Algae is an informal term for a large, diverse group of photosynthetic eukaryotic organisms that are not closely related, is thus polyphyletic. Including organisms ranging from unicellular microalgae genera, such as Chlorella and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 m in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata and phloem, which are found in land plants; the largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example and the stoneworts. No definition of algae is accepted. One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Although cyanobacteria are referred to as "blue-green algae", most authorities exclude all prokaryotes from the definition of algae.
Algae constitute a polyphyletic group since they do not include a common ancestor, although their plastids seem to have a single origin, from cyanobacteria, they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga. Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction. Algae lack the various structures that characterize land plants, such as the phyllids of bryophytes, rhizoids in nonvascular plants, the roots and other organs found in tracheophytes. Most are phototrophic, although some are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy; some unicellular species of green algae, many golden algae, euglenids and other algae have become heterotrophs, sometimes parasitic, relying on external energy sources and have limited or no photosynthetic apparatus.
Some other heterotrophic organisms, such as the apicomplexans, are derived from cells whose ancestors possessed plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago. The singular alga retains that meaning in English; the etymology is obscure. Although some speculate that it is related to Latin algēre, "be cold", no reason is known to associate seaweed with temperature. A more source is alliga, "binding, entwining"; the Ancient Greek word for seaweed was φῦκος, which could mean either the seaweed or a red dye derived from it. The Latinization, fūcus, meant the cosmetic rouge; the etymology is uncertain, but a strong candidate has long been some word related to the Biblical פוך, "paint", a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean.
It could be any color: black, green, or blue. Accordingly, the modern study of marine and freshwater algae is called either phycology or algology, depending on whether the Greek or Latin root is used; the name Fucus appears in a number of taxa. The algae contain chloroplasts. Chloroplasts contain circular DNA like that in cyanobacteria and are interpreted as representing reduced endosymbiotic cyanobacteria. However, the exact origin of the chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events; the table below describes the composition of the three major groups of algae. Their lineage relationships are shown in the figure in the upper right. Many of these groups contain some members; some retain plastids, but not chloroplasts. Phylogeny based on plastid not nucleocytoplasmic genealogy: Linnaeus, in Species Plantarum, the starting point for modern botanical nomenclature, recognized 14 genera of algae, of which only four are considered among algae.
In Systema Naturae, Linnaeus described the genera Volvox and Corallina, a species of Acetabularia, among the animals. In 1768, Samuel Gottlieb Gmelin published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the new binomial nomenclature of Linnaeus, it included elaborate illustrations of seaweed and marine algae on folded leaves. W. H. Harvey and Lamouroux were the first to divide macroscopic algae into four divisions based on their pigmentation; this is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: red algae, brown algae, green algae, Diatomaceae. At this time, microscopic algae were discovered and reported by a different group of workers studying the Infusoria. Unlike macroalgae, which were viewed as plants, microalgae were considered animals because they are motile; the nonmotile microalgae were sometimes seen as stages of the lifecycle of plants, macroalgae, or animals. Although used as a taxonomic category in some pre-D
The Fahrenheit scale is a temperature scale based on one proposed in 1724 by Dutch–German–Polish physicist Daniel Gabriel Fahrenheit. It uses the degree Fahrenheit as the unit. Several accounts of how he defined his scale exist; the lower defining point, 0 °F, was established as the freezing temperature of a solution of brine made from equal parts of ice and salt. Further limits were established as the melting point of ice and his best estimate of the average human body temperature; the scale is now defined by two fixed points: the temperature at which water freezes into ice is defined as 32 °F, the boiling point of water is defined to be 212 °F, a 180 °F separation, as defined at sea level and standard atmospheric pressure. At the end of the 2010s, Fahrenheit was used as the official temperature scale only in the United States, its associated states in the Western Pacific, the Bahamas, the Cayman Islands and Liberia. Antigua and Barbuda and other islands which use the same meteorological service, such as Anguilla, the British Virgin Islands and Saint Kitts and Nevis, as well as Bermuda and the Turks and Caicos Islands, use Fahrenheit and Celsius.
