Actinopterygii

Actinopterygii (/ˌæktɪˌnɒptəˈrɪdʒiaɪ/), members of which are known as ray-finned fishes, is a class or subclass of the bony fishes.[1]

Ray-finned fish
Temporal range: Late Silurian - recent
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Superclass: Osteichthyes
Class: Actinopterygii
Klein, 1885
Subclasses

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).

Numerically, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish.[2] 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 8 mm (0.3 in), to the massive ocean sunfish, at 2,300 kg (5,070 lb), and the long-bodied oarfish, at 11 m (36 ft).

Characteristics
Anatomy of a typical ray-finned fish (cichlid)
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−ventral 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 a typical ray-finned fish are shown in the adjacent diagram.

Ray-finned fishes have many different types of scales; but all teleosts, the most advanced actinopterygians, 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.

Fin arrangements
Further information: Fish fin

Ray-finned fish are very varied in size and shape, and in the number of their ray-fins and the manner in which they arrange them.

Reproduction
Three-spined stickleback 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.[3] 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.[4] Most families use external rather than internal fertilization.[5] Of the oviparous teleosts, most (79%) do not provide parental care.[6] 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.[6] Viviparity is relatively rare and is found in about 6% of teleost species; male care is far more common than female care.[6][7] Male territoriality "preadapts" a species for evolving male parental care.[8][9]

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.[10]

Fossil record
See also: Evolution of fish

The earliest known fossil actinopterygian is Andreolepis hedei, dating back 420 million years (Late Silurian). Remains have been found in Russia, Sweden, and Estonia.[11]

Classification

Actinopterygia is divided into the subclasses Chondrostei and Neopterygii. The Neopterygii, in turn, is divided into the infraclasses Holostei and Teleostei. During the Mesozoic and Cenozoic the teleosts in particular diversified widely, and as a result, 96% of all known fish species are teleosts. The cladogram shows the major groups of actinopterygians and their relationship to the terrestrial vertebrates (tetrapods) that evolved from a related group of fish.[12][13][14] Approximate dates are from Near et al., 2012.[12]

Osteichthyes
Sarcopterygii
 

Coelacanths, Lungfish

 
Tetrapods
 

Amphibians

 
Amniota
 

Mammals

 
 

Sauropsids (reptiles, birds)

 
 
 
 
Actinopterygii
Cladistia

Polypteriformes (bichirs, reedfishes)

 
Actinopteri
Chondrostei

Acipenseriformes (sturgeons, paddlefishes)

 
Neopterygii
Holostei
 

Lepisosteiformes (gars)

 
 

Amiiformes (bowfins)

 
275 mya
 

Teleostei

310 mya
360 mya
 
400 mya
 

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.[12]

Chondrostei
Atlantic sturgeon
 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 were 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.[15] 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.[15]
Neopterygii
Atlantic salmon
 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.[16]
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
Blue catfish skeleton
Fossil of a ray-finned perch (Priscacara serrata) from the Lower Eocene about 50 million years ago

The listing below follows Phylogenetic Classification of Bony Fishes[13][17] with notes when this differs from Nelson,[18] ITIS[19] and FishBase[20] and extinct groups from Van der Laan 2016.[21]

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References
  1. Kardong, Kenneth (2015). Vertebrates: Comparative Anatomy, Function, Evolution. New York: McGraw-Hill Education. pp. 99–100. ISBN 978-0-07-802302-6.
  2. (Davis, Brian 2010).
  3. Dorit, R.L.; Walker, W.F.; Barnes, R.D. (1991). Zoology. Saunders College Publishing. p. 819. ISBN 978-0-03-030504-7.
  4. Avise, J.C.; Mank, J.E. (2009). "Evolutionary perspectives on hermaphroditism in fishes". Sexual Development. 3 (2–3): 152–163. doi:10.1159/000223079. PMID 19684459.
  5. Pitcher, T (1993). The Behavior of Teleost Fishes. London: Chapman & Hall.
  6. Reynolds, John; Nicholas B. Goodwin; Robert P. Freckleton (19 March 2002). "Evolutionary Transitions in Parental Care and Live Bearing in Vertebrates". Philosophical Transactions of the Royal Society B: Biological Sciences. 357 (1419): 269–281. doi:10.1098/rstb.2001.0930. PMC 1692951. PMID 11958696.
  7. Clutton-Brock, T. H. (1991). The Evolution of Parental Care. Princeton, NJ: Princeton UP.
  8. Werren, John; Mart R. Gross; Richard Shine (1980). "Paternity and the evolution of male parentage". Journal of Theoretical Biology. 82 (4): 619–631. doi:10.1016/0022-5193(80)90182-4. PMID 7382520. Retrieved 15 September 2013.
  9. Baylis, Jeffrey (1981). "The Evolution of Parental Care in Fishes, with reference to Darwin's rule of male sexual selection". Environmental Biology of Fishes. 6 (2): 223–251. doi:10.1007/BF00002788.
  10. Wootton, Robert J.; Smith, Carl (2014). Reproductive Biology of Teleost Fishes. Wiley. ISBN 978-1-118-89139-1.
  11. "Fossilworks: Andreolepis". Archived from the original on 12 February 2010. Retrieved 14 May 2008.
  12. Thomas J. Near; et al. (2012). "Resolution of ray-finned fish phylogeny and timing of diversification". PNAS. 109 (34): 13698–13703. Bibcode:2012PNAS..10913698N. doi:10.1073/pnas.1206625109. PMC 3427055. PMID 22869754. Archived from the original on 11 April 2015. Retrieved 11 April 2014.
  13. Betancur-R, Ricardo; et al. (2013). "The Tree of Life and a New Classification of Bony Fishes". PLOS Currents Tree of Life. 5 (Edition 1). doi:10.1371/currents.tol.53ba26640df0ccaee75bb165c8c26288. PMC 3644299. PMID 23653398. Archived from the original on 13 October 2013.
  14. 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.
  15. "Chondrosteans: Sturgeon Relatives". paleos.com. Archived from the original on 25 December 2010.
  16. Theodore Holmes Bullock; Carl D. Hopkins; Arthur N. Popper (2005). Electroreception. Springer Science+Business Media, Incorporated. p. 229. ISBN 978-0-387-28275-6.
  17. Betancur-Rodriguez; et al. (2017). "Phylogenetic Classification of Bony Fishes Version 4". BMC Evolutionary Biology. 17 (1): 162. doi:10.1186/s12862-017-0958-3. PMC 5501477. PMID 28683774.
  18. Nelson, Joseph, S. (2016). Fishes of the World. John Wiley & Sons, Inc. ISBN 978-1-118-34233-6.
  19. "Actinopterygii". Integrated Taxonomic Information System. Retrieved 3 April 2006.
  20. R. Froese and D. Pauly, editors (February 2006). "FishBase". Archived from the original on 5 July 2018. Retrieved 8 January 2020.
  21. Van der Laan, Richard (2016). Family-group names of fossil fishes. doi:10.13140/RG.2.1.2130.1361.
  22. In Nelson, Polypteriformes is placed in its own subclass Cladistia.
  23. In Nelson and ITIS, Syngnathiformes is placed as the suborder Syngnathoidei of the order Gasterosteiformes.
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