Actinopterygii

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Template:Short description Template:Use dmy dates Template:Automatic taxobox Actinopterygii (Template:IPAc-en; Template:Etymology), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish<ref>Template:Cite book</ref> that constitute nearly 99% of the over 30,000 living species of fish.<ref>(Davis, Brian 2010).</ref> The vast majority of extant actinopterygian species are teleosts, and by species count they dominate the subphylum Vertebrata, comprising over 50% of all living vertebrates.<ref name="NelsonFoW">Template:Cite book</ref> They are the most abundant nektonic aquatic animals and are ubiquitous throughout freshwater, brackish and marine environments from the deep sea to subterranean waters to the highest mountain streams. Extant species can range in size from Paedocypris, at Template:Cvt, to the giant sunfish, at Template:Cvt, and the giant oarfish, at Template:Cvt (or possibly Template:Cvt). The largest ever known ray-finned fish, the extinct Leedsichthys from the Jurassic, is estimated to have grown to Template:Cvt.

Ray-finned fish are so called because of their lightly built fins made of webbings of skin supported by radially extended thin bony spines called lepidotrichia, as opposed to the bulkier, fleshy fins of the sister clade Sarcopterygii (lobe-finned fish). Resembling folding fans, the actinopterygian fins can easily change shape, orientation and wetted area, providing superior thrust-to-weight ratios per movement compared to sarcopterygian and chondrichthyian fins. The fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the articulation between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).

Characteristics

Template:Center A: dorsal fin, B: fin rays, C: lateral line, D: kidney, E: swim bladder, F: Weberian apparatus, G: inner ear, H: brain, I: nostrils, L: eye, M: gills, N: heart, O: stomach, P: gall bladder, Q: spleen, R: internal sex organs (ovaries or testes), S: pelvic fins, T: spine, U: anal fin, V: tail (caudal fin). Possible other parts not shown: barbels, adipose fin, external genitalia (gonopodium)

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 a more derived structure and used for buoyancy.<ref name="Funk">Template:Cite journal</ref> Except from the bichirs, which just like the lungs of lobe-finned fish have retained the ancestral condition of ventral budding from the foregut, the swim bladder in ray-finned fishes derives from a dorsal bud above the foregut.<ref name="Funk" /> In early forms the swim bladder could still be used for breathing, a trait still present in Holostei (bowfins and gars).<ref>Template:Cite journal</ref> In some fish like the arapaima, the swim bladder has been modified for breathing air again,<ref>Template:Cite journal</ref> and in other lineages it has been completely lost.<ref>Template:Cite journal</ref> The teleosts have urinary and reproductive tracts that are fully separated, while the Chondrostei have common urogenital ducts, and partially connected ducts are found in Cladistia and Holostei.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite web</ref> Teleosts and chondrosteans (sturgeons and paddlefish) also differ from the bichirs and holosteans (bowfin and gars) in having gone through a whole-genome duplication (paleopolyploidy). The WGD is estimated to have happened about 320 million years ago in the teleosts, which on average has retained about 17% of the gene duplicates, and around 180 (124–225) million years ago in the chondrosteans. It has since happened again in some teleost lineages, like Salmonidae (80–100 million years ago) and several times independently within the Cyprinidae (in goldfish and common carp as recently as 14 million years ago).<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Template:Clear left

Body shapes and fin arrangements

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

Reproduction

Three-spined stickleback (Gasterosteus aculeatus) males (red belly) build nests and compete to attract females to lay eggs in them. Males then defend and fan the eggs. Painting by Alexander Francis Lydon, 1879

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.<ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref> Most families use external rather than internal fertilization.<ref name=Pitcher>Template:Cite book</ref> Of the oviparous teleosts, most (79%) do not provide parental care.<ref name=Reynolds>Template:Cite journal</ref> 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.<ref name=Reynolds /> The oldest case of viviparity in ray-finned fish is found in Middle Triassic species of Template:ExtinctSaurichthys.<ref>Template:Cite journal</ref> Viviparity is relatively rare and is found in about 6% of living teleost species; male care is far more common than female care.<ref name=Reynolds /><ref name=Clutton-Brock>Template:Cite book</ref> Male territoriality "preadapts" a species for evolving male parental care.<ref name=Werren>Template:Cite journal</ref><ref name=Baylis>Template:Cite journal</ref> 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 Template:Convert and can fertilise eggs that are then spawned by the female. This maintains genetic variability in a species that is otherwise highly inbred.<ref name=Wootton>Template:Cite book</ref> Template:Clear