All other countries in the world now use the Celsius scale, named after Swedish astronomer Anders Celsius. On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit and the boiling point is 212 °F; this puts the freezing points of water 180 degrees apart. Therefore, a degree on the Fahrenheit scale is 1⁄180 of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart. A temperature interval of 1 °F is equal to an interval of 5⁄9 degrees Celsius; the Fahrenheit and Celsius scales intersect at −40°. Absolute zero is −273.15 °C or −459.67 °F. The Rankine temperature scale uses degree intervals of the same size as those of the Fahrenheit scale, except that absolute zero is 0 °R — the same way that the Kelvin temperature scale matches the Celsius scale, except that absolute zero is 0 K; the Fahrenheit scale uses the symbol ° to denote a point on the temperature scale and the letter F to indicate the use of the Fahrenheit scale, as well as to denote a difference between temperatures or an uncertainty in temperature.
For an exact conversion, the following formulas can be applied. Here, f is the value in Fahrenheit and c the value in Celsius: f °Fahrenheit to c °Celsius: °F × 5°C/9°F = /1.8 °C = c °C c °Celsius to f °Fahrenheit: + 32 °F = °F + 32 °F = f °FThis is an exact conversion making use of the identity −40 °F = −40 °C. Again, f is the value in Fahrenheit and c the value in Celsius: f °Fahrenheit to c °Celsius: − 40 = c. C °Celsius to f °Fahrenheit: − 40 = f. Fahrenheit proposed his temperature scale in 1724, basing it on two reference points of temperature. In his initial scale, the zero point was determined by placing the thermometer in a mixture "of ice, of water, of ammonium chloride or of sea salt"; this combination forms a eutectic system which stabilizes its temperature automatically: 0 °F was defined to be that stable temperature. The second point, 96 degrees, was the human body's temperature. According to a story in Germany, Fahrenheit chose the lowest air temperature measured in his hometown Danzig in winter 1708/09 as 0 °F, only had the need to be able to make this value reproducible using brine.
According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer's scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, water boils at 60 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and make the scale more fine-grained, he re-calibrated his scale using the melting point of ice and normal human body temperature. Fahrenheit soon after observed; the use of the freezing and boiling points of water as thermometer fixed reference points became popular following the work of Anders Celsius and these fixed points were adopted by a committee of the Royal Society led by Henry Cavendish in 1776. Under this system, the Fahrenheit scale is redefined so that the freezing point of water is 32 °F, the boiling point is 212 °F or 180 degrees higher, it is for this reason that normal human body temperature is 98° on the revised scale. In the present-day Fahrenheit scale, 0 °F no longer corresponds to the eutectic temperature of ammonium chloride brine as described above.
Instead, that eutectic is at 4 °F on the final Fahrenheit scale. The Rankine temperature s
Cobitidae known as the True loaches, is a family of Old World freshwater fish. They occur throughout Eurasia and in Morocco, inhabit riverine ecosystems. Today, most "loaches" are placed in other families; the family includes about 260 described species. New species are being described regularly; the body forms of the Cobitidae tend to be vermiform – worm-shaped and thin. Most true loaches do not have true scales, like many other Cypriniformes or catfishes, they have barbels at their mouths; some other traits found in this family are a small bottom-facing mouth suited to their scavenging benthic lifestyle, an erectile spine below the eye, a single row of pharyngeal teeth. True loaches are scavengers and are omnivorous not picky about their food, they may eat aquatic crustaceans and other small invertebrates, as well as scraps of organic detritus. Many live in eutrophic waters of poor quality and feed on tubifex worms and similar benthos associated with such habitat; some of these loaches have adapted to low oxygen levels in warm, muddy rivers or dirty ponds by being able to gulp up atmospheric oxygen.
Some species from the genera Cobitis and Misgurnus, are sensitive to changing air pressure. They change their behavior accordingly, as these changes in activity are followed by a change in weather, they are known as "weather fishes" or "weather loaches"; some Cobitidae have been introduced to foreign lands, where they may pose problems to local wildlife as invasive species. Other true loaches, many of them migratory fish, have been affected by habitat destruction, chemical pollution, damming, are considered threatened species today; some migratory species are popular aquarium fish and since they are hard to raise in captivity, overfishing has depleted once-common stocks in several cases. The other "loaches" used to be included in this family, but nowadays are recognized as well-distinct members of the order Cypriniformes. Together with the suckers, the "loaches" made up the superfamily Cobitoidea. However, the sucking loaches were recognizable as relatives of the suckers; the hillstream loaches, though more similar to the true loaches than the other two presumed Cobitoidea, were recognized as distinct enough to be better regarded a family in their own right - Balitoridae.
