Actinopterygii

Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.

The ray-finned fishes are so called because their fins are webs of skin supported by bony or horny spines (rays), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii (lobe-finned fish). 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 (e.g., pelvic and pectoral girdles).

By species count, actinopterygians dominate the vertebrates, and they constitute 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 , to the massive ocean sunfish, at , and the long-bodied oarfish, at . The vast majority of Actinopterygii (~99%) are teleosts.

Characteristics

Ray-finned fishes occur in many variant forms. The main features of typical ray-finned fish are shown in the adjacent diagram. The swim bladder is the more derived structure.

Ray-finned fishes have many different types of scales; but all teleosts have leptoid scales. The outer part of these scales fan out with bony ridges, while the inner part is crossed with fibrous connective tissue. Leptoid scales are thinner and more transparent than other types of scales, and lack the hardened enamel or dentine-like layers found in the scales of many other fish. Unlike ganoid scales, which are found in non-teleost actinopterygians, new scales are added in concentric layers as the fish grows.

Ray-finned and lobe-finned fishes sometimes possesses lungs used for aerial respiration. Only bichirs retain ventrally budding lungs.

Body shapes and fin arrangements

Ray-finned fish vary in size and shape, in their feeding specializations, and in the number and arrangement of their ray-fins.

Reproduction

In nearly all ray-finned fish, the sexes are separate, and in most species the females spawn eggs that are fertilized externally, typically with the male inseminating the eggs after they are laid. Development then 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 (79%) 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 (21%) of the 422 teleost families; no care is likely the ancestral condition. The oldest case of viviparity in ray-finned fish is found in Middle Triassic species of '' Saurichthys''. Viviparity is relatively rare and is found in about 6% of living teleost species; male care is far more common than female care. Male territoriality "preadapts" a species for evolving male parental care.

There are a few examples of fish that self-fertilise. 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 occasionally produced at temperatures below and can fertilise eggs that are then spawned by the female. This maintains genetic variability in a species that is otherwise highly inbred.

Classification and fossil record

Actinopterygii is divided into the classes Cladistia and Actinopteri. The latter comprises the subclasses Chondrostei and Neopterygii. The Neopterygii, in turn, is divided into the infraclasses Holostei and Teleostei. During the Mesozoic (Triassic, Jurassic, Cretaceous) and Cenozoic the teleosts in particular diversified widely. As a result, 96% of living fish species are teleosts (40% of all fish species belong to the teleost subgroup Acanthomorpha), while all other groups of actinopterygians represent depauperate lineages.

The classification of ray-finned fishes can be summarized as follows:

* Cladistia, which include bichirs and reedfish
* Actinopteri, which include:
** Chondrostei, which include Acipenseriformes (paddlefishes and sturgeons)
** Neopterygii, which include:
***Teleostei (most living fishes)
***Holostei, which include:
****Lepisosteiformes (gars)
****Amiiformes (bowfin)

The cladogram below shows the main clades of living actinopterygians and their evolutionary relationships to other extant groups of fishes and the four-limbed vertebrates (tetrapods). The latter include mostly terrestrial species but also groups that became secondarily aquatic (e.g. Whales and Dolphins). Tetrapods evolved from a group of bony fish during the Devonian period. Approximate divergence dates for the different actinopterygian clades (in millions of years, mya) are from Near et al., 2012.

The polypterids (bichirs and reedfish) are the sister lineage of all other actinopterygians, the Acipenseriformes (sturgeons and paddlefishes) are the sister lineage of Neopterygii, and Holostei (bowfin and gars) are the sister lineage of teleosts. The Elopomorpha (eels and tarpons) appear to be the most basal teleosts.

The earliest known fossil actinopterygian is ''Andreolepis hedei'', dating back 420 million years (Late Silurian), remains of which have been found in Russia, Sweden, and Estonia. Crown group actinopterygians most likely originated near the Devonian-Carboniferous boundary. The earliest fossil relatives of modern teleosts are from the Triassic period ('' Prohalecites'', '' Pholidophorus''), although it is suspected that teleosts originated already during the Paleozoic Era.

Taxonomy

The listing below is a summary of all extinct (indicated by a dagger, †) and living groups of Actinopterygii with their respective taxonomic rank. The taxonomy follows Phylogenetic Classification of Bony Fishes with notes when this differs from Nelson, ITIS and FishBase and extinct groups from Van der Laan 2016 and Xu 2021.

