The first ray-finned fish appeared during the Devonian Period and underwent significant expansion and diversification during the Carboniferous Period. The first actinopterygiians evolved in freshwater but they later expanded into marine habitats. During the Late Triassic a group of ray-finned fish called teleosts first appeared. Teleosts are today the most numerous group of fish species alive today, with an estimated 23,000 of the 24,000 species.
Ray-finned fish have a bony skeleton made of true bone. They also have an upper jaw that consists of two bones. The two bones that form the upper jaw of ray-finned fish are the the maxilla and the premaxilla. Ray-finned fish have fins that are supported by bony spines. Their fins consist of a set of bony spines that are covered with a thin layer of skin, which contrasts with the sarcopterygiian fish, a group of fish whose fins consist of fleshy lobes.
The two-boned structure of the upper jaw in actinopterygians has proven highly adaptable. This structure makes the upper jaw protusible (easily extended) and as a result, a wide range of feeding adaptations have developed in actinopterygiians.
Ray-finned fishes exhibit a variety of ornamentation and color variety. The widest variation in coloration and appearance occurs in the coral reef habitas. Some coloration is intended to advertise while other coloration serves to conceal.
Ray-finned fishes range in size from the tiny gobies, which are among the smallest of all vertebrates at just under 8 millimeters in length and Acipenser huso, a huge sturgeon that inhabits the Adriatic Black and Caspian seas and grows to lengths of more than 4 meters.
Ray-finned fishes inhabit both marine and freshwater habitats and are worldwide in their distribution. Ray-finned fishes occupy a wide range of habitats, some of which are considerably harsh such as the deep sea, caves, turbulent rivers, desert springs, and high-altitude lakes. Species have adapted to a wide range of conditions. Water temperatures they live in range from -1.8°C to 40°C, pH values range from 4 to 10, salinitiy from 0 to 90 ppm. Ray-finned fishes inhabit waters of depths from the shallow shores to 7,000 meters deep.
Regions with high diversity of freshwater species include the Amazon River Basin and the rivers and wetlands of Southeast Asia. Marine species are most diverse around coral reef systems such as the Great Barrier Reef. Some species of ray-finned fishes are capable of migrating between fresh and salt water, and are therefore considered diadromous. Some species such as salmon make a migration from saltwater upstream to freshwater where they spawn. Other species such as freshwater eels migrate from freshwater to salt water to spawn.