And as it seems the "sucking Cobitoidea" are quite distant indeed even markedly closer to the Cyprinidae, thus the old superfamily Catostomoidea warrants revalidation. The puzzling mountain carps were most considered the distinct family Psilorhynchidae in recent times. In a number of systematic schemes, they were placed in the Balitoridae. In fact, they seem to belong in the Cyprinidae. In 2012, Maurice Kottelat reviewed the loaches and elevated the former subfamily Botiinae to its own family and established the family Serpenticobitidae for the genus Serpenticobitis; some true loaches are popular as food fish in East Asian countries such as Japan. These are of importance in the fisheries or being raised in aquaculture. Small species may be caught for bait. Many of the more brightly colored species are popular with freshwater aquarists, so are therefore of importance in the aquarium trade; some Cobitidae encountered in aquarium trade include: Dojo loach, Misgurnus anguillicaudatus Horseface loach, Acantopsis choirorhynchus Kuhli loach, Pangio kuhlii List of fish families
In scientific nomenclature, a synonym is a scientific name that applies to a taxon that goes by a different scientific name, although the term is used somewhat differently in the zoological code of nomenclature. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies; this name is no longer in use: it is now a synonym of the current scientific name, Picea abies. Unlike synonyms in other contexts, in taxonomy a synonym is not interchangeable with the name of which it is a synonym. In taxonomy, synonyms have a different status. For any taxon with a particular circumscription and rank, only one scientific name is considered to be the correct one at any given time. A synonym cannot exist in isolation: it is always an alternative to a different scientific name. Given that the correct name of a taxon depends on the taxonomic viewpoint used a name, one taxonomist's synonym may be another taxonomist's correct name. Synonyms may arise whenever the same taxon is named more than once, independently.
They may arise when existing taxa are changed, as when two taxa are joined to become one, a species is moved to a different genus, a variety is moved to a different species, etc. Synonyms come about when the codes of nomenclature change, so that older names are no longer acceptable. To the general user of scientific names, in fields such as agriculture, ecology, general science, etc. A synonym is a name, used as the correct scientific name but, displaced by another scientific name, now regarded as correct, thus Oxford Dictionaries Online defines the term as "a taxonomic name which has the same application as another one, superseded and is no longer valid." In handbooks and general texts, it is useful to have synonyms mentioned as such after the current scientific name, so as to avoid confusion. For example, if the much advertised name change should go through and the scientific name of the fruit fly were changed to Sophophora melanogaster, it would be helpful if any mention of this name was accompanied by "".
Synonyms used in this way may not always meet the strict definitions of the term "synonym" in the formal rules of nomenclature which govern scientific names. Changes of scientific name have two causes: they may be taxonomic or nomenclatural. A name change may be caused by changes in the circumscription, position or rank of a taxon, representing a change in taxonomic, scientific insight. A name change may be due to purely nomenclatural reasons, that is, based on the rules of nomenclature. Speaking in general, name changes for nomenclatural reasons have become less frequent over time as the rules of nomenclature allow for names to be conserved, so as to promote stability of scientific names. In zoological nomenclature, codified in the International Code of Zoological Nomenclature, synonyms are different scientific names of the same taxonomic rank that pertain to that same taxon. For example, a particular species could, over time, have had two or more species-rank names published for it, while the same is applicable at higher ranks such as genera, orders, etc.
In each case, the earliest published name is called the senior synonym, while the name is the junior synonym. In the case where two names for the same taxon have been published the valid name is selected accorded to the principle of the first reviser such that, for example, of the names Strix scandiaca and Strix noctua, both published by Linnaeus in the same work at the same date for the taxon now determined to be the snowy owl, the epithet scandiaca has been selected as the valid name, with noctua becoming the junior synonym. One basic principle of zoological nomenclature is that the earliest published name, the senior synonym, by default takes precedence in naming rights and therefore, unless other restrictions interfere, must be used for the taxon. However, junior synonyms are still important to document, because if the earliest name cannot be used the next available junior synonym must be used for the taxon. For other purposes, if a researcher is interested in consulting or compiling all known information regarding a taxon, some of this may well have been published under names now regarded as outdated and so it is again useful to know a list of historic synonyms which may have been used for a given current taxon name.