* 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 † Haplolepidiformes Westoll 1944
* Order † Aeduelliformes Heyler 1969
* Order † Platysomiformes Aldinger 1937
* Order † Dorypteriformes Cope 1871
* Order † Eurynotiformes Sallan & Coates 2013
* Class Cladistia Pander 1860
** Order † Guildayichthyiformes Lund 2000
** Order Polypteriformes Bleeker 1859 (bichirs and reedfishes)
* Class Actinopteri Cope 1972 s.s.
** Order † Elonichthyiformes Kazantseva-Selezneva 1977
** Order † Phanerorhynchiformes
** Order † Bobasatraniiformes Berg 1940
** Order †Saurichthyiformes Aldinger 1937
** Subclass Chondrostei Müller, 1844
*** Order † Birgeriiformes Heyler 1969
*** Order † Chondrosteiformes Aldinger, 1937
*** Order Acipenseriformes Berg 1940 (includes sturgeons and paddlefishes)
** Subclass Neopterygii Regan 1923 sensu Xu & Wu 2012
*** Order † Pholidopleuriformes Berg 1937
***Order † Redfieldiiformes Berg 1940
***Order †Platysiagiformes Brough 1939
***Order † Polzbergiiformes Griffith 1977
***Order † Perleidiformes Berg 1937
***Order † Louwoichthyiformes Xu 2021
*** Order † Peltopleuriformes Lehman 1966
*** Order † Luganoiiformes Lehman 1958
*** Order †Pycnodontiformes Berg 1937
*** Infraclass Holostei Müller 1844
**** Division Halecomorphi Cope 1872 sensu Grande & Bemis 1998
***** Order †Parasemionotiformes Lehman 1966
***** Order † Ionoscopiformes Grande & Bemis 1998
***** Order Amiiformes Huxley 1861 sensu Grande & Bemis 1998 ( bowfins)
**** Division Ginglymodi Cope 1871
***** Order † Dapediiformes Thies & Waschkewitz 2015
***** Order †Semionotiformes Arambourg & Bertin 1958
***** Order Lepisosteiformes Hay 1929 (gars)
*** 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 † Pholidophoriformes Berg 1940
***** Order † Dorsetichthyiformes Nelson, Grand & Wilson 2016
***** Order † Leptolepidiformes
***** Order †Crossognathiformes Taverne 1989
***** Order † Ichthyodectiformes Bardeck & Sprinkle 1969
***** Teleocephala de Pinna 1996 s.s.
******Megacohort Elopocephalai Patterson 1977 sensu Arratia 1999 (Elopomorpha Greenwood et al. 1966)
******* Order Elopiformes Gosline 1960 (ladyfishes and tarpon)
******* Order Albuliformes Greenwood et al. 1966 sensu Forey et al. 1996 (bonefishes)
******* Order Notacanthiformes Goodrich 1909 ( halosaurs and spiny eels)
******* Order Anguilliformes Jarocki 1822 sensu Goodrich 1909 (true eels)
****** Megacohort Osteoglossocephalai sensu Arratia 1999
*******Supercohort Osteoglossocephala sensu Arratia 1999 (Osteoglossomorpha Greenwood et al. 1966)
******** Order † Lycopteriformes Chang & Chou 1977
******** Order Hiodontiformes McAllister 1968 sensu Taverne 1979 (mooneye and goldeye)
******** Order Osteoglossiformes Regan 1909 sensu Zhang 2004 (bony-tongued fishes)
******* Supercohort Clupeocephala Patterson & Rosen 1977 sensu Arratia 2010
********Cohort Otomorpha Wiley & Johnson 2010 ( Otocephala; Ostarioclupeomorpha)
********* Subcohort Clupei Wiley & Johnson 2010 ( Clupeomorpha Greenwood et al. 1966)
********** Order † Ellimmichthyiformes Grande 1982
********** Order Clupeiformes Bleeker 1859 ( herrings and anchovies)
********* Subcohort Alepocephali
********** Order Alepocephaliformes Marshall 1962
********* Subcohort Ostariophysi Sagemehl 1885
********** Section Anotophysa (Rosen & Greenwood 1970) Sagemehl 1885
*********** Order † Sorbininardiformes Taverne 1999
*********** Order Gonorynchiformes Regan 1909 (milkfishes)
********** Section Otophysa Garstang 1931
*********** Order Cypriniformes Bleeker 1859 sensu Goodrich 1909 ( barbs, carp, danios, goldfishes, loaches, minnows, rasboras)
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