Actinopterygii

Actinopterygii (/ˌæktɪnɒptəˈrɪdʒiaɪ/; from actino- 'having rays', and Ancient Greekπτέρυξ (ptérux) 'wing, fins'), members of which are known as ray-finned fishes, is a clade (traditionally class or subclass) of the bony fishes.^[1] They comprise over 50% of living vertebratespecies.^[2]

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).

By species count, actinopterygians dominate the vertebrates, and they comprise nearly 99% of the over 30,000 species of fish.^[3] 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). The vast majority of Actinopterygii (~95%) are teleosts.

Ray-finned fishes occur in many variant forms. The main features of a typical ray-finned fish are shown in the adjacent diagram. The swim bladder is the more derived structure.^[4]

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.^[5]

Ray-finned and lobe-finned fishes, including tetrapods, possessed lungs used for aerial respiration. Only bichirs retain ventrally budding lungs.^[4]

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)

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

Fossil of the Devonian †cheirolepidiform †Cheirolepis canadensis

Fossil of the Carboniferous †elonichthyiform †Elonichthys peltigerus

Fossil of the Permian †aeduelliform †Aeduella blainvillei

Fossil of the Permian †palaeonisciform †Palaeoniscum freieslebeni

Fossil of the Triassic †bobasatraniiform †Bobasatrania canadensis

Fossil of the Triassic †perleidiform †Thoracopterus magnificus

Fossils of the Triassic †prohaleciteiform †Prohalecites sp., the earliest teleosteomorph

Fossil of the Jurassic †aspidorhynchiform †Aspidorhynchus sp.

Fossil of the Jurassic †pachycormiform †Pachycormus curtus

Fossil of the Cretaceous acipenseriform †Yanosteus longidorsalis

Fossil of the Cretaceous aulopiform †Nematonotus longispinus

Fossil of the Cretaceous †ichthyodectiform †Thrissops formosus

Fossil of the Cretaceous carangiform †Mene oblonga

Fossil of the Cretaceous pleuronectiform †Amphistium paradoxum

Fossil of a ray-finned perch (†Priscacara serrata) from the Lower Eocene about 50 million years ago

Fossil of the Miocene syngnathiform †Nerophis 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

Blue catfish skeleton