Teleost

Teleosts are members of the Teleostei infraclass, which contains 96 percent of all extant fish species in the Actinopterygii (ray-finned fishes) class. Teleosts are divided into 448 families and 40 orders. There are around 26,000 species that have been identified. Teleosts range in size from 7.6 m or more enormous oarfish and ocean sunfish averaging over 2 t (2.0 long tonnes; 2.2 short tonnes) to the 6.2 mm long male anglerfish Photocorynus spiniceps. Teleosts could be flattened vertically or horizontally, be extended cylinders, or acquire unique shapes, such as anglerfish and seahorses, and include not only torpedo-shaped fish optimised for speed.

[Image will be Uploaded Soon]

Teleosts vary from other bony fish primarily in their jawbones; they have moveable premaxilla and matching alterations in the jaw muscles that allow them to extend their jaws outwardly from the mouth. This gives them a huge advantage since it allows them to grip prey and pull it into their mouth. The expanded premaxilla is the principal tooth-bearing bone of more developed teleosts, and the maxilla, which is linked to the lower jaw, works as a lever, pulling and pushing the premaxilla like the mouth opens and closes. Food is ground and swallowed by other bones in the rear of the mouth. An additional distinction is that the tail (caudal) fin's top and lower lobes are roughly similar in size. The spine of such a group of fish terminates at the caudal peduncle, whereas the spine of other fish continues into the upper lobe of the tail fin.

Teleosts have used a variety of reproduction methods. External fertilisation is used by the majority of species: the female deposits a batch of eggs, the male is responsible for fertilizing them, and the larvae grow without parental intervention. A large number of teleosts are sequential hermaphrodites, meaning they start as females and eventually convert to males, with some species undoing the process. Certain teleosts are viviparous, and several offer parental care, with the male fish protecting the nest and fanning the eggs to maintain them well-oxygenated.

Anatomy

Teleosts are distinguished by their movable premaxilla, enlarged neural arches at the caudal fin's end, and unpaired basibranchial tooth plates. The premaxilla is a tooth that protrudes from the mouth and creates a circular aperture. It is unattached to the neurocranium (braincase). This reduces the pressure inside the mouth, allowing the prey to be sucked in. After that, the lower jaw and maxilla are drawn back to seal the mouth, allowing the fish to grip the prey. Simply closing the jaws, on the other hand, risks forcing food out from the mouth. The premaxilla is larger and has teeth in mature teleosts, whereas the maxilla is toothless. Both the premaxilla and the lower jaw are pushed forward by the maxilla. Furthermore, the maxilla rotates little, pushing a bony process forward which interlocks with the premaxilla.

Teleosts' pharyngeal jaws, a second set of jaws located inside the throat, are made up of five branchial arches, which are bone loops that support the gills. A single basibranchial is surrounded by two hypobranchial, ceratobranchial, epibranchial, and pharyngobranchials in the first three arches. A tooth plate covers the median basibranchial. The fourth arch is made up of pairs of ceratobranchials and epibranchials, as well as pharyngeal branchials and a basibranchial on occasion. The fifth ceratobranchial forms the base of the lower pharyngeal jaws, whereas the second, third, and fourth pharyngeal bronchioles serve as the base of the top. The pharyngeal jaws of more primitive teleosts are made up of well-separated thin components that adhere to the pectoral girdle, neurocranium, and hyoid bar. 

Their primary job is to carry food, and they rely heavily on lower pharyngeal jaw activity to do it. The jaws of more developed teleosts are stronger, with the ceratobranchial fusing to form a single lower jaw and the pharyngeal branchial fusing to form a huge upper jaw, which articulates with the neurocranium. They've also created a muscle that permits the pharyngeal jaws to do more than just carry food.