Objective synonyms refer to taxa with same rank. This may be species-group taxa of the same rank with the same type specimen, genus-group taxa of the same rank with the same type species or if their type species are themselves objective synonyms, of family-group taxa with the same type genus, etc. In the case of subjective synonyms, there is no such shared type, so the synonymy is open to taxonomic judgement, meaning that th
Actinopterygii, or the ray-finned fishes, constitute a class or subclass of the bony fishes. The ray-finned fishes are so called because their fins are webs of skin supported by bony or horny spines, as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii; these actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton. Numerically, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish, they are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm, to the massive ocean sunfish, at 2,300 kg, the long-bodied oarfish, at 11 m. Ray-finned fishes occur in many variant forms; the main features of a typical ray-finned fish are shown in the adjacent diagram. In nearly all ray-finned fish, the sexes are separate, in most species the females spawn eggs that are fertilized externally with the male inseminating the eggs after they are laid.
Development proceeds with a free-swimming larval stage. However other patterns of ontogeny exist, with one of the commonest being sequential hermaphroditism. In most cases this involves protogyny, fish starting life as females and converting to males at some stage, triggered by some internal or external factor. Protandry, where a fish converts from male to female, is much less common than protogyny. Most families use external rather than internal fertilization. Of the oviparous teleosts, most do not provide parental care. Viviparity, ovoviviparity, or some form of parental care for eggs, whether by the male, the female, or both parents is seen in a significant fraction of the 422 teleost families. Viviparity is rare and is found in about 6% of teleost species. Male territoriality "preadapts" a species for evolving male parental care. There are a few examples of fish; the mangrove rivulus is an amphibious, simultaneous hermaphrodite, producing both eggs and spawn and having internal fertilisation.
This mode of reproduction may be related to the fish's habit of spending long periods out of water in the mangrove forests it inhabits. Males are produced at temperatures below 19 °C and can fertilise eggs that are spawned by the female; this maintains genetic variability in a species, otherwise inbred. The earliest known fossil actinopterygiian is Andreolepis hedei. Remains have been found in Russia and Estonia. Actinopterygians are divided into the subclasses Neopterygii; the Neopterygii, in turn, are divided into the infraclasses Teleostei. During the Mesozoic and Cenozoic the teleosts in particular diversified and as a result, 96% of all known fish species are teleosts; the cladogram shows the major groups of actinopterygians and their relationship to the terrestrial vertebrates that evolved from a related group of fish. Approximate dates are from al.. 2012. The polypterids are the sister lineage of all other actinopterygians, the Acipenseriformes are the sister lineage of Neopterygii, Holostei are the sister lineage of teleosts.
The Elopomorpha appears to be the most basic teleosts. The listing below follows Phylogenetic Classification of Bony Fishes with notes when this differs from Nelson, ITIS and FishBase and extinct groups from Van der Laan 2016. Order †? Asarotiformes Schaeffer 1968 Order †? Discordichthyiformes Minikh 1998 Order †? Paphosisciformes Grogan & Lund 2015 Order †? Scanilepiformes Selezneya 1985 Order †Cheirolepidiformes Kazantseva-Selezneva 1977 Order †Paramblypteriformes Heyler 1969 Order †Rhadinichthyiformes Order †Palaeonisciformes Hay 1902 Order †Tarrasiiformes sensu Lund & Poplin 2002 Order †Ptycholepiformes Andrews et al. 1967 Order †Redfieldiiformes Berg 1940 Order †Haplolepidiformes Westoll 1944 Order †Aeduelliformes Heyler 1969 Order †Platysomiformes Aldinger 1937 Order †Dorypteriformes Cope 1871 Order †Eurynotiformes Sallan & Coates 2013 Subclass Cladistii Pander 1860 Order †Guildayichthyiformes Lund 2000 Order Polypteriformes Bleeker 1859 Clade Actinopteri Cope 1972 s.s. Order †Elonichthyiformes Kazantseva-Selezneva 1977 Order †Phanerorhynchiformes Order †Saurichthyiformes Berg 1937 Subclass Chondrostei Order †Birgeriiformes Jin 2001 Order †Chondrosteiformes Order Acipenseriformes Berg 1940 Subclass Neopterygii Regan 1923 sensu Xu & Wu 2012 Order †Pholidopleuriformes Berg 1937 Order †Peltopleuriformes Lehman 1966 Order †Perleidiformes Berg 1937 Order †Luganoiiformes Lehman 1958 Order †Pycnodontiformes Berg 1937 Infraclass Holostei Muller 1844 Division Halecomorpha Cope 1872 sensu Grande & Bemis 1998 Order †Parasemionotiformes Lehman 1966 Order †Ionoscopiformes Grande & Bemis 1998 Order Amiiformes Huxley 1861 sensu Grande & Bemis 1998 Division Ginglymodi Cope 1871 Order †Dapediiformes Thies & Waschkewitz 2015 Order †Semionotiformes Arambourg & Bertin 1958 Order Lepisosteiformes Hay 1929 Clade Teleosteomorpha Arratia 2000 sensu Arratia 2013 Order †Prohaleciteiformes Arratia 2017 Division Aspidorhynchei Nelson, Grand & Wilson 2016 Order †Aspidorhynchiformes Bleeker 1859 Order †Pachycormiformes Berg 1937 Infraclass Teleostei Müller 1844 sensu Arratia 2013 Order †?