Classification and fossil record

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Actinopterygii is divided into the subclasses Cladistia, 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.<ref name="Sallan 2014">Template:Cite journal</ref> 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).<ref name=PNAS>Template:Cite journal</ref><ref name=TOL>Template:Cite journal</ref> 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.<ref name=laurin&reisz1995>Template:Cite journal</ref> Approximate divergence dates for the different actinopterygian clades (in millions of years, mya) are from Near et al., 2012.<ref name=PNAS /> Template:Clade 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.<ref name=PNAS /> 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.<ref>Template:Cite web</ref> Crown group actinopterygians most likely originated near the Devonian-Carboniferous boundary.<ref name="HendersonDunneFaseyGiles2022">Template:Cite journal</ref> The earliest fossil relatives of modern teleosts are from the Triassic period (Prohalecites, Pholidophorus),<ref name="Arratia 2015">Template:Cite journal</ref><ref name="Romano et al 2016">Template:Cite journal</ref> although it is suspected that teleosts originated already during the Paleozoic Era.<ref name=PNAS />

Chondrostei Template:Center Chondrostei (cartilage bone) is a subclass of primarily cartilaginous fish showing some ossification. Earlier definitions of Chondrostei are now known to be paraphyletic, meaning that this subclass does not contain all the descendants of their common ancestor. There used to be 52 species divided among two orders, the Acipenseriformes (sturgeons and paddlefishes) and the Polypteriformes (reedfishes and bichirs). Reedfish and birchirs are now separated from the Chondrostei into their own sister lineage, the Cladistia. It is thought that the chondrosteans evolved from bony fish but lost the bony hardening of their cartilaginous skeletons, resulting in a lightening of the frame. Elderly chondrosteans show beginnings of ossification of the skeleton, suggesting that this process is delayed rather than lost in these fish.<ref name="chondro">Template:Cite web</ref> This group had once been classified with the sharks: the similarities are obvious, as not only do the chondrosteans mostly lack bone, but the structure of the jaw is more akin to that of sharks than other bony fish, and both lack scales (excluding the Polypteriforms). Additional shared features include spiracles and, in sturgeons, a heterocercal tail (the vertebrae extend into the larger lobe of the caudal fin). However the fossil record suggests that these fish have more in common with the Teleostei than their external appearance might suggest.<ref name="chondro" />
Neopterygii Template:Center Neopterygii (new fins) is a subclass of ray-finned fish that appeared somewhere in the Late Permian. There were only few changes during its evolution from the earlier actinopterygians. Neopterygians are a very successful group of fishes because they can move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient. While electroreception and the ampullae of Lorenzini is present in all other groups of fish, with the exception of hagfish, neopterygians have lost this sense, though it later re-evolved within Gymnotiformes and catfishes, who possess nonhomologous teleost ampullae.<ref>Template:Cite book</ref>
Fossil of the Devonian Template:Extinctcheirolepidiform Template:ExtinctCheirolepis canadensis
Fossil of the Carboniferous Template:Extinctelonichthyiform Template:ExtinctElonichthys peltigerus
Fossil of the Permian Template:Extinctaeduelliform Template:ExtinctAeduella blainvillei
Fossil of the Permian Template:Extinctpalaeonisciform Template:ExtinctPalaeoniscum freieslebeni
Fossil of the Triassic Template:Extinctbobasatraniiform Template:ExtinctBobasatrania canadensis
Fossil of the Triassic Template:Extinctperleidiform Template:ExtinctThoracopterus magnificus
Fossils of the Triassic Template:Extinctprohaleciteiform Template:ExtinctProhalecites sp., the earliest teleosteomorph
Fossil of the Jurassic Template:Extinctaspidorhynchiform Template:ExtinctAspidorhynchus sp.
Fossil of the Jurassic Template:Extinctpachycormiform Template:ExtinctPachycormus curtus
Fossil of the Cretaceous acipenseriform Template:ExtinctYanosteus longidorsalis
Fossil of the Cretaceous aulopiform Template:ExtinctNematonotus longispinus
Fossil of the Cretaceous Template:Extinctichthyodectiform Template:ExtinctThrissops formosus
Fossil of the Eocene carangiform Template:ExtinctMene oblonga
Fossil of the Eocene pleuronectiform Template:ExtinctAmphistium paradoxum
Fossil of a ray-finned perch (Template:ExtinctPriscacara serrata) from the Lower Eocene about 50 million years ago
Fossil of the Miocene syngnathiform Template:ExtinctNerophis zapfei
Skeleton of the angler fish, Lophius piscatorius. The first spine of the dorsal fin of the anglerfish is modified so it functions like a fishing rod with a lure
Skeleton of another ray-finned fish, the lingcod
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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 Eschmeyer's Catalog of Fishes<ref name=":13222">Template:Cite web</ref> and Phylogenetic Classification of Bony Fishes<ref name=TOL /> with notes when this differs from Nelson,<ref name="NelsonFoW"/> ITIS<ref>Template:ITIS</ref> and FishBase<ref>Template:Cite web</ref> and extinct groups from Van der Laan 2016<ref>Template:Cite book</ref> and Xu 2021.<ref>Template:Cite journal</ref>

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References

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Template:Actinopterygii Template:Chordata Template:Evolution of fish Template:Fins, limbs and wings Template:Taxonbar Template:Authority control

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