Ichthyoplankton are the eggs and larvae of fish. They are found in the sunlit zone of the water column, less than 200 metres deep, sometimes called the epipelagic or photic zone. Ichthyoplankton are planktonic, meaning they cannot swim under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, are unambiguously planktonic. Early stage larvae swim poorly, but stage larvae swim better and cease to be planktonic as they grow into juveniles. Fish larvae are part of the zooplankton that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals. Fish can produce high numbers of eggs which are released into the open water column. Fish eggs have a diameter of about 1 millimetre; the newly hatched young of oviparous fish are called larvae. They are poorly formed, carry a large yolk sac and are different in appearance from juvenile and adult specimens; the larval period in oviparous fish is short, larvae grow and change appearance and structure to become juveniles.
During this transition larvae must switch from their yolk sac to feeding on zooplankton prey, a process which depends on inadequate zooplankton density, starving many larvae. Ichthyoplankton can be a useful indicator of the health of an aquatic ecosystem. For instance, most late stage larvae in ichthyoplankton have been preyed on, so ichthyoplankton tends to be dominated by eggs and early stage larvae; this means that when fish, such as anchovies and sardines, are spawning, ichthyoplankton samples can reflect their spawning output and provide an index of relative population size for the fish. Increases or decreases in the number of adult fish stocks can be detected more and sensitively by monitoring the ichthyoplankton associated with them, compared to monitoring the adults themselves, it is usually easier and more cost effective to sample trends in egg and larva populations than to sample trends in adult fish populations. Interest in plankton originated in Britain and Germany in the nineteenth century when researchers discovered there were microorganisms in the sea, that they could trap them with fine-mesh nets.
They started testing different net configurations. Ichthyoplankton research started in 1864 when the Norwegian government commissioned the marine biologist G. O. Sars to investigate fisheries around the Norwegian coast. Sars found fish eggs cod eggs, drifting in the water; this established that fish eggs could be pelagic, living in the open water column like other plankton. Around the beginning of the twentieth century, research interest in ichthyoplankton became more general when it emerged that, if ichthyoplankton was sampled quantitatively the samples could indicate the relative size or abundance of spawning fish stocks. Research vessels collect ichthyoplankton from the ocean using fine mesh nets; the vessels either tow the nets through the sea or pump sea water onboard and pass it through the net. There are many types of plankton tows:Neuston net tows are made at or just below the surface using a nylon mesh net fitted to a rectangular frame The PairoVET tow, used for collecting fish eggs, drops a net about 70 metres into the sea from a stationary research vessel and drags it back to the vessel.
Ring net tows involve a nylon mesh net fitted to a circular frame. These have been replaced by bongo nets, which provide duplicate samples with their dual-net design; the bongo tow drags nets shaped like bongo drums from a moving vessel. The net is lowered to about 200 metres and allowed to rise to the surface as it is towed. In this way, a sample can be collected across the whole photic zone where most ichthyoplankton is found. MOCNESS tows and Tucker trawls utilize multiple nets that are mechanically opened and closed at discrete depths in order to provide insights into the vertical distribution of the plankton The manta trawl tows a net from a moving vessel along the surface of the water, collecting larvae, such as grunion, mahi-mahi, flying fish which live at the surface. After the tow the plankton is flushed with a hose to the cod end of the net for collection; the sample is placed in preservative fluid prior to being sorted and identified in a laboratory. Plankton pumps: Another method of collecting ichthyoplankton is to use a Continuous Underway Fish Egg Sampler.
Water from a depth of about three metres is filtered with a net. This method can be used. Ichthyoplankton researchers use the terminology and development stages introduced in 1984 by Kendall and others; this consists of two transitional stages. Recruitment of fish is regulated by larval fish survival. Survival is regulated by prey abundance and hydrology. Fish eggs and larvae are eaten by many marine organisms. For example, they may be fed upon by marine invertebrates, such as copepods, arrow worms, amphipods, marine snails and krill; because they are so abundant, marine invertebrates inflict high overall mortality rates. Adult fish prey on fish eggs and larvae. For example, haddock were observed satiating themselves with herring eggs back in 1922. Another study found cod in a herring spawning area with 20,000 herring eggs in their stomachs, concluded that they could prey on half of the total egg production. Fish cannibalise their own eggs. For example, separate studies found northern anchovy were respons
A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail: these five anatomical features define this phylum. Chordates are bilaterally symmetric; the Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata. There are extinct taxa such as the Vetulicolia. Hemichordata has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically, vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull.