Araripichthyiformes Order †? Ligulelliiformes Taverne 2011 Order †? Tselfatiiformes Nelson 1994 Order †Pholidophori
A snail is, in loose terms, a shelled gastropod. The name is most applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name snail is used for most of the members of the molluscan class Gastropoda that have a coiled shell, large enough for the animal to retract into; when the word "snail" is used in this most general sense, it includes not just land snails but numerous species of sea snails and freshwater snails. Gastropods that lack a shell, or have only an internal shell, are called slugs, land snails that have only a small shell are called semi-slugs. Snails have considerable human relevance, including as food items, as pests, as vectors of disease, their shells are used as decorative objects and are incorporated into jewelry; the snail has had some cultural significance, has been used as a metaphor. Snails that respire using a lung belong to the group Pulmonata; as traditionally defined, the Pulmonata were found to be polyphyletic in a molecular study per Jörger et al. dating from 2010.
But snails with gills form a polyphyletic group. Both snails that have lungs and snails that have gills have diversified so over geological time that a few species with gills can be found on land and numerous species with lungs can be found in freshwater. A few marine species have lungs. Snails can be found in a wide range of environments, including ditches and the abyssal depths of the sea. Although land snails may be more familiar to laymen, marine snails constitute the majority of snail species, have much greater diversity and a greater biomass. Numerous kinds of snail can be found in fresh water. Most snails have thousands of microscopic tooth-like structures located on a banded ribbon-like tongue called a radula; the radula works like a file. Many snails are herbivorous, eating plants or rasping algae from surfaces with their radulae, though a few land species and many marine species are omnivores or predatory carnivores. Snails cannot absorb colored pigments when eating paper or cardboard so their feces are colored.
Several species of the genus Achatina and related genera are known as giant African land snails. The largest living species of sea snail is Syrinx aruanus; the snail Lymnaea makes decisions by using only two types of neuron: one deciding whether the snail is hungry, the other deciding whether there is food in the vicinity. The largest known land gastropod is the African giant snail Achatina achatina, the largest recorded specimen of which measured 39.3 centimetres from snout to tail when extended, with a shell length of 27.3 cm in December 1978. It weighed 900 g. Named Gee Geronimo, this snail was owned by Christopher Hudson of Hove, East Sussex, UK, was collected in Sierra Leone in June 1976. Gastropods that lack a conspicuous shell are called slugs rather than snails; some species of slug have a red shell, some have only an internal vestige that serves as a calcium repository, others have no shell at all. Other than that there is little morphological difference between slugs and snails. There are however important differences in habitats and behavior.
A shell-less animal is much more maneuverable and compressible, so quite large land slugs can take advantage of habitats or retreats with little space, retreats that would be inaccessible to a similar-sized snail. Slugs squeeze themselves into confined spaces such as under loose bark on trees or under stone slabs, logs or wooden boards lying on the ground. In such retreats they are in less danger from either predators or desiccation, those are suitable places for laying their eggs. Slugs as a group are far from monophyletic; the reduction or loss of the shell has evolved many times independently within several different lineages of gastropods. The various taxa of land and sea gastropods with slug morphology occur within numerous higher taxonomic groups of shelled species. Land snails are known as an agricultural and garden pest but some species are an edible delicacy and household pets. There are a variety of snail-control measures that gardeners and farmers use in an attempt to reduce damage to valuable plants.
