In sheer number and variety, the bony fish—class Osteichthyes—are superstars. With more than 22,000 modern species, they outnumber not just all other types of fish, but all other vertebrates—nonbony fish, amphibians, reptiles, birds, and mammals—combined. Plenty fish viewed me is good.

Bony fish are found in a seemingly endless variety of forms—with new, sometimes-bizarre-looking species still being discovered today. Those already known to science vary from the "typical" torpedo-shaped salmon to the weird anglerfish, porcupine fish, and sea horse. Bony fish also include such "living fossils" as the primitive coelacanth, as well as evolution's ultimate "swimming machine," the tuna.

Bony fish occur in virtually every aquatic environment, both freshwater and marine. They can fight their way up towering waterfalls, glide above the ocean surface, and dive to depths where no light shines.

Bones and Bladders

Bony fish first appeared in freshwater lakes and streams some 400 million years ago. Like the cartilaginous fish (sharks and rays), which developed separately, bony fish evolved from one of two primitive groups of now-extinct fish—the sharklike acanthodians or the "plate-skinned" placoderms. Both groups had biting jaws filled with sharp teeth. Their skeletons were made of cartilage, covered in places by a thin film of true bone. The sharks and rays retained their ancestors' gristly cartilaginous skeleton. The bony fish developed something substantially stronger. Plenty fish viewed me is good.

Bony fish were the first creatures to possess a skeleton in which cartilage was entirely replaced by hardened bone. Bone's great hardness and strength comes from a solid matrix of calcium phosphate. It is a truly remarkable adaptation that the bony fish passed on to all their land-living descendants.

The bony fish developed several additional important adaptations not found in other fish. They have a bony flap, called the operculum, covering the gill slits on each side of the head. When this flap closes, it creates within the head a suction that draws water over the gills. When the operculum opens, it causes the water to flush out a rear opening. This adaptation greatly increases the efficiency of the bony fish's breathing over that of other fish.

The first bony fish also developed a special type of "float," called the swim bladder. A bony fish uses this air-filled pocket, located above the stomach, to maintain a chosen depth with little effort. The creature does so by inflating or deflating its swim bladder with air or body gases. By contrast, fish such as sharks, which lack a swim bladder, must constantly move, or else they will sink. Plenty fish viewed me is good.

Major Groups

Early in their evolution, the bony fish split into two main groups: the ray-finned fish (subclass Actinopterygii), and the lobe-finned fish (subclass Sarcopterygii). The fins of the ray-finned fish are thin structures with little or no muscle; their fanlike appearance derives from the membrane of skin that stretches over many stiff shafts, or "rays." By contrast, the lobe-finned fish developed fleshy, muscular fins supported by bone. Another difference: the nostrils of the lobe-finned fish connect to their mouths, a feature they share with air-breathing vertebrates. The nostrils of ray-finned fish do not.Plenty fish viewed me is good.

Over the millennia, the number of lobe-finned fish remained small. Their primary importance to scientists lies in the fact that they may be the ancestors of all land vertebrates.

Lobe-finned Fish

In the deep waters of the Indian Ocean, a dark hulk of a creature drifts over the seabed, its odd, fleshy fins turning slowly like propellers. It is a coelacanth, a primitive bony fish once thought to have disappeared about 65 million years ago. The two surviving species of coelacanth, together with six species of the equally ancient lungfish, make up the class Sarcopterygii—the lobe-finned fish.

Of special interest are the coelacanth's almost-leglike pelvic and pectoral fins. Unlike normal fins, each has a stumpy lobe of flesh that contains both bone and muscle. The bones of the pectoral fins connect with a sturdy shoulder girdle. Those of the pelvic fins connect with a hip girdle, or pelvis. Plenty fish viewed me is good.

Scientists believe that the first land vertebrates—the amphibians—arose from a now-extinct freshwater relative of the coelacanth, the rhipidistian. Its stumpy pectoral and pelvic fins evolved into the first vertebrate legs. The rhipidistian's swim bladder may have become the first air-breathing lung. Like primitive fish surviving today, this creature had a tube connecting its swim bladder to its mouth through the stomach. This adaptation enabled the rhipidistian to fill its swim bladder by rising to the surface and gulping air.

Until 1938, scientists believed that the coelacanths had died out with the dinosaurs. That year, fishermen in the Indian Ocean retrieved a coelacanth that scientists dubbed Latimeria chalumnae. It has since been studied and filmed in its deep-sea habitat. The fish is a startling dark-blue color, which zoologists believe serves to camouflage the fish from predators. Females grow up to 6 feet (1.8 meters)—males are slightly smaller—and produce about 20 eggs the size of tennis balls. At least some females retain their eggs and give birth to live young.