TELEOST

any member of the infraclass Teleostei, a large and extremely diverse group of ray-finned fishes. Along with the chondrosteans and the holosteans, they are one of the three major subdivisions of the class Actinopterygii, the most advanced of the bony fishes. The teleosts include virtually all of the world's important sport and commercial fishes, as well as a much larger number of lesser known species. The infraclass is distinguished primarily by the presence of a homocercal tail; i.e., one in which the upper and lower halves are about equal. The teleosts comprise some 20,000 species (about equal to all other vertebrate groups combined), with new species being discovered each year. Common mola, or ocean sunfish (Mola mola). The great abundance of some large species, such as the tunas and halibuts, and of smaller species, such as the various herrings, have made teleost fishes extremely important to mankind as a food supply. In almost every part of the world local fishes are used as food by men at all stages of economic development. In addition to being a commercial food resource, teleost fishes provide enjoyment to millions of people and in many countries of the world support a large sport fishing industry. As aquarium subjects both marine and, especially, small freshwater teleosts provide esthetic beauty for millions of aquarists, supporting a multimillion dollar industry. Part of the interest in teleosts as aquarium subjects is derived from the great diversity of their anatomical structures, functions, and colour. Indeed, these fishes vary more in structure and behaviour than do all the mammals, birds, reptiles, and amphibians combined. Teleosts range in size from tiny gobies less than an inch long when fully adult to large marlins exceeding 3.4 metres (11 feet) in length and 550 kilograms (1,200 pounds) in weight. Another large fish, the ocean sunfish (Mola; see photograph), reaches at least three metres (10 feet) and may weigh more than 900 kilograms (2,000 pounds). Different species and groups have widely varying life cycles and behaviours. Teleost fishes are adapted to widely varied habitats from cold Arctic and Antarctic oceans that remain colder than the freezing point of fresh water to desert hot springs that reach temperatures over 38 C (100 F). Usually a particular species will have a restricted temperature tolerance, often between 10 to 20 F above and below the mean temperature of its environment. In terms of physical habitat, the advanced bony fishes are adapted to a wide and interesting variety of conditions, from fast, rock-laden torrential streams in the Himalaya Mountains to the lightless depths of ocean trenches 10,670 metres (35,000 feet) below the surface, where many species manufacture their own light. A species of teleost fish is usually restricted to one kind of habitat at any given stage of its life cycle. It may occupy this habitat throughout its life, or it may change habitats as it grows older. The various species living in rocky marine shores, mud flats, sandy shores, and coral reefs are usually all different, a diversity of habitat adaptation reflected in the life cycle, behaviour, locomotion, anatomy, and reproduction. Some cyprinodonts or killifishes, for example, confined to annual ponds in Africa and South America, live only the few months during the rainy season that their ponds retain water. They hatch from eggs buried in the mud, grow up, lay their own eggs in the mud in the short space of four to eight months. Such fishes are known as annuals. Others, such as the Pacific salmon, hatch from eggs laid in the gravel of cool, temperate zone streams, spend their first year growing in the streams, then enter the sea to grow and migrate for two, three, or four years, finally returning to the streams where they first grew up. There they lay their eggs and die. Such fishes, spawning in fresh water and living most of their lives in the ocean, are called anadromous. Many freshwater and marine teleosts lay eggs on rocks or aquatic plants, the male and sometimes the female defending the eggs and even the young against predators. Many of these fishes will live two, three, or four years or more, usually spawning in the spring in temperate regions and in the rainy season in the tropics. There are large numbers of teleosts, especially those in the ocean, whose breeding habits are unknown. Most fishes lay numerous eggs, often simply scattering them over plants or in the open ocean, where they provide food for many organisms, only a few young surviving to adulthood. Most offshore marine teleosts lay planktonic (free-floating) eggs, whereas most freshwater fishes lay demersal eggs; i.e., eggs that sink to the bottom. Some teleosts, such as certain of the perchlike African cichlids, some catfishes, and some marine fishes (e.g., cardinal fishes) are oral brooders, the male or female incubating the eggs in its mouth. Some fishes, for example, some of the sea perches (Serranidae), are functional hermaphrodites, one individual producing both sperm and eggs. Self-fertilization is evidently possible in some cases, but more often the fish plays the male and female roles alternately. In some species an individual is a male during the early part of its adult life and a female later. About a dozen families of teleosts produce living young. In some the eggs are abundantly supplied with yolk and merely hatch in the ovary (ovoviviparous), in others the eggs have little yolk, the young hatch at a relatively undeveloped state and are nourished by a placenta-like structure of the ovary (viviparous). The behaviour of teleosts is as varied as their other attributes. Some oceanic fishes travel