Distribution

Teleosts can be found in a variety of aquatic habitats, including warm and cold oceans, flowing and still rivers, and even isolated, occasionally hot and saline water bodies in deserts, like in the situation of the desert pupfish. At very high latitudes, teleost diversity plummets; at Franz Josef Land, up to 82°N, ice cover and water temperatures under 0 °C (32 °F) for much of the year constrain the number of species; 75 percent of the species discovered there are Arctic endemics.

The Elopomorpha, Clupeomorpha, and Percomorpha (perches, tunas, and a variety of other teleosts) all seem to have a global reach and are primarily marine; the Ostariophysi and Osteoglossomorpha are both worldwide but yet primarily freshwater, with the latter primarily in the tropics; and the Atherinomorpha (guppies, and other surface-dwelling teleosts) tend to have a worldwide distribution. The Esociformes (pikes) are restricted to freshwater in the Northern Hemisphere, but the Salmoniformes (salmon, trout) are located in freshwater in both the Northern and Southern temperate zones, with certain species moving to and from the sea. The Paracanthopterygii (cods etc.) are a group of fish from the Northern Hemisphere that includes both saltwater and freshwater species. 

Certain teleosts are migratory; several freshwater species migrate within river systems on a yearly basis; others, such as salmon and striped bass, are anadromous, living their days at sea and returning inland to breed. Many, such as the eel, are catadromous, meaning they swim backwards. Like an adult, the European freshwater eel migrates throughout the Atlantic Ocean to mate in floating seaweed in the Sargasso Sea. The adults spawn and die here, however the growing young are carried towards Europe by the Gulf Stream. They are little fish when they arrive, and they enter estuaries and ascend rivers, defeating challenges along the way to reach the streams and ponds where they will spend the rest of their lives.

Physiology

Respiration

The transfer of gases over the surface of the gills when water is brought in by the mouth and pushed out through the gills is the primary mode of respiration in teleosts, as it is in most other fish. The body has no oxygen reserves other than the swim bladder, which stores a limited amount of air, therefore respiration must be constant throughout the fish's life. Certain teleosts thrive in low-oxygen environments, like stagnant water or moist mud, and have developed supplementary tissues and organs to aid gas exchange in such environments.

Sensory Systems

Teleosts have extremely well-developed sense organs. Almost all daylight fish possess a colour vision that is comparable to that of a typical human. Chemoreceptors, which are responsible for intense sensations of taste and smell in many fish, are also present. The lateral line system, which senses gentle currents and vibrations as well as the motion of neighbouring fish and prey, is found in most fish. The lateral line, the swim bladder, and, in certain species, the Weberian apparatus are all used by fish to detect sound. Fish employ landmarks to orient themselves, and they may create mental maps depending on different landmarks or symbols. Fish have the spatial memory required to create such a mental map, according to experiments with mazes.

Osmoregulation

The teleost's skin is mostly impermeable to water, and the gills are the primary interface between the fish's body and its surroundings. Teleost fish receive water through their gills through osmosis in freshwater, but lose it in seawater. In freshwater, salts flow outwards across the gills, while saltwater diffuses inwards. The European flounder spends the majority of its life in the sea, but it does migrate to estuaries and rivers on occasion. This could obtain Na+ ions correspond to 40% of its total free salt content in one hour in the water, with 75% coming via the gills and the remaining through drinking. In rivers, on the other hand, only 2% of the body's Na+ concentration is exchanged per hour.

There is an active mechanism from across gills for the removal of salt in seawater and its absorption in freshwater, in addition to being able to choose to restrict salt and water exchanged by diffusion.

Thermoregulation

Fish are cold-blooded, and their body temperatures are often similar to those of their surroundings. They lose or gain heat via their skin and breathing, and they can control their circulation by increasing or decreasing blood flow to their gills in reaction to changes in water temperature. When exposed to cold, metabolic heat created in the muscles or intestines is swiftly lost through the gills, with blood redirected away from the gills. Several teleosts could only thrive in a narrow range of water temperatures due to their inability to regulate the temperature of the blood.

