Osteichthyes, or bony fishes, includes two major groups: Sarcopterygii, or lobe-finned fishes, and Actinopterygii, or ray-finned fishes. The characteristics that unite this diverse group include lungs or a gas-filled swim bladder derived from lungs, segmented fin rays, bone, and bony scales. Although the tetrapods (including birds, reptiles, mammals, and amphibians) are formally within the Sarcopterygii, they are discussed within their own entries; only animals commonly thought of as "fishes" are discussed here.
Sarcopterygii (Lobe-Finned Fishes)
Although only eight sarcopterygian fish species exist today, they are interesting because scientists believe they are the likely descendants of the fishes that gave rise to the terrestrial vertebrates , or tetrapods. The defining feature supporting this notion is a limblike fin with supporting bones that attach to the pelvic and pectoral girdles. There are six species of lungfish (Dipnoi or Dipneusti) found in Africa, South America, and Australia. Lungfish have true lungs, which allow them to live in stagnant water. African lungfish can survive for many months in a dry lake bed, protected by a mucus cocoon.
Coelacanths (Crossopterygii) were thought to have gone extinct 70 million years ago along with the dinosaurs until they were rediscovered near South Africa in 1938. There are two species, both large, up to 2 meters (7 feet) long, which prey on fish and squid in the deep waters of the Indian Ocean. Coelacanths are thought to rest on their lobed fins on the ocean floor. They have apparently evolved from a shallow-water, air-breathing ancestor, but their lunglike swim bladders are now filled with fat.
Actinopterygii (Ray-Finned Fishes)
With over 21,000 species distributed over the fresh and salt waters of the world, actinopterygians match the diversity of birds, mammals, reptiles, and amphibians put together. The fins of ray-finned fishes are attached to their body by fin rays, rather than lobes.
About thirty-five species are "primitive" actinopterygians. The Chondrostei have a cartilaginous skeleton and lack true scales, and they have an asymmetrical tail like a shark's. They include the sturgeons, noted for their caviar and among the largest fishes found in freshwater, and the filter-feeding paddlefish of the Mississippi River. Gars (Semionotiformes, family Lepisosteidae) are sit-and-wait predators restricted to the fresh and brackish waters of North America. Gars have a cylindrical body covered with armorlike scales and a long snout lined with sharp teeth. Their large swim bladder functions as a lung, allowing them to live in stagnant water. The alligator gar, found in the southern United States and Mexico, reaches 3 meters (10 feet) in length. The single existing species in the last major group of "primitive" actinopterygians, the bowfin (Amiiformes), is also a sit-and-wait predator restricted to North American freshwaters.
The remaining actinopterygians are teleosts, characterized by a symmetrical tail, highly maneuverable fins, and jaws adapted for sucking.
Bony-Tongues, Eels, and Herring
Most bony-tongues (Osteoglossomorpha) are African electric fishes (Mormyridae), which forage and communicate by producing electrical fields. The group also includes the large arapaima of South America and the mooneye and goldeneye of the upper Mississippi drainage. Tarpon and eels are familiar representatives of the Elopomorpha, characterized by larvae called leptocephali, which are transparent, very slender-bodied, and leaflike. Eels are long, small-headed fishes with sharp teeth, adapted for living in tight crevices or burrows.
Freshwater eels (family Anguillidae) spend most of their lives preying on fishes and invertebrates in freshwater. They then migrate to specific breeding grounds in the ocean, usually a distance of thousands of kilometers, and can even travel on land if conditions are damp enough. European and North American eels from the Atlantic slope all migrate to the deep waters of the Sargasso Sea area of the North Atlantic Ocean to spawn. The clupeomorphs include some of the most abundant vertebrates, shad, herring (Clupeidae), and anchovies (Engraulidae). Most species in this group school (live in groups) in open water. They filter-feed on plankton that they catch on modified gill rakers, specialized structures associated with the gills.
Pikes and Salmon
Pikes (family Esocidae) are sit-and-wait predators restricted to northern Eurasia and North America. The largest of the pikes, the muskellunge of the North America Great Lakes region, can be as much as 1.8 meters (6 feet) long and weigh as much as 36 kilograms (80 pounds). These long, sharp-toothed fishes grab their prey sideways, turn it around, and swallow it headfirst.
There are only about 150 species of salmonids (Salmoniformes), but they are the dominant fishes of cold-water streams and lakes in northern regions and are of substantial economic importance for food and sport fishing. Familiar salmonids include trout, largely restricted to freshwater lakes and streams, and salmon, most of which spend most of their lives at sea. They use a sophisticated sense of smell to detect the stream they were born in and return there to spawn a single time and die.
About one-quarter of all known fishes belong to this diverse group, which dominates Earth's freshwaters. Three main features characterize the Ostariophysi: (1) Pharyngeal teeth, located in the throat behind the gills, are used for processing food once swallowed and allow for specialization on different food types. (2) Weberian ossicles, a series of bones connecting the swim bladder (which acts like an eardrum) to the inner ear, allow for sensitive hearing. (3) Schreckstoff, or "fright substance," is a chemical they give off when injured that causes other fish of the same species to dive for cover or swim closer together. These three features may account for the success of Ostariophysi, particularly in murky lakes and streams where visibility is limited.