in close-ordered schools, seemingly responding to predation from larger fishes almost as a single organism. Larger, predatory fishes are usually solitary and hunt or wait for prey alone. Many marine shore and freshwater fishes establish territories during the breeding season, and some may travel in relatively loose schools or shoals when not breeding. Many kinds of teleosts enter into symbiotic relationships with other species of fishes and organisms, for example, a small, blind goby along the California coast lives together with a shrimp in the shrimp's cavelike tubular dwelling. The shrimp carries food to the goby while the goby keeps the shrimp's burrow clean. Many species of wrasse pick parasites from larger fishes, even entering the mouths of these fishes to clean the gill chambers. On the other hand, in South America some small catfishes, 2.5 to 10 centimetres (one to four inches) long, appear to be parasitic on certain other species of catfishes that reach lengths of over 2.4 metres (eight feet). The smaller fishes enter the mouths of the larger catfishes and feed on gill tissue. Unfortunately, little is known about the behaviour of most teleosts, but study of the details of their behaviour has been greatly increased in the last several decades. Defining teleost fishes by functional morphology is hard, for they have evolved into many diverse shapes; but if a relatively simple teleost, a trout, is examined, its basic swimming motion can be determined. Forward motion is provided by bending of the body and caudal fin; waves of muscular action pass from the head to the tail, pushing the sides of the body and tail against the water and forcing the fish forward. The structure of the tail and the efficiency of the swimming mechanism is the prime character that distinguishes teleosts from other, lower, fishes. The dorsal fin and the anal fin (a ventral median fin) are used partly to aid in stability and in turning, and partly in forward locomotion. The paired pelvic or ventral fins and the paired pectoral fins behind the head are used to help stabilize the body and to turn the fish. The fusiform shape of the trout reduces the turbulence and drag of water flowing over the fish's body, offering least resistance to the water. The head of the fish must be adapted for feeding, breathing, and detecting prey and enemies. At the same time it must be relatively streamlined, offering as little resistance to the water as possible. The head of a trout is well formed for these functions by being fusiform but expandable, where necessary, to take in food and water. The fish forces the water in one direction, into the mouth, over the gills, and out the gill slits. Back flow is prevented by valves at the mouth and by the gill covers. The fish, however, can eject undesirable particles and water out the mouth by special action. The teleost head is efficient in having eyes and organs for the sense of smell located in optimum spots for seeing and smelling food. At the same time these organs offer little resistance to water flowing over the head. Stanley H. Weitzman any member of the infraclass Teleostei, a large and extremely varied group of ray-finned fishes (class Actinopterygii) that are considered to be the most advanced of the bony fishes. The teleosts comprise about 20,000 species and account for more than 95 percent of all extant species of fish. Virtually all of the important commercial and sport fishes are teleosts. The most distinctive feature of teleosts is their homocercal taili.e., one whose upper and lower halves are of about equal size. The fins are supported by rays or spines, and the tooth-bearing bones of the jaw are attached to the front end of the snout. The dorsal and anal fins aid in swimming and turning, and paired pectoral and pelvic fins stabilize the body. Teleosts vary more widely in their body structure, size, functions, colour, and behaviour than any other major group of vertebrate animals. They range in size from tiny gobies less than one inch long to large marlins exceeding 3.4 m (11 feet) in length and 550 kg (1,200 pounds) in weight. Among the teleosts are eels, sea horses, sunfishes, flounders, and scorpion fishes, as well as many species of more conventional body type. Such teleosts as tuna, herring, anchovy, salmon, trout, perch, carp, catfish, and cod are extremely important to humans as a food supply. Other, smaller species of teleosts are popular aquarium fish. Teleosts inhabit a wider variety of aquatic environments than any other major group of fishes. They are found in all the world's oceans and major bodies of freshwater; they inhabit tropical, temperate, Arctic, and Antarctic latitudes; and they range in habitat from torrential mountain watercourses to the deep ocean floor. Most species live in temperate or tropical waters, either marine or freshwater. In general, teleosts remain in one habitat throughout their life cycle, though some, such as the salmon, hatch and develop in freshwater, then migrate to the sea before returning to their birthplace to spawn and die. Most teleosts lay eggs that either float in the ocean or sink to the bottom. A few species hatch the eggs in their mouths, carrying them through the entire incubation period, and a few other species give birth to live young. A few hermaphroditic species such as the sea bass are able to produce both eggs and sperm. The behaviour of teleosts is as varied as their other attributes. Some live in closely packed schools, while others live and hunt alone. Additional reading W.A. Gosline, Functional Morphology and Classification of Teleostean Fishes (1971), a study of evolutionary relationships among orders of teleost fish.