Buoyancy

Because a teleost's body is denser than water, it must balance for the difference or sink. Numerous teleosts have a swim bladder, which regulates their buoyancy by manipulating gases, allowing them to remain at the present water depth, as well as climb and descend without wasting energy swimming. 

The swim bladder of some primitive animals, such as minnows, is accessible to the oesophagus and serves as a lung. Fast-swimming fish, including tuna and mackerel, are generally devoid of it. The rete mirabilis, a network of blood arteries that serves as a countercurrent gas exchange between the swim bladder and the blood, regulates the gas level in fish with closed swim bladders. Sturgeons and other Chondrostei possess swim bladders, but they appear to have developed separately: other Actinopterygii, including the bowfin and bichir, may not, therefore swim bladders seem to have arisen twice, and the teleost swim bladder is not homologous with the chondrostean swim bladder.

Locomotion

Sturgeons and other Chondrostei possess swim bladders, but they appear to have developed separately: other Actinopterygii, including the bowfin and bichir, may not, therefore swim bladders seem to have arisen twice, and the teleost swim bladder is not homologous with the chondrostean swim bladder. There are several variations to this type of mobility, particularly where speed is not the primary goal; for example, sluggish swimming with excellent manoeuvrability may be advantageous among rocks and coral reefs. 

Eels move around by wiggling their whole bodies. The seahorse, which lives among seagrasses and algae, maintains an upright posture and swims by flapping its pectoral fins, whereas the pipefish, which is closely related, moves by rippling its extended dorsal fin. Gobies "hop" along the substrate, using their pectoral fins to support themselves up and propel themselves forward.

Sound Production

Certain teleosts make sounds, either by stridulation or by shaking their swim bladder, to attract mates. The muscles which connect to the swim bladder of the Sciaenidae make it oscillate quickly, producing drumming sounds. Grunts, sea horses, and marine catfish stridulate via rubbing skeletal components, teeth, and spines collectively. The swim bladder of such fish can function as a resonator. Stridulation noises have a frequency range of 1000 – 4000 Hz, while sounds modulated by the swim bladder have a lower frequency range.

Reproduction

The majority of teleost species are oviparous, meaning that both eggs and sperm are discharged into the water for fertilisation. Internal fertilisation is seen in 500 to 600 teleost species, however, it is most common in Chondrichthyes and numerous tetrapods. An intromittent organ is used by the male to inseminate the female. Less than one in a million externally fertilised eggs thrive to become mature fish, while the children of members of approximately a dozen viviparous families have a significantly greater chance of living. The eggs are fertilised internally in these and remain in the female throughout development. 

A few of these species, such as live-bearing aquarium fish (teleost fish) in the Poeciliidae family, are ovoviviparous, meaning that each egg does have a yolk sac that feeds the developing embryo, and then when the yolk sac is depleted, the egg hatches and the larva is released into the water column. Certain species, such as splitfins of the Goodeidae family, are completely viviparous, with the developing foetus receiving nourishment from the maternal blood supply through a placenta-like structure that grows in the uterus. Certain species, like Nomorhamphus ebrardtii, practise oophagy, wherein the mother deposits unfertilized eggs on which the developing larvae eat in the uterus, and intrauterine cannibalism has been observed in certain halfbeaks.

Hermaphroditism:

Hermaphroditic teleost species can be found in two forms: simultaneous and sequential. In the former, the gonads contain both spermatozoa and eggs. Simultaneous hermaphroditism is most common among creatures that reside deep in the ocean, where possible mates are scarce. Self-fertilization is uncommon, with only two species, Kryptolebias marmoratus and Kryptolebias hermaphroditus, having been observed. Individuals with sequential hermaphroditism may behave like one sex early in adulthood and then switch later. Parrotfish, sea breams, sea basses, flatheads, wrasses, and light fishes are among the species affected.

