BONY FISH

any member of the class Osteichthyes in the subphylum Vertebrata, including the great majority of living fishes and virtually all the world's sport and commercial fishes. The scientific term Pisces has also been used to identify this group of fishes. Osteichthyes excludes the jawless fishes of the class Agnatha (hagfishes and lampreys) and the cartilaginous fishes constituting the class Chondrichthyes (sharks, skates, and rays) but includes the 20,000 species and more than 400 families of modern bony fishes (infraclass Teleostei) of the world, as well as a few primitive forms. The primary characteristic of bony fishes is a skeleton at least partly composed of true bone (as opposed to cartilage). Other features include, in most forms, the presence of a swim bladder (an air-filled sac to give buoyancy), gill covers over the gill chamber, bony platelike scales, a skull with sutures, and external fertilization of eggs. Common mola, or ocean sunfish (Mola mola). Bony fishes occur in all freshwater and ocean environments, including caves, deep-sea habitats, and thermal springs. The variety of shapes and behavioral habits is remarkable. Their body sizes range from tiny species such as the pygmy goby (Pandaka pygmaea; 12 mm [0.5 inch]) to the enormous marlins and swordfishes (family Istiophoridae) with lengths up to 4.5 m (15 feet) and the ocean sunfish (Mola mola; see photograph), which may weigh over 900 kg (1 ton).