FISH

any of a variety of cold-blooded, vertebrate animals found in the fresh and salt waters of the world. Living species range from the primitive, jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Fishes are enormously varied in shape, size, and colour. Their bodies are generally fusiform (tapered to each end), and they can range in length from 10 mm (0.4 inch) to more than 20 m (60 feet). Most fishes that inhabit surface or midwater regions are streamlined or flattened side-to-side, while most bottom-dwellers are flattened top-to-bottom. Tropical species are often brightly coloured, and others may have a drab appearance so as to blend in with their surrounding environment. Most fishes have paired fins, and their skins are covered with either bony or toothlike (placoid) scales; some have bony plates embedded in the skin or lack scales. Respiration is generally through gills. Most bony fish have a swim bladder, a gas-filled organ used to adjust swimming depth. In a few species the swim bladder has evolved into a lunglike respiratory organ, enabling these fishes to breathe air. Most fish reproduce by laying eggs, which may be fertilized externally or internally. Some species are hermaphroditic, although examples of fishes that are self-fertilizing are rare. A few fishes bear live young. The mortality rate of eggs and hatching is generally very high; only a few individuals reach adulthood out of a batch of hundreds or even millions of eggs. The central nervous system of fishesthe brain and spinal cordcontrols body activity. Most fishes have a well-developed sense of smell; the olfactory, or nasal, organ is located on the dorsal surface of the snout. Many fishes have taste buds in their mouth cavities. Most can see well, and experiments have shown that many fishes, especially those that swim near the surface, have colour vision. Hearing organs are located within the skull, on either side of the brain. The lateral-line system, consisting of highly innervated fluid-filled canals that run the length of the body, detects vibrations in the water current. Fish first appeared more than 450 million years ago. Since that time they have evolved to fit almost all freshwater and saltwater habitats. any of a variety of cold-blooded vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive, jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animalsamphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) are the only fishes that have a suctorial, or filter-feeding, mouth, a feature that makes them dependent on an essentially parasitic way of life. They have either no fins or poorly developed ones. Extant examples of the agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (chondr, cartilage, and ichthyes, fish) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny sea horse to the 450-kilogram (1,000-pound) blue marlin, from the flat soles and flounders to the boxy puffers and sunfishes. Unlike those of the cartilaginous fishes, the scales of bony fishes, when present, grow throughout life and are made up of thin, overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits. The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world's food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases. Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. To many, aquarium fishes provide a personal challenge, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing support multimillion-dollar industries throughout the world. Stanley H. Weitzman The Editors of the Encyclopdia Britannica Additional reading General works E.S. Herald, Living Fishes of the World (1961, reissued 1972), a clearly written and extensively illustrated introduction to fishes; G.U. Lindberg, Fishes of the World (1974; originally published in Russian, 1971), a comprehensive work with an extensive bibliography; J.S. Nelson, Fishes of the World, 3rd ed. (1994), a treatment of all families that includes maps showing distribution; P.P. Grass (ed.), Trait de zoologie, vol. 13, Agnathas et poissons, 3 parts (1958), a classic and authoritative review in French of the classification, anatomy, and biology of fishes. J.R. Norman, A History of Fishes, 3rd ed. by P.H. Greenwood (1975); N.B. Marshall, The Life of Fishes (1965); K.F. Lagler et al., Ichthyology, 2nd ed. (1977), college-level introductory texts of general ichthyology; J.E. Webb, J.A. Wallwork, and J.H. Elgood, Guide to Living Fishes (1981); and Tim M. Berra, An Atlas of Distribution of the Freshwater Fish Families of the World (1981). Regional works A.H. Leim and W.B. Scott, Fishes of the Atlantic Coast of Canada (1966), a good general account, completely illustrated; H.B. Bigelow et al., Fishes of the Western North Atlantic, 5 vol. (194866), a comprehensive treatment of the biology of western North Atlantic fishes; J.E. Bhlke and C.C.G. Chaplin, Fishes of the Bahamas and Adjacent Tropical Waters, 2nd ed. (1993); W.B. Scott, Freshwater Fishes of Canada (1973); W.A. Clemens and G.V. Wilby, Fishes of the Pacific Coast of Canada, 2nd ed. (1961); J. and G. Lythgoe, Fishes of the Sea: The Coastal Waters of the British Isles, Northern Europe and the Mediterranean (1975); W.A. Gosline and V.E. Brock, Handbook of Hawaiian Fishes (1960), an excellent handbook of fishes from the central Pacific Ocean; T.C. Marshall, Fishes of the Great Barrier Reef and Coastal Waters of Queensland (1964); T. Kamohara, Fishes of Japan in Color (1967; originally published in Japanese, 1955); J.T. Nichols, The Fresh-Water Fishes of China (1943), somewhat old but complete coverage of fishes of eastern Asia; I.S.R. Munro, The Marine and Freshwater Fishes of Ceylon (1955, reprinted 1982), an inclusive illustrated account of fishes from the Indian Ocean area; J.L.B. Smith, The Sea Fishes of Southern Africa, 5th ed. (1965); R.H. Lowe-McConnel, Fish Communities in Tropical Freshwaters: Their Distribution, Ecology and Evolution (1975), a broad review of fishes of Africa, South America, and Asia. Somewhat more local, but applicable to much of North America, are the following: M.B. Trautman, The Fishes of Ohio with Illustrated Keys, rev. ed. (1981); F.B. Cross, Handbook of Fishes of Kansas (1967), and, with J.T. Collins, a companion vol., Fishes in Kansas (1975); H.D. Hoese and R.H. Moore, Fishes