Protandry is when a man becomes a woman, whereas protogyny is when a woman becomes a man. Protandry is more prevalent than protogyny. Changing sex can happen in a variety of situations. The largest and most dominant female in the Bluestreak cleaner wrasse's harem of up to 10 females displays male-like behaviour and eventually tests when the male is absent. If she's fired, the next highest-ranking female will take her position. If a particular number of males are eliminated from a colony of the species Anthias squamipinnis, when females outnumber males by a wide margin, the very same number of females change gender and substitute them.

Individual clownfish reside in groups, and only the two largest females and males in each group breed: the largest female and the largest male. If the female dies, the male swaps sexes and is replaced by the next largest male. 

Mating Tactics

Teleosts have a number of different mating strategies. Certain species are promiscuous, with both males and females breeding with multiple partners and no evident mate preferences. Baltic herring, humbug damselfish, Nassau groupers, guppies, cichlids, and creole wrasses have all been found to have this trait. Polygamy, or having numerous partners of one sex, can take multiple shapes.

Polyandry occurs when a single adult female breeds with numerous males who only breed with her. This is uncommon in teleosts and fish in general, however, it can be found in clownfish. Furthermore, it is possible that it exists to some level among anglerfish, in which some females do have several males hooked to them. Polygyny is significantly more common, in which one man breeds with numerous females. 

Multiple females might visit a territorial male who protects and looks after eggs and babies in sculpins, damselfish, darters, sunfish, and cichlids. A male defending a harem of several ladies is another example of polygyny. Damselfishes, parrotfishes, wrasses, triggerfishes, surgeonfishes, and teleost fish are examples of coral reef species that exhibit this behaviour.

Growth and Development

The egg, larva, juvenile, and adult are the four major life stages of teleosts. Species can begin their lives in either a pelagic or a demersal habitat (near the seabed). Pelagic eggs are light, translucent, buoyant, and have thin coverings in most marine teleosts. Pelagic eggs depend on ocean currents to disseminate and are not cared for by their parents. The larvae are planktonic and unable to swim after they hatch. They are linked to a yolk sac, which gives nourishment. Demersal eggs are thick, coloured, moderately heavy, and able to adhere to substrates in most freshwater species. Freshwater fish are significantly more likely to be cared for by their parents. 

Demersal larvae, unlike their pelagic relatives, can swim and feed as early as they hatch. Adult teleosts and larval teleosts have radically diverse appearances, especially in maritime species. Certain larvae have even been thought to be of a distinct species from the adults. Larvae have a high mortality rate; most perish during the first week due to hunger or predation. Survival rates rise as they mature, and they develop increased physiological tolerance and sensitivity, as well as behavioural and ecological competence.

Economic Importance

Teleosts are economically significant in a variety of ways. They are hunted for food all around the globe. A few species, including tuna, herring, anchovy, pollock, cod, and mackerel, contribute millions of tonnes of food each year, whereas many others are caught in smaller quantities. They provide a significant share of the sport fish captured. Millions of people are employed as a result of commercial and leisure fishing.

Carp, salmon, tilapia, and catfish are just a few of the commercially farmed species that produce millions of tonnes of protein-rich food each year. According to the UN's Edible and Agriculture Organization, by 2030, farming will account for approximately 62 percent of all food fish.

Fresh fish is eaten, but it can also be preserved using traditional methods such as a combination of smoking, drying, and salting, as well as fermentation. Freezing, freeze-drying, and heat processing are all modern preservation procedures.

Breaded or battered fillets, fishcakes and fish fingers are all examples of frozen fish items. Fish meal has been used to augment the diets of farmed fish and animals. Fish oils are produced from either fish liver, which is particularly high in vitamins A and D, or the bodies of oily fish like sardines and herring, and are used as dietary supplements and to cure vitamin deficiencies. Aquarium specimens and pets include several smaller and much more colourful species. The leather industry employs sea wolves. Thread fish and drum fish are used to make isinglass.