Actinopterygii (/), or the ray-finned fishes, constitute a class or subclass of the bony fishes.
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. 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).
Anatomy of a typical ray-finned fish (cichlid
−gall bladder, Q
−internal sex organs (ovaries or testes), S
−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 actinopterygii, 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 actinopterygii, new scales are added in concentric layers as the fish grows.