Ray-finned fish are so called because of their lightly builtfins made of webbings ofskin supported by radially extended thin bonyspines calledlepidotrichia, as opposed to the bulkier, fleshy fins of thesistercladeSarcopterygii (lobe-finned fish). Resemblingfolding fans, the actinopterygian fins can easily change shape, orientation andwetted area, providing superiorthrust-to-weight ratios per movement compared to sarcopterygian andchondrichthyian fins. The fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent thearticulation between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).
Ray-finned fishes occur in many variant forms. The main features of typical ray-finned fish are shown in the adjacent diagram.Theswim bladder is a more derived structure and used forbuoyancy.[5] Except from thebichirs, which just like thelungs oflobe-finned fish have retained the ancestral condition of ventral budding from theforegut, the swim bladder in ray-finned fishes derives from a dorsal bud above the foregut.[5] In early forms the swim bladder could still be used for breathing, a trait still present inHolostei (bowfins andgars).[6] In some fish like thearapaima, the swim bladder has been modified for breathing air again,[7] and in other lineages it has been completely lost.[8]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.[9]Ray-finned fishes have many different types ofscales; but allteleosts haveleptoid 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 hardenedenamel- ordentine-like layers found in the scales of many other fish. Unlikeganoid scales, which are found in non-teleost actinopterygians, new scales are added in concentric layers as the fish grows.[10]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 theCyprinidae (in goldfish and common carp as recently as 14 million years ago).[11][12][13][14][15]
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 byAlexander 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.[16] However other patterns ofontogeny exist, with one of the commonest beingsequential hermaphroditism. In most cases this involvesprotogyny, 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.[17]Most families useexternal rather thaninternal fertilization.[18] Of theoviparous teleosts, most (79%) do not provide parental care.[19]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.[19] The oldest case of viviparity in ray-finned fish is found inMiddle Triassic species of†Saurichthys.[20] Viviparity is relatively rare and is found in about 6% of living teleost species; male care is far more common than female care.[19][21] Male territoriality"preadapts" a species for evolving male parental care.[22][23]There are a few examples of fish that self-fertilise. Themangrove 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 19 °C (66 °F) and can fertilise eggs that are then spawned by the female. This maintains genetic variability in a species that is otherwise highly inbred.[24]
Actinopterygii is divided into the subclassesCladistia,Chondrostei andNeopterygii. TheNeopterygii, in turn, is divided into the infraclassesHolostei andTeleostei. During theMesozoic (Triassic,Jurassic,Cretaceous) andCenozoic the teleosts in particulardiversified widely. As a result, 96% of living fish species are teleosts (40% of all fish species belong to the teleost subgroupAcanthomorpha), while all other groups of actinopterygians represent depauperate lineages.[25]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)
The polypterids (bichirs and reedfish) are thesister 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. TheElopomorpha (eels andtarpons) appear to be the mostbasal teleosts.[26]The earliest knownfossil actinopterygian isAndreolepis hedei, dating back 420 million years (Late Silurian), remains of which have been found inRussia,Sweden, andEstonia.[29] Crown group actinopterygians most likely originated near the Devonian-Carboniferous boundary.[30] The earliest fossil relatives of modern teleosts are from theTriassicperiod (Prohalecites,Pholidophorus),[31][32] although it is suspected that teleosts originated already during thePaleozoicEra.[26]
Chondrostei(cartilage bone) is a subclass of primarilycartilaginous fish showing someossification. Earlier definitions of Chondrostei are now known to beparaphyletic, meaning that this subclass does not contain all the descendants of their common ancestor. There used to be 52 species divided among two orders, theAcipenseriformes (sturgeons andpaddlefishes) and thePolypteriformes (reedfishes andbichirs). Reedfish and birchirs are now separated from the Chondrostei into their own sister lineage, theCladistia. 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.[33] This group had once been classified with thesharks: 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 lackscales (excluding the Polypteriforms). Additional shared features includespiracles and, in sturgeons, a heterocercal tail (thevertebrae extend into the larger lobe of thecaudal fin). However the fossil record suggests that these fish have more in common with theTeleostei than their external appearance might suggest.[33]
Neopterygii(new fins) is a subclass of ray-finned fish that appeared somewhere in the LatePermian. 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. Whileelectroreception and theampullae of Lorenzini is present in all other groups of fish, with the exception ofhagfish, neopterygians have lost this sense, though it later re-evolved withinGymnotiformes andcatfishes, who possess nonhomologous teleost ampullae.[34]
^Maxwell; et al. (2018). "Re-evaluation of the ontogeny and reproductive biology of the Triassic fishSaurichthys (Actinopterygii, Saurichthyidae)".Palaeontology.61:559–574.doi:10.5061/dryad.vc8h5.
^Clutton-Brock, T. H. (1991).The Evolution of Parental Care. Princeton, NJ: Princeton UP.
^Laurin, M.; Reisz, R.R. (1995). "A reevaluation of early amniote phylogeny".Zoological Journal of the Linnean Society.113 (2):165–223.doi:10.1111/j.1096-3642.1995.tb00932.x.
^Arratia, G. (2015). "Complexities of early teleostei and the evolution of particular morphological structures through time".Copeia.103 (4):999–1025.doi:10.1643/CG-14-184.S2CID85808890.
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