The Craniata and Tunicata compose the clade Olfactores. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: A notochord, a stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, in wholly aquatic species this helps the animal to swim by flexing its tail. A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. Pharyngeal slits; the pharynx is the part of the throat behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. Post-anal tail. A muscular tail that extends backwards behind the anus. An endostyle; this is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.
It stores iodine, may be a precursor of the vertebrate thyroid gland. There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: All chordates are deuterostomes; this means. All chordates are based on a bilateral body plan. All chordates are coelomates, have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm; the following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships, it retains the traditional ranks used in Linnaean taxonomy. Phylum Chordata †Vetulicolia? Subphylum Cephalochordata – Class Leptocardii Clade Olfactores Subphylum Tunicata – Class Ascidiacea Class Thaliacea Class Appendicularia Class Sorberacea Subphylum Vertebrata Infraphylum incertae sedis Cyclostomata Superclass'Agnatha' paraphyletic Class Myxini Class Petromyzontida or Hyperoartia Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi Infraphylum Gnathostomata Class †Placodermi Class Chondrichthyes Class †Acanthodii Superclass Osteichthyes Class Actinopterygii Class Sarcopterygii Superclass Tetrapoda Class Amphibia Class Sauropsida Class Synapsida Craniates, one of the three subdivisions of chordates, all have distinct skulls.
They include the hagfish. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or all deuterostomes, are by their tails". Most craniates are vertebrates; these consist of a series of bony or cartilaginous cylindrical vertebrae with neural arches that protect the spinal cord, with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved; however the cladistic exclusion of hagfish from the vertebrates is controversial, as they ma
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated
Crustaceans form a large, diverse arthropod taxon which includes such familiar animals as crabs, crayfish, krill and barnacles. The crustacean group is treated as a subphylum, because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, comprises all animals in the Pancrustacea clade other than hexapods; some crustaceans are more related to insects and other hexapods than they are to certain other crustaceans. The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm, to the Japanese spider crab with a leg span of up to 3.8 m and a mass of 20 kg. Like other arthropods, crustaceans have an exoskeleton, they are distinguished from other groups of arthropods, such as insects and chelicerates, by the possession of biramous limbs, by their larval forms, such as the nauplius stage of branchiopods and copepods. Most crustaceans are free-living aquatic animals, but some are terrestrial, some are parasitic and some are sessile; the group has an extensive fossil record, reaching back to the Cambrian, includes living fossils such as Triops cancriformis, which has existed unchanged since the Triassic period.
More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as fished, but may be the animals with the greatest biomass on the planet, form a vital part of the food chain; the scientific study of crustaceans is known as carcinology, a scientist who works in carcinology is a carcinologist. The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head, the pereon or thorax, the pleon or abdomen; the head and thorax may be fused together to form a cephalothorax, which may be covered by a single large carapace. The crustacean body is protected by the hard exoskeleton, which must be moulted for the animal to grow; the shell around each somite can be divided into a dorsal tergum, ventral sternum and a lateral pleuron. Various parts of the exoskeleton may be fused together; each somite, or body segment can bear a pair of appendages: on the segments of the head, these include two pairs of antennae, the mandibles and maxillae.
The abdomen bears pleopods, ends in a telson, which bears the anus, is flanked by uropods to form a tail fan. The number and variety of appendages in different crustaceans may be responsible for the group's success. Crustacean appendages are biramous, meaning they are divided into two parts, it is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Trilobites, for instance possessed biramous appendages; the main body cavity is an open circulatory system, where blood is pumped into the haemocoel by a heart located near the dorsum. Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods and branchiopods have haemoglobins; the alimentary canal consists of a straight tube that has a gizzard-like "gastric mill" for grinding food and a pair of digestive glands that absorb food. Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, a collection of major ganglia is found below the gut.
In many decapods, the first pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs; the majority of crustaceans are aquatic, living in either marine or freshwater environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, woodlice. Marine crustaceans are as ubiquitous in the oceans; the majority of crustaceans are motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts, adult barnacles live a sessile life – they are attached headfirst to the substrate and cannot move independently. Some branchiurans are able to withstand rapid changes of salinity and will switch hosts from marine to non-marine species. Krill are the bottom layer and the most important part of the food chain in Antarctic animal communities.