The 1,600 species in Cyprinidae (minnows and carps) are the dominant freshwater fishes in Eurasia, Africa, and North America but are absent from South America and Australia. They are mostly small, tapered, silvery fishes, although some, like the Colorado squawfish, can attain lengths of 2 meters (7 feet). Male North American shiners (genus Notropis ) turn bright red or orange and defend nests. Suckers (Catostomidae), found in North America and Asia, have extendable, fleshy lips specialized for sucking algae and other food from the bottom. The order Characiformes, which includes tetras, piranhas, and pencilfishes, is a diverse group of 1,200 species restricted to South America, southern North America, and Africa. The order Siluriformes (2,000 species) is made up of the catfish, most of which are nocturnal and characterized by sensitive barbels on the snout that look like cat whiskers.
Cod, Anglerfishes, Killifishes, and Livebearers
The diverse group Paracanthopterygii includes the abundant and economically important codfishes (Gadiformes, 700 species), elongated fishes with three dorsal fins and a chin barbel, which are mostly found in open oceanic waters. Anglerfishes and frogfishes (Lophiiformes) are cryptic, bottom-dwelling fishes. They have a structure that looks like a fishing pole growing out of the head. The top of the "fishing pole" looks like a fish or invertebrate, and the fish uses the pole and "fish" to entice prey. The Atheriniformes include many small, colorful freshwater fishes popular in the aquarium trade. The Atherinidae include the silversides of North American brackish waters and the Australian rainbowfishes. Killifishes (Cyprinodontidae and related families) are often found in very confined habitats such as desert springs. Some species live in tiny puddles in the rain forest and jump from puddle to puddle during rains.
The African and South American species restricted to temporary pools have the shortest life spans of any vertebrate. Eggs can remain dormant in dry mud for most of the year, but the fish hatch and grow rapidly to maturity during the rainy season, sometimes living only three months before ponds dry up again. Livebearers (Poeciliidae and Goodeidae), restricted to the New World, include the mosquitofish, introduced worldwide and a threat to native wildlife, and the colorful guppies and swordtails.
With about 9,000 species, this group of "spiny-rayed fishes" is composed of the dominant fishes of the oceans as well as numerous inhabitants of freshwater. The Gasterosteiformes are mostly long, covered in armored plates, and characterized by males that share parental duties. They include the sticklebacks (Gasterosteidae), whose males build and defend nests, as well as the seahorses and pipefishes (Syngnathidae), whose males carry the eggs and hatchlings in specialized pouches on their belly. The Scorpaeniformes include 1,000 species of mostly bottom-dwelling fishes such as sculpins and rockfish. The group is characterized by very spiny dorsal and anal fins, often associated with venom glands, which in some tropical scorpionfishes can cause death to humans. The 500 species of flatfishes (Pleuronectiformes) such as sole, halibut, and flounders, are uniquely adapted to life on the ocean bottom. When they are larvae they look like most other fish larvae, but as they develop they undergo a metamorphosis in which one eye migrates to the other side of the head. As a result, the adult flatfish has a bottom side with no eyes and a top side with two eyes. The top side can sometimes change color to match the background of the ocean bottom. The Tetraodontiformes include the pufferfishes and triggerfishes, which are slow-swimming, heavily armored fishes with "beaks" that they use to feed on coral and invertebrates. Some, like the balloonfish, can inflate rapidly with water when confronted with a predator.
The ruling perches, or Perciformes, are 7,000 species characterized by spiny fins and a two-part dorsal fin. They include the North American basses and sunfishes (Centrarchidae) and the family Percidae, which includes the perches of Eurasia and the walleyes and brightly colored darters of North America. Drums (Sciaenidae) are mostly coastal fishes that use low-frequency sounds during courtship. Cichlids (Cichlidae) have greatly diversified throughout the New World and African tropics; the Rift Lakes of Africa contain hundreds of species of cichlids restricted to each lake, many of which are highly specialized to a particular feeding task. For example, some fishes eat only the scales on the left sides of other fishes, while others have mouths specialized for sucking out the eyes of other fishes. Closely related marine fishes include the colorful wrasses (Labridae) and parrotfishes (Scaridae), in which some individuals can change sex from female to male. The cleaner wrasse specializes in removing parasites from other fishes. The colorful damselfishes (Pomacentridae) are unusual among marine fish in that they care for their young; males in some species build and defend nests. The group includes the anemonefish, which have developed an immunity to the sting of the anemone and can live and reproduce within its tentacles. The more than 800 species of gobies (Gobiidae) are small fishes found in fresh and salt water; their pelvic fins are fused to form a sucking disk. These fishes have colonized habitats such as small crevices among rocks, tide pools, and streams above waterfalls, and some, like the mudskipper, can travel on land. Tuna and mackerel (Scombridae) are "warm-blooded" for efficient muscle activity and rapid swimming.
The Stenopterygii include the bristlemouths, small, luminescent filter feeders that live in the deep oceans and may be the world's most abundant vertebrate. Lanternfishes (Scopelomorpha) use luminous lures to catch prey. Lizardfishes (Cyclosquamata) are well-camouflaged sit-and-wait predators who live on coral reefs.
see also Phylogenetic Relationships of Major Groups.
Gil G. Rosenthal
Moyle, Peter B. Fish: An Enthusiast's Guide. Berkeley: University of California Press, 1993.
Moyle, Peter B., and Joseph J. Cech, Jr. Fishes: An Introduction to Ichthyology, 3rd ed. Upper Saddle River, NJ: Prentice-Hall, 1996.
Nelson, Joseph S. Fishes of the World, 3rd ed. New York: John Wiley & Sons, 1994.