Including not only torpedo-shaped fish built for speed, teleosts can be flattened vertically or horizontally, be elongated cylinders or take specialised shapes as in anglerfish and seahorses.

A fair proportion of teleosts are sequential hermaphrodites, starting life as females and transitioning to males at some stage, with a few species reversing this process.

A small percentage of teleosts are viviparous and some provide parental care with typically the male fish guarding a nest and fanning the eggs to keep them well-oxygenated.

Distinguishing features of the teleosts are mobile premaxilla, elongated neural arches at the end of the caudal fin and unpaired basibranchial toothplates.

In the more basal teleosts the pharyngeal jaws consist of well-separated thin parts that attach to the neurocranium, pectoral girdle, and hyoid bar.

[11] Müller based this classification on certain soft tissue characteristics, which would prove to be problematic, as it did not take into account the distinguishing features of fossil teleosts.

[10][12] The oldest fossils of teleosteomorphs (the stem group from which teleosts later evolved) date back to the Triassic period (Prohalecites, Pholidophorus).

[15] Coelacanths Lungfish Lissamphibia Mammals Sauropsida (reptiles, birds) Polypteriformes (bichirs, reedfishes) Acipenseriformes (sturgeons, paddlefishes) Lepisosteiformes (gars) Amiiformes (bowfin) Teleostei The phylogeny of the teleosts has been subject to long debate, without consensus on either their phylogeny or the timing of the emergence of the major groups before the application of modern DNA-based cladistic analysis.

[20][21] Hiodontiformes (mooneyes) Osteoglossiformes (bonytongues, elephantfishes) Elopiformes (tenpounders, tarpons) Albuliformes (Japanese gissus and bonefishes) Notacanthiformes (deep sea spiny eels) Anguilliformes (true eels) Clupeiformes (herrings) Alepocephaliformes (slickheads) Gonorynchiformes (milkfish) Cypriniformes (minnows, carps, loaches) Characiformes (tetras, piranhas) Gymnotiformes (knifefish and electric eels) Siluriformes (catfish) Lepidogalaxiiformes (salamanderfish) Argentiniformes (marine smelts) Galaxiiformes (whitebait, mudfishes) Esociformes (pike) Salmoniformes (salmon, trout) Stomiiformes (dragonfish) Osmeriformes (smelt) Ateleopodiformes (jellynoses) Aulopiformes (lizardfish) Myctophiformes (lanternfish) Lampriformes (oarfish, opah, ribbonfish) Percopsiformes (troutperches) Zeiformes (dories) Stylephoriformes (tube-eyes/thread-fins) Gadiformes (cods) Polymixiiformes (beardfish) Beryciformes (alfonsinos, whalefishes) Trachichthyiformes (pinecone fishes, slimeheads) Holocentriformes (squirrelfish, soldier fishes) Percomorpha The most diverse group of teleost fish today are the Percomorpha, which include, among others, the tuna, seahorses, gobies, cichlids, flatfish, wrasse, perches, anglerfish, and pufferfish.

Fossil evidence shows that there was a major increase in size and abundance of teleosts immediately after the mass extinction event at the Cretaceous-Paleogene boundary ca.

[5] The adipose fin, which is present in over 6,000 teleost species, is often thought to have evolved once in the lineage and to have been lost multiple times due to its limited function.

[30] Teleosts are found in almost every aquatic environment and have developed specializations to feed in a variety of ways as carnivores, herbivores, filter feeders and parasites.

Reef fish live in a complex, relatively confined underwater landscape and for them, manoeuvrability is more important than speed, and many of them have developed bodies which optimize their ability to dart and change direction.

By the time they arrive, they are small fish and enter estuaries and ascend rivers, overcoming obstacles in their path to reach the streams and ponds where they spend their adult lives.

Apart from the swim bladder, which contains a small amount of air, the body does not have oxygen reserves, and respiration needs to be continuous over the fish's life.

They gain and lose heat through their skin, and regulate their circulation in response to changes in water temperature by increasing or reducing the blood flow to the gills.

[58] When cold acclimated, teleost fish show physiological changes in skeletal muscle that include increased mitochondrial and capillary density.

[59] This reduces diffusion distances and aids in the production of aerobic ATP, which helps to compensate for the drop in metabolic rate associated with colder temperatures.

Other adaptations of tuna for speed include a streamlined, spindle-shaped body, fins designed to reduce drag,[60] and muscles with a raised myoglobin content, which gives these a reddish colour and makes for a more efficient use of oxygen.

[67] There are many exceptions to this method of locomotion, especially where speed is not the main objective; among rocks and on coral reefs, slow swimming with great manoeuvrability may be a desirable attribute.

Oophagy is practised by a few species, such as Nomorhamphus ebrardtii; the mother lays unfertilised eggs on which the developing larvae feed in the uterus, and intrauterine cannibalism has been reported in some halfbeaks.

[78] Salmon of the genus Oncorhynchus are well known for this feature; they hatch in fresh water and then migrate to the sea for up to four years before travelling back to their place of birth where they spawn and die.

This is recorded in Sculpins, sunfish, darters, damselfish and cichlids where multiple females may visit a territorial male that guards and takes care of eggs and young.

[79] Courtship in teleosts plays a role in species recognition, strengthening pair bonds, spawning site position and gamete release synchronisation.

It is often more efficient to gather food by working as a group, and individual fish optimise their strategies by choosing to join or leave a shoal.

[107] A small number of productive species including carp, salmon,[108] tilapia and catfish are farmed commercially, producing millions of tons of protein-rich food per year.

Among many recorded instances, overfishing caused the complete collapse of the Atlantic cod population off Newfoundland in 1992, leading to Canada's indefinite closure of the fishery.

Water pollution caused local extinction of teleost populations in many northern European lakes in the second half of the twentieth century.

The zebrafish is the most commonly used laboratory vertebrate,[106] offering the advantages of genetic similarity to mammals, small size, simple environmental needs, transparent larvae permitting non-invasive imaging, plentiful offspring, rapid growth, and the ability to absorb mutagens added to their water.

Haeckel had become convinced by Goethe and Alexander von Humboldt that by making accurate depictions of unfamiliar natural forms, such as from the deep oceans, he could not only discover "the laws of their origin and evolution but also to press into the secret parts of their beauty by sketching and painting".