Bony fish (Osteichthyes) are distinguished from other fish species that have a cartilaginous skeleton (Chondrichthyes—sharks, rays and chimaeras, for example) by the presence of true bone—a mixture of calcium phosphates and carbonates—in their skeletons. Other differences between the two groups are modifications in the structure and arrangement of the scales and fins and the presence of more specialized teeth in bony fish. When feeding, bony fish display a far wider range of adaptations than cartilaginous species: the former may be either carnivorous (like most cartilaginous species), plant-eating, or both. Combined, these features have helped them to exploit a much wider range of feeding and living habitats.
Fish breathe and feed in order to obtain sufficient energy to meet their daily physical requirements. Much of this energy is required for swimming, which can range from simple short distances to much greater seasonal migrations. However, considerable amounts of energy are also required for finding food and mates, as well as for avoiding predators. To conserve energy, bony fish have evolved a special swim bladder which is a gas-filled chamber that provides buoyancy and helps keeps them weightless in the water column. Through this system they may remain at the same level of water for several hours without expending too much valuable energy.
With few exceptions, all bony fish require a constant source of oxygen for respiration. Dissolved oxygen is freely available in salt and freshwater and to extract this, fish pass the water through specialised gill chambers that are richly supplied with blood vessels. As the water passes over the highly convoluted surface of the gills, the oxygen passes across the thin membranes and enters the red hemoglobin cells of the blood stream.
The classification of bony fish is complex and outside the scope of this present account. Basically there are two main groups of bony fish: the Crossopterygii and Actinopterygii. The latter contains some additional small groups of distinct fish such as the Polypteridae (bichirs), Acipenseridae (sturgeons), Polyodontidae (paddle fish) and Lepisosteidae (gar or pikes). It also includes the teleost fish, which are by far the most numerous of all fishes, with more than 20,000 species identified from a wide range of habitats—aquatic, marine, and terrestrial. Although these species vary considerably in size, appearance and structure, they are all much lighter than primitive species, largely through the loss of heavy body armor and thickened scales. The smallest known teleost fish is the Philippine goby (Pandaka pygmaea) which reaches a length of just 0.5 in (12 mm); the largest is the arapamia or pirarucu (Arapamia gigas) of the Amazonian waterways, which has been known to measure 16 ft (5 m).
In addition to the wide range of modern bony fish, a few primitive representatives still survive and, for taxonomic purposes, have been grouped together in the Crossopterygii. One of these is the coelacanth (Latimera chalumnae), a large blue-gray fish that may reach a length of 6.5 ft (2 m). Known only from the deep ocean trenches off the tiny Comoros Archipelago around northwest Madagascar, this sluggish species lives in complete darkness and preys on other fish. Many of its features are similar to fossil species, the most notable of which are its arrangement of fins. The rays of the second dorsal fin, the anal fin, and the paired fins rest on a muscular, scale-covered lobe, while the powerful tail fin is symmetrical in appearance. Almost nothing is known about the ecology of this species. A recent discovery showed that coelacanths actually give birth to live young.
Other unusual species of this same group are the lungfish of Africa, Australia, and South America, the only living representatives of a widespread group of fish that lived on Earth some 350 million years ago. In some of these, the dorsal, anal, and caudal fins are fused and modified to form a continuous median fin. Some species come to the surface to gulp in air, while others have developed a gill system that enables them to breathe when submerged. During periods of dry weather, the African and South American species dig deeply into the mud at the base of lakes and swamps and envelop themselves in a thick coat of mucus. As this dries out it provides a protective covering for the fish, enabling it to survive periods of drought in a state of aestivation. When the rains return, the mucus coating is softened and the fish reemerges.
Features such as these enabled some species to withstand periods of adverse weather or avoid excessive predation through the development of toughened skins that were often reinforced with bulky scales. As the world's climate changed and new species continued to evolve, most of these primitive features were lost and an explosion of new life forms spread throughout the seas and freshwater ecosystems. Witness to this is the present staggering diversity and numbers of bony fish that occur in almost every aquatic habitat on Earth, from the warm tropical waters to the frozen seas of the poles and from the margins of the tides to the deepest oceanic trenches.