Some crustaceans are significant invasive species, such as the Chinese mitten crab, Eriocheir sinensis, the Asian shore crab, Hemigrapsus sanguineus. The majority of crustaceans have separate sexes, reproduce sexually. A small number are hermaphrodites, including barnacles and Cephalocarida; some may change sex during the course of their life. Parthenogenesis is widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male; this occurs in many branchiopods, some os
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
South Africa the Republic of South Africa, is the southernmost country in Africa. It is bounded to the south by 2,798 kilometres of coastline of Southern Africa stretching along the South Atlantic and Indian Oceans. South Africa is the largest country in Southern Africa and the 25th-largest country in the world by land area and, with over 57 million people, is the world's 24th-most populous nation, it is the southernmost country on the mainland of the Eastern Hemisphere. About 80 percent of South Africans are of Sub-Saharan African ancestry, divided among a variety of ethnic groups speaking different African languages, nine of which have official status; the remaining population consists of Africa's largest communities of European and multiracial ancestry. South Africa is a multiethnic society encompassing a wide variety of cultures and religions, its pluralistic makeup is reflected in the constitution's recognition of 11 official languages, the fourth highest number in the world. Two of these languages are of European origin: Afrikaans developed from Dutch and serves as the first language of most coloured and white South Africans.
The country is one of the few in Africa never to have had a coup d'état, regular elections have been held for a century. However, the vast majority of black South Africans were not enfranchised until 1994. During the 20th century, the black majority sought to recover its rights from the dominant white minority, with this struggle playing a large role in the country's recent history and politics; the National Party imposed apartheid in 1948. After a long and sometimes violent struggle by the African National Congress and other anti-apartheid activists both inside and outside the country, the repeal of discriminatory laws began in 1990. Since 1994, all ethnic and linguistic groups have held political representation in the country's liberal democracy, which comprises a parliamentary republic and nine provinces. South Africa is referred to as the "rainbow nation" to describe the country's multicultural diversity in the wake of apartheid; the World Bank classifies South Africa as an upper-middle-income economy, a newly industrialised country.
Its economy is the second-largest in Africa, the 34th-largest in the world. In terms of purchasing power parity, South Africa has the seventh-highest per capita income in Africa; however and inequality remain widespread, with about a quarter of the population unemployed and living on less than US$1.25 a day. South Africa has been identified as a middle power in international affairs, maintains significant regional influence; the name "South Africa" is derived from the country's geographic location at the southern tip of Africa. Upon formation, the country was named the Union of South Africa in English, reflecting its origin from the unification of four separate British colonies. Since 1961, the long form name in English has been the "Republic of South Africa". In Dutch, the country was named Republiek van Zuid-Afrika, replaced in 1983 by the Afrikaans Republiek van Suid-Afrika. Since 1994, the Republic has had an official name in each of its 11 official languages. Mzansi, derived from the Xhosa noun umzantsi meaning "south", is a colloquial name for South Africa, while some Pan-Africanist political parties prefer the term "Azania".
South Africa contains human-fossil sites in the world. Archaeologists have recovered extensive fossil remains from a series of caves in Gauteng Province; the area, a UNESCO World Heritage site, has been branded "the Cradle of Humankind". The sites include one of the richest sites for hominin fossils in the world. Other sites include Gondolin Cave Kromdraai, Coopers Cave and Malapa. Raymond Dart identified the first hominin fossil discovered in Africa, the Taung Child in 1924. Further hominin remains have come from the sites of Makapansgat in Limpopo Province and Florisbad in the Free State Province, Border Cave in KwaZulu-Natal Province, Klasies River Mouth in Eastern Cape Province and Pinnacle Point and Die Kelders Cave in Western Cape Province; these finds suggest that various hominid species existed in South Africa from about three million years ago, starting with Australopithecus africanus. There followed species including Australopithecus sediba, Homo ergaster, Homo erectus, Homo rhodesiensis, Homo helmei, Homo naledi and modern humans.
