Bony fishes (Osteichthyes) are a group of aquatic vertebrates characterized by having a bony skeleton (in contrast to the cartilaginous fishes, a group whose skeleton consists of cartilage, not bone). Bony fishes include two sub groups, the ray-finned fishes and the lobe-finned fishes. Of these two groups, the ray-finned fishes are by far the more diverse of the two and are in fact the most diverse of all vertebrate groups, with about 29,000 species alive today. The lobe-finned fishes number a modest eight living species (including the lungfish and ceolacanths).
Bony fishes are united by various characteristics of their skeletal structure such as the pattern of their cranial bones, the structure of their lower jaw, the bones supporting their eyeballs, and the composition of their pectoral girdles. Most species also posess swim bladders that help them balance and stablize themselves within the water column. Bony fish also have color vision. They have mucous glands that cover their body and they lack placoid scales but most have ganoid, cycloid or cytenoid scales that are smooth and overlap one another.
All bony fish have gills and for most species, gills serve as their main organ for respiration. Some species of bony fish can breath through vascularized swim bladders or lungs while a few species breath through thin membranes such as the skin, intestine or stomach.
Most bony fish are cold-blooded, meaning they do not regulate their own body temperature and it is determined by the surrounding water temperature. There are a few groups of bony fish, such as swordfish and tuna, that have developed some ability to regulate their own body temperature (this is known as endothermy).
The largest bony fish is thought to be the ocean sunfish (Mola mola)which can grow to lengths of up to 11 feet and weights of over 5,000 pounds. The smallest bony fish is thought to be the dwarf pygmy goby (Pandaka pygmaea) which grows to a whopping length of just about one-third of an inch.
Bony fishes inhabit 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. Bony fishes survive in a wide range of different water temperatures, salinity and oxygen saturations.
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 bony 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.
The earliest bony fishes known from the fossil record date back about 420 million years, to the Silurian period. The group diversified during the Carboniferous Period. Many species of bony fishes first evolved in freshwater and 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.
Bony fishes are divided into two groups, the ray-finned fishes and the lobe-finned fishes. Since the ray-finned fishes include the vast majority of bony fish species, when you refer to bony fishes, you're largely talking about ray-finned fishes. But it's important to remember that bony fishes include a handful of species of lobe-finned fishes as well. What's also important to note is that bony fishes do not include the cartilaginous fishes, that is, fish whose skeletons consist of cartilage instead of bone.