the necessity to harvest corals from the wild. Under adverse conditions, certain species of coral resort to another type of asexual reproduction in
10878-481: The sea where fertilisation takes place, and the planula larvae drift as part of the plankton , but a few species brood their eggs. Asexual reproduction is mostly by fragmentation , when part of a colony becomes detached and reattaches elsewhere. Stony corals occur in all the world's oceans. Much of the framework of modern coral reefs is formed by scleractinians. Reef-building or hermatypic corals are mostly colonial; most of these are zooxanthellate and are found in
10989-438: The sense of genetic recombination in meiosis . Prokaryotes ( Archaea and Bacteria ) reproduce asexually through binary fission , in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi ) may reproduce in a functionally similar manner by mitosis ; most of these are also capable of sexual reproduction. Multiple fission at
11100-507: The septa were arranged serially rather than cyclically. However, it may be that similarities of scleractinians to rugosans are due to a common non-skeletalized ancestor in the early Paleozoic. Alternatively, scleractinians may have developed from a Corallimorpharia -like ancestor. It seems that skeletogenesis may have been associated with the development of symbiosis and reef formation, and may have occurred on more than one occasion. DNA sequencing appears to indicate that scleractinian corals are
11211-484: The septal trabeculae . In 1943, the American zoologists Thomas Wayland Vaughan and John West Wells , and Wells again 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 . The 1952 classification by French zoologist J. Alloiteau
11322-529: The shallow waters into which sunlight penetrates. Other corals that do not form reefs may be solitary or colonial; some of these occur at abyssal depths where no light reaches. Stony corals first appeared in the Middle Triassic , but their relationship to the tabulate and rugose corals of the Paleozoic is currently unresolved. In modern times stony corals numbers are expected to decline due to
11433-740: The social pathway, they form a multi-cellular slug which then forms a fruiting body with asexually generated spores. In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells. The hyphae of the common mold ( Rhizopus ) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexual and asexual reproduction. A number of plants use both sexual and asexual means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual under varying environmental conditions. In
11544-509: The species were also capable of self-fertilization to varying extents. There is little evidence on which to base a hypothesis about the origin of the scleractinians; plenty is known about modern species but very little about fossil specimens, which first appeared in the record in the Middle Triassic ( 240 million years ago ). It was not until 25 million years later that they became important reef builders, their success likely
11655-497: The species, but some of the branching species can increase in height or length by around 10 cm (4 in) a year (about the same rate as human hair grows). Other corals, like the dome and plate species, are more bulky and may only grow 0.3 to 2 cm (0.1 to 0.8 in) per year. The rate of aragonite deposition varies diurnally and seasonally. Examination of cross sections of coral can show bands of deposition indicating annual growth. Like tree rings, these can be used to estimate
11766-447: The traditional families were plausible but that the suborders were incorrect. They 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. Veron suggested that both morphological and molecular systems be used in future classification schemes. All Scleractinian corals (excluding fossils) are listed under Appendix II of
11877-440: The usual pattern is synchronized release of eggs and sperm into the water during brief spawning events, often related to the phases of the moon. In tropical regions, reproduction may occur throughout the year. In many cases, as in the genus Acropora , the eggs and sperm are released in buoyant bundles which rise to the surface. This increases the concentration of sperm and eggs and thus the likelihood of fertilization , and reduces
11988-407: The world's primary reef -builders. Ahermatypic corals are either colonial or solitary and are found in all regions of the ocean and do not build reefs. Some live in tropical waters but some inhabit temperate seas, polar waters, or live at great depths, from the photic zone down to about 6,000 m (20,000 ft). Scleractinians fall into one of two main categories: In reef-forming corals,
12099-494: The zygote is formed solely with genetic material from the father, resulting in offspring genetically identical to the male organism. This has been noted in many plants like Nicotiana , Capsicum frutescens , Cicer arietinum , Poa arachnifera , Solanum verrucosum , Phaeophyceae , Pripsacum dactyloides , Zea mays , and occurs as the regular reproductive method in Cupressus dupreziana . This contrasts with
12210-694: Was built on these earlier systems but included more microstructural observations and did not involve the anatomical characters of the polyp. Alloiteau recognized eight suborders. In 1942, W.H. Bryan and D. Hill 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 there were many examples of convergent evolution between fossils and recent taxa . The rise of molecular techniques at
12321-478: Was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes. The female boa could have chosen any number of male partners (and had successfully in the past) but on this occasion she reproduced asexually, creating 22 female babies with WW sex-chromosomes. Polyembryony
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