Actinopterygii (/ˌæktɪnɒptəˈrɪdʒiaɪ/; from actino- 'having rays', and Ancient Greekπτέρυξ (ptérux) 'wing, fins'), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish[2] that comprise over 50% of living vertebrate species.[3] They 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 lobed fins of the sister class Sarcopterygii (lobe-finned fish). Resembling folding fans, the actinopterygian fins can easily change shape 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).
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.[5] 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.[6][5] In early forms the swim bladder could still be used for breathing, a trait still present in Holostei (bowfins and gars).[7] In some fish like the arapaima, the swim bladder has been modified for breathing air again,[8] and in other lineages it have been completely lost.[9]
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.[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 the Cyprinidae (in goldfish and common carp as recently as 14 million years ago). [11][12][13][14][15]
Gars (along with the bowfin) are the only surviving members of the Holostei
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.[16] 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.[17]
Most families use external rather than internal fertilization.[18] Of the oviparous 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 in Middle 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. 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 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]
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.[26]
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.[33] 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.[33]
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.[34]
^Maxwell; et al. (2018). "Re-evaluation of the ontogeny and reproductive biology of the Triassic fish Saurichthys (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.
^Romano, Carlo; Koot, Martha B.; Kogan, Ilja; Brayard, Arnaud; Minikh, Alla V.; Brinkmann, Winand; Bucher, Hugo; Kriwet, Jürgen (February 2016). "Permian-Triassic Osteichthyes (bony fishes): diversity dynamics and body size evolution". Biological Reviews. 91 (1): 106–147. doi:10.1111/brv.12161. PMID25431138. S2CID5332637.
Tuna are streamlined for straight line speed with a deeply forked tail
The swordfish is even faster and more streamlined than the tuna
The four-eyed fish Anableps anableps can see both below and above the water surface
Fangtooth are indifferent swimmers who try to ambush their prey
The first spine of the dorsal fin of anglerfish is modified like a fishing rod with a lure
European conger are ray-finned fish
Seahorses are in the extended pipefish family
The hoodwinker sunfish Mola tecta has no caudal fin
Lactoria fornasini is a poisonous species of boxfish
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