Traditional pesticides are still used, as are many less toxic control options such as concentrated garlic or wormwood solutions. Copper metal is a snail repellent, thus a copper band around the trunk of a tree will prevent snails from climbing up and reaching the foliage and fruit. Placing crushed egg shells on the soil around garden plants can deter snails from coming to the plants; the decollate snail will capture and eat garden snails, because of this it has sometimes been introduced as a biological pest control agent. However, this is not without problems, as the decollate snail is just as to attack and devour other gastropods that may represent a valuable part of the native fauna of the region. In French cuisine, edible snails are served for instance in Escargot à la Bourguignonne; the practice of rearing snails for food is known as heliciculture. For purposes of cultivation, the snails are kept in a
Fish farming or pisciculture involves raising fish commercially in tanks or enclosures such as fish ponds for food. It is the principal form of aquaculture. A facility that releases juvenile fish into the wild for recreational fishing or to supplement a species' natural numbers is referred to as a fish hatchery. Worldwide, the most important fish species produced in fish farming are carp, tilapia and catfish. Demand is increasing for fish and fish protein, which has resulted in widespread overfishing in wild fisheries. China provides 62% of the world's farmed fish; as of 2016, more than 50% of seafood was produced by aquaculture. Farming carnivorous fish, such as salmon, does not always reduce pressure on wild fisheries. Carnivorous farmed fish are fed fishmeal and fish oil extracted from wild forage fish; the 2008 global returns for fish farming recorded by the FAO totaled 33.8 million tonnes worth about $US 60 billion. Aquaculture makes use of local photosynthetic production or fish that are fed with external food supply.
Growth is limited by available food zooplankton feeding on pelagic algae or benthic animals, such as crustaceans and mollusks. Tilapia filter feed directly on phytoplankton. Photosynthetic production can be increased by fertilizing pond water with artificial fertilizer mixtures, such as potash, phosphorus and microelements. Another issue is the risk of algal blooms; when temperatures, nutrient supply, available sunlight are optimal for algal growth, algae multiply at an exponential rate exhausting nutrients and causing a subsequent die-off in fish. The decaying algal biomass depletes the oxygen in the pond water because it blocks out the sun and pollutes it with organic and inorganic solutes, which can lead to massive loss of fish. An alternate option is to use a wetland system, such as that used in the commercial fish farm Veta La Palma, Spain. To tap all available food sources in the pond, the aquaculturist chooses fish species that occupy different places in the pond ecosystem, e.g. a filter algae feeder such as tilapia, a benthic feeder such as carp or [catfish, a zooplankton feeder or submerged weeds feeder such as grass carp.
Despite these limitations, significant fish farming industries use these methods. In the Czech Republic, thousands of natural and semi-natural ponds are harvested each year for trout and carp; the large Rožmberk Pond near Trebon, built in 1590, is still in use. In these kinds of systems fish production per unit of surface can be increased at will, as long as sufficient oxygen, fresh water and food are provided; because of the requirement of sufficient fresh water, a massive water purification system must be integrated in the fish farm. One way to achieve this is to combine hydroponic horticulture and water treatment, see below; the exception to this rule are cages which are placed in a river or sea, which supplements the fish crop with sufficient oxygenated water. Some environmentalists object to this practice; the cost of inputs per unit of fish weight is higher than in extensive farming because of the high cost of fish feed. It must contain a much higher level of protein than cattle feed and a balanced amino acid composition, as well.
These higher protein-level requirements are a consequence of the higher feed efficiency of aquatic animals. Fish such as salmon have an FCR around 1.1 kg of feed per kg of salmon whereas chickens are in the 2.5 kg of feed per kg of chicken range. Fish do not use energy to keep warm, eliminating some carbohydrates and fats in the diet, required to provide this energy; this may be offset, though, by the lower land costs and the higher production which can be obtained due to the high level of input control. Aeration of the water is essential; this is achieved by cascade flow, or aqueous oxygen. Clarias spp. can breathe atmospheric air and can tolerate much higher levels of pollutants than trout or salmon, which makes aeration and water purification less necessary and makes Clarias species suited for intensive fish production. In some Clarias farms, about 10% of the water volume can consist of fish biomass; the risk of infections by parasites such as fish lice, intestinal worms and protozoa is similar to that in animal husbandry at high population densities.
However, animal husbandry is a larger and more technologically mature area of human agriculture and has developed better solutions to pathogen problems. Intensive aquaculture has to provide adequate water quality levels to minimize stress on the fish; this requirement makes control of the pathogen problem more difficult. Intensive aquaculture requires a high level of expertise of the fish farmer. Very-high-intensity recycle aquaculture systems, where all the production parameters are controlled, are being used for high-value species. By recycling water, little is used per unit of production. However, the process has high operating costs; the higher cost structures mean that RAS is economical only for high-value products, such as broodstock for egg production, fingerlings for net pen aquaculture operations, sturgeon production, research animals, some special niche markets such as live fish. Raising ornamental coldwater fish, although theoretically much more profitable due to the higher income per weig