Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of Osteichthyes[(Davis, Brian 2010).] The vast majority of extant taxon actinopterygian species are , and by species count they dominate the subphylum Vertebrata, comprising over 50% of all living vertebrates.
Ray-finned fish are so called because of their lightly built fish fin made of webbings of skin supported by radially extended thin bony spines called lepidotrichia, as opposed to the bulkier, fleshy fins of the sister taxon clade Sarcopterygii (lobe-finned fish). Resembling , 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
[[File:Anatomia dei pesci.jpg|thumb|left|upright=1.2|
A:
dorsal fin,
B:
,
C:
lateral line,
D: kidney,
E:
swim bladder,
F: Weberian apparatus,
G:
inner ear,
H: brain,
I: nostrils,
L: eye,
M:
,
N: heart,
O: stomach,
P: gall bladder,
Q: spleen,
R: internal sex organs (ovaries or testes),
S:
,
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.
Except from the
, which just like the
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.
In early forms the swim bladder could still be used for breathing, a trait still present in
Holostei (
and
).
In some fish like the
arapaima, the swim bladder has been modified for breathing air again,
and in other lineages it has been completely lost.
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.
Ray-finned fishes have many different types of
fish scale; but all
have
. 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
tooth enamel- or
dentine-like layers found in the scales of many other fish. Unlike
, which are found in non-teleost actinopterygians, new scales are added in concentric layers as the fish grows.
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).
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
oviparity teleosts, most (79%) do not provide parental care.
,
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
exaptation 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 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.
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 taxon groups of
fishes and the four-limbed vertebrates (
tetrapods).
The latter include mostly terrestrial
species but also groups that became secondarily aquatic (e.g.
Cetacea). Tetrapods evolved from a group of
bony fish during the
Devonian period.
Approximate divergence dates for the different actinopterygian clades (in
myr, mya) are from Near et al., 2012.
The polypterids (bichirs and reedfish) are the Sister group 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 ( and ) appear to be the most basal teleosts.
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 and Phylogenetic Classification of Bony Fishes
with notes when this differs from Nelson,
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
-
Subclass Cladistia Pander 1860
-
Order †Guildayichthyiformes Lund 2000
-
Order Polypteriformes Bleeker 1859 ( and )
[In Nelson, Polypteriformes is placed in its own subclass Cladistia.]
-
Subclass Actinopteri Cope 1972 s.s.
-
Order †Elonichthyiformes Kazantseva-Selezneva 1977
-
Order †Phanerorhynchiformes
-
Order †Bobasatraniiformes Berg 1940
-
Order †Saurichthyiformes Aldinger 1937
-
Subclass Chondrostei Müller, 1844
-
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
-
Clade Teleosteomorpha Arratia 2000 sensu Arratia 2013
-
Order †Prohaleciteiformes Arratia 2017
-
Division Aspidorhynchei Nelson, Grand & Wilson 2016
-
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)
-
Megacohort Osteoglossocephalai sensu Arratia 1999
-
Supercohort Osteoglossocephala sensu Arratia 1999 (Osteoglossomorpha Greenwood et al. 1966)
-
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)
-
Subcohort Alepocephali
-
Order Alepocephaliformes Marshall 1962
-
Subcohort Ostariophysi Sagemehl 1885
-
Section Anotophysa (Rosen & Greenwood 1970) Sagemehl 1885
-
Section Otophysa Garstang 1931
-
Order Cypriniformes Bleeker 1859 sensu Goodrich 1909 (barbs, carp, danios, , loaches, , )
-
Order Characiformes Goodrich 1909 (, , hatchetfishes, , , Golden dorado and pacu)
-
Order Gymnotiformes Berg 1940 ( and knifefishes)
-
Order Siluriformes Cuvier 1817 sensu Hay 1929 ()
-
Cohort Euteleosteomorpha (Greenwood et al. 1966) (Euteleostei Greenwood 1967 sensu Johnson & Patterson 1996)
-
Subcohort Lepidogalaxii
-
Order Lepidogalaxiiformes Betancur-Rodriguez et al. 2013 (salamanderfish)
-
Subcohort Protacanthopterygii Greenwood et al. 1966 sensu Johnson & Patterson 1996
-
Subcohort Stomiati
-
Subcohort Neoteleostei Nelson 1969
-
Infracohort Ateleopodia
-
Infracohort Eurypterygia Rosen 1973
-
Section Aulopa Cyclosquamata
-
Section Ctenosquamata Rosen 1973
-
Subsection Myctophata Scopelomorpha
-
Subsection Acanthomorpha Betancur-Rodriguez et al. 2013
-
Division Lampridacea Betancur-Rodriguez et al. 2013 Lampridomorpha;
-
Division Paracanthomorphacea sensu Grande et al. 2013 (Paracanthopterygii Greenwood 1937)
-
Division Polymixiacea Betancur-Rodriguez et al. 2013 (Polymyxiomorpha; Polymixiipterygii)
-
Order †Pattersonichthyiformes Gaudant 1976
-
Order †Ctenothrissiformes Berg 1937
-
Order Polymixiiformes Lowe 1838 ()
-
Division Euacanthomorphacea Betancur-Rodriguez et al. 2013 (Euacanthomorpha sensu Johnson & Patterson 1993; Acanthopterygii Gouan 1770 sensu])
-
Order Trachichthyiformes ( and )
-
Subdivision Berycimorphaceae Betancur-Rodriguez et al. 2013
-
Subdivision Percomorphaceae Betancur-Rodriguez et al. 2013 (Percomorpha sensu Miya et al. 2003; Acanthopteri)
-
Series Ophidiimopharia Betancur-Rodriguez et al. 2013
-
Series Batrachoidimopharia Betancur-Rodriguez et al. 2013
-
Series Gobiomopharia Betancur-Rodriguez et al. 2013
-
Series Scombrimopharia Betancur-Rodriguez et al. 2013
-
Series Carangimopharia Betancur-Rodriguez et al. 2013
-
Subseries Anabantaria Betancur-Rodriguez et al. 2014
-
Subseries Carangaria Betancur-Rodriguez et al. 2014
-
Subseries Ovalentaria Smith & Near 2012 (Stiassnyiformes sensu Li et al. 2009)
-
Series Eupercaria Betancur-Rodriguez et al. 2014 (Percomorpharia Betancur-Rodriguez et al. 2013)
External links