Modern humans have inhabited Southern Africa for at least 170,000 years. Various researchers have located pebble tools within the Vaal River valley. Settlements of Bantu-speaking peoples, who were iron-using agriculturists and herdsmen, were present south of the Limpopo River by the 4th or 5th century CE, they displaced and absorbed the original Khoisan speakers, the Khoikhoi and San peoples. The Bantu moved south; the earliest ironworks in modern-day KwaZulu-Natal Province are believed to date from around 1050. The southernmost group was the Xhosa people, whose language incorporates certain linguistic traits from the earlier Khoisan people; the Xhosa reached the Great Fish River, in today's Eastern Cape Province. As they migrated, these larger Iron Age populations
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, other forms were described by Antonie van Leeuwenhoek in 1703. Most rotifers are around 0.1–0.5 mm long, are common in freshwater environments throughout the world with a few saltwater species. Some rotifers are free swimming and planktonic, others move by inchworming along a substrate, some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are either sessile or planktonic. Rotifers are an important part of the freshwater zooplankton, being a major foodsource and with many species contributing to the decomposition of soil organic matter. Most species of the rotifers are cosmopolitan, but there are some endemic species, like Cephalodella vittata to Lake Baikal. Recent barcoding evidence, suggests that some'cosmopolitan' species, such as Brachionus plicatilis, B. calyciflorus, Lecane bulla, among others, are species complexes.
In some recent treatments, rotifers are placed with acanthocephalans in a larger clade called Syndermata. Rev. John Harris first described the rotifers in 1696 as "an animal like a large maggot which could contract itself into a spherical figure and stretch itself out again. In 1702, Antonie van Leeuwenhoek gave a detailed description of Rotifer vulgaris and subsequently described Melicerta ringens and other species, he was the first to publish observations of the revivification of certain species after drying. Other forms were described by other observers, but it wasn't until the publication of Christian Gottfried Ehrenberg's Die Infusionsthierchen als vollkommene Organismen in 1838 that the rotifers were recognized as being multicellular animals. About 2200 species of rotifers have been described, their taxonomy is in a state of flux. One treatment places them in the phylum Rotifera, with three classes: Seisonidea and Monogononta; the largest group is the Monogononta, with about 1500 species, followed by the Bdelloidea, with about 350 species.
There are only two known genera with three species of Seisonidea. The Acanthocephala considered to be a separate phylum, have been demonstrated to be modified rotifers; the exact relationship to other members of the phylum has not yet been resolved. One possibility is that the Acanthocephala are closer to the Bdelloidea and Monogononta than to the Seisonidea; the Rotifera speaking, are confined to the Bdelloidea and the Monogononta. Rotifera and Seisonida make up a clade called Syndermata; the word "rotifer" is derived from a Latin word meaning "wheel-bearer", due to the corona around the mouth that in concerted sequential motion resembles a wheel. Rotifers have a variety of different shapes; the body of a rotifer is divided into a head and foot, is somewhat cylindrical. There is a well-developed cuticle, which may be thick and rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape. Rigid cuticles are composed of multiple plates, may bear spines, ridges, or other ornamentation.
Their cuticle is formed from sclerotized proteins. The most distinctive feature of rotifers is the presence of a ciliated structure, called the corona, on the head. In the more primitive species, this forms a simple ring of cilia around the mouth from which an additional band of cilia stretches over the back of the head. In the great majority of rotifers, this has evolved into a more complex structure. Modifications to the basic plan of the corona include alteration of the cilia into bristles or large tufts, either expansion or loss of the ciliated band around the head. In genera such as Collotheca, the corona is modified to form a funnel surrounding the mouth. In many species, such as Testudinella, the cilia around the mouth have disappeared, leaving just two small circular bands on the head. In the bdelloids, this plan is further modified, with the upper band splitting into two rotating wheels, raised up on a pedestal projecting from the upper surface of the head; the trunk forms the major part of the body, encloses most of the internal organs.
The foot projects from the rear of the trunk, is much narrower, giving the appearance of a tail. The cuticle over the foot forms rings, making it appear segmented, although the internal structure is uniform. Many rotifers can retract the foot or wholly into the trunk; the foot ends in from one to four toes, which, in sessile and crawling species, contain adhesive glands to attach the animal to the substratum. In many free-swimming species, the foot as a whole is reduced in size, may be absent; the coronal cilia create a current. The mouth opens into a characteristic chewing pharynx, sometimes via a ciliated tube, sometimes directly; the pharynx has a powerful muscular wall and contains tiny, jaw-like structures called trophi, which are the only fossilizable parts of a rotifer. The shape of the trophi varies between different species, depending on the nature of their diet. In suspension feeders, the trophi are covered in grinding ridges, while in more carnivorous species, they may be shaped like forceps to help bite into prey.
In some e