Temporal range: Early Triassic–Recent 
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Retroculus lapidifer (Perciformes: Cichlidae)
Hairy Blenny, Labrisomus nuchipinnis
Blue Tang, Acanthurus coeruleus
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 26,840 extant species in about 40 orders and 448 families; most living fishes are members of this group. The other two infraclasses, Holostei and Chondrostei, may be paraphyletic.
Teleosts have a movable maxilla and premaxilla and corresponding modifications in the jaw musculature. These modifications make it possible for teleosts to protrude their jaws outwards from the mouth. The caudal fin is homocercal, meaning the upper and lower lobes are about equal in size. The spine ends at the caudal peduncle, distinguishing this group from those in which the spine extends into the upper lobe of the caudal fin, such as most fish from the Paleozoic.
Reproduction and parental care
Most teleost families use external fertilization rather than internal fertilization. Of the oviparous teleosts, most (79%) do not provide parental care. Viviparity, ovoviviparity, or some form of parental care for eggs is seen in a significant fraction (21%) of the 422 teleost families. Viviparity is relatively rare and is found in about 6% of teleost species, whereas it is found in around half of shark and ray species.
Evolution of parental care
Teleosts exhibit all of the four potential care states (no care, male care, biparental care, and female care), with no care being by far the most frequent and female care being least frequent. No care is the most likely ancestral state and is still seen in most teleost families. Numerous species of teleost fishes have transitioned from no care toward uniparental care. Viviparity or parental care of eggs is seen more often in teleost species that breed in freshwater or shallow coastal waters because in such environments, unattended eggs are more likely to die of predation and/or variable water conditions. Because there are also fewer optimal spawning sites in those environments, males territoriality and defense of such sites is evolutionarily favored. Biparental care of eggs or young is rare, even though sharing the costs of parental duties often improves survival of the offspring. It is typically unnecessary for fish parents to feed their young, compared to birds where feeding the young and biparental care are more common. The lighter workload in fish means it is unnecessary for both parents to give care. It is likely that in the Cichlidae family, female only care developed from biparental care.
Male parental care
Among teleosts that show parental care of eggs or young, male parental care is far more common than female parental care. Teleosts that have evolved exclusive male care are characterized by male territoriality and low costs of care. These conditions probably surfaced from the relatively limited spawning grounds, usually in freshwater or shallow coastal water, where it would benefit males to guard territories. That in turn would reduce the cost of guarding clutches of eggs (thus providing parental care), and would allow females to spawn repeatedly to produce multiple clutches. Male territoriality "preadapts" a species for evolving male parental care. Examples of teleost species with male parental care include the mottled sculpin, the three-spined stickleback, and the pumpkinseed sunfish. There seems to be a correlation between teleost species that show male parental care and female fecundity that increases as body length increases.
Some teleost species show no signs of mate selection beyond the correct sex and species, while some species display male preference for higher female fecundity, which is usually related to size. In many species, females select for better nest sites which are usually occupied by larger males. In species showing male parental care on dense spawning grounds, males on adjacent territories often fight among themselves, most likely for females. Some species, like the desert pupfish, exhibit lek mating, wherein males aggregate along spawning territories and may display competitive behavior in order to entice visiting females that survey prospective partners for copulation.
Systematics and evolution
The oldest teleost fossils date back to early Triassic, possibly evolving from fish related to the bowfin in the clade Holostei. During the Mesozoic and Cenozoic they diversified and as a result, 96% of all known fish species are teleosts. Teleosts are here divided into twelve superorders, but this system is unlikely to be entirely correct and is in the process of being studied.
- Superorder Osteoglossomorpha
- Superorder Elopomorpha
- Superorder Clupeomorpha
- Superorder Ostariophysi
- Order Gonorynchiformes, including the milkfishes
- Order Cypriniformes, including barbs, carp, danios, goldfishes, loaches, minnows, rasboras
- Order Characiformes, including characins, pencilfishes, hatchetfishes, piranhas, tetras
- Order Gymnotiformes, including electric eels and knifefishes
- Order Siluriformes, the catfishes
- Superorder Protacanthopterygii
- Superorder Stenopterygii (may belong in Protacanthopterygii)
- Superorder Cyclosquamata (may belong in Protacanthopterygii)
- Superorder Scopelomorpha
- Superorder Lampridiomorpha
- Superorder Polymyxiomorpha
- Superorder Paracanthopterygii
- Superorder Acanthopterygii
- Order Mugiliformes, the mullets
- Order Atheriniformes, including silversides and rainbowfishes
- Order Beloniformes, including the flyingfishes
- Order Cetomimiformes, the whalefishes
- Order Cyprinodontiformes, including livebearers, killifishes
- Order Stephanoberyciformes, including the ridgeheads
- Order Beryciformes, including the fangtooths and pineconefishes
- Order Zeiformes, including the dories
- Order Gobiesociformes, the clingfishes
- Order Gasterosteiformes including sticklebacks, pipefishes, seahorses
- Order Syngnathiformes, including the seahorses and pipefishes
- Order Synbranchiformes, including the swamp eels
- Order Tetraodontiformes, including the filefish, pufferfish, triggerfish, porcupinefish, and sunfish
- Order Pleuronectiformes, the flatfishes
- Order Scorpaeniformes, including scorpionfishes and the sculpins
- Order Perciformes, 40% of all fish including anabantids, basses, cichlids, gobies, gouramis, mackerel, perches, scats, whiting, wrasses
Because most extant fish species are members of this group, teleostei are found world-wide and in most aquatic environments. About 60% live in marine environments and 40% live in freshwater.
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- In ITIS, Gobiesociformes is placed as the suborder Gobiesocoidei of the order Perciformes.
- In ITIS, Syngnathiformes is placed as the suborder Syngnathoidei of the order Gasterosteiformes.