of the Gulf of Mexico: Texas, Louisiana and Adjacent Waters (1977); W.F. Smith-Vaniz, Freshwater Fishes of Alabama (1968); C.L. Hubbs and C. Lagler, Fishes of the Great Lakes Region, rev. ed. (1958, reissued 1967); James E. Morrow, The Freshwater Fishes of Alaska (1980); and Robert J. Naiman and David L. Soltz (eds.), Fishes in North American Deserts (1981). Natural history C.M. Breder and D.E. Rosen, Modes of Reproduction in Fishes (1966), a summary of reproductive behaviour of fishes; N.B. Marshall, Aspects of Deep Sea Biology (1954), a college-level introduction to deep-sea biology; B.W. Halstead, Poisonous and Venomous Marine Animals of the World, vol. 2 and 3, 2nd rev. ed. (1988), an extensive treatment of poisonous and venomous marine fishes, beautifully illustrated in colour; H.S. Davis, Culture and Diseases of Game Fishes (1953, reissued 1967), a general aid to the culture of North American game fishes. See also Michael Goulding, The Fishes and the Forest: Explorations in Amazonian Natural History (1980). Form and function R.M. Alexander, Functional Design in Fishes, 3rd ed. (1974), a short college-level book on functional anatomy of fishes; M.E. Brown (ed.), The Physiology of Fishes, 2 vol. (1957); and W.S. Hoar and D.J. Randall (eds.), Fish Physiology, 6 vol. (196971), advanced general texts; NATO, Environmental Physiology of Fishes (1980). Paleontology and classification L.S. Berg, System der rezenten und fossilen Fischartigen und Fische (1958), a German translation from the second Russian edition, a revised edition of Berg's 1940 classification of contemporary and fossil fishes; A.S. Romer, Vertebrate Paleontology, 3rd ed. (1966), a college-level text with a good review of fish evolution; W.A. Gosline, Functional Morphology and Classification of Teleostean Fishes (1971), a study of evolutionary relationships among orders of teleost fish. Bibliography B. Dean et al. (eds.), A Bibliography of Fishes, 3 vol. (191623, reprinted 1972), an almost complete bibliography of works on contemporary and fossil fishes up to about 1923. Classification Distinguishing taxonomic features In forming hypotheses about the evolution of fishes and in establishing classifications based on these hypotheses, ichthyologists place special emphasis on the comparative study of the skeleton. There are two primary advantages of this approach. First, direct comparison between extant and fossil groups is possible, the latter usually represented only by bony remains. The second advantage is that the bones of living fishes are relatively easy to observe and to study, compared with other body structures. Proper preservation and special preparation of the nervous system, for example, is difficult and expensive when the fishes compared are from the far ends of the earth. In the study of the relationships of species within a group major use has been made of similarities and differences in the dimensions of external features such as head and body length, and of counts of external characters, such as teeth, fin rays, and scales. Colour pattern is also important. In recent years valuable data on classification of fishes has been obtained from studies of comparative behaviour, physiology, genetics and functional anatomy. Annotated classification The following classification has been derived primarily from the works of C. Patterson, Miles, P.H. Greenwood and co-workers, D.E. Rosen and C. Patterson, and K.S. Thomson. Fish classification has undergone major revisions in recent years and further modifications can be expected in the future. Ichthyologists frequently disagree on major as well as minor concepts of phyletic relationships. There remains much to learn about both living and fossil fishes. The geographical distribution given for a poorly known fossil group usually represents only the location of fossil finds not necessarily the true distribution of the group. In the classification presented here groups indicated by a dagger () are known only from fossils. Stanley H. Weitzman Evolution and paleontology Although a great many fossil fishes have been found and described, they represent a tiny portion of the long and complex evolution of fishes and knowledge of fish evolution remains relatively fragmentary. In the classification presented in this article fishlike vertebrates are divided into seven classes, the members of each having a different basic structural organization and different physical and physiological adaptations for the problems presented by the environment. The broad basic pattern has been one of successive replacement of older groups by newer, better adapted groups. One or a few members of a group evolved a basically more efficient means of feeding, breathing, swimming, or several better ways of living. These better adapted groups then forced the extinction of members of the older group with which they competed for available food, breeding places, or other necessities of life. As the new fishes became well established, some of them evolved further and adapted to other habitats, where they continued to replace members of the old group already there. The process was repeated until all or almost all members of the old group in a variety of habitats had been replaced by members of the newer evolutionary line. Agnatha: early jawless fishes The earliest vertebrate fossils of certain relationships are fragments of dermal armour of jawless fishes (class Agnatha, order Heterostraci) from the Middle Ordovician Period in North America, about 450,000,000 years in age. Early Ordovician toothlike fragments from the U.S.S.R. are less certainly remains of the class Agnatha. It is uncertain whether the North American jawless fishes inhabited shallow coastal marine waters, where their remains became fossilized, or were freshwater vertebrates washed into coastal deposits by stream action. Jawless fishes probably arose from ancient small, softbodied filter-feeding organisms much like and probably also ancestral to the modern sand-dwelling filter feeders, the Cephalochordata ( Amphioxus and its relatives). The body in the ancestral animals was probably stiffened by a notochord. Although a vertebrate origin in fresh water is much debated by paleontologists, it is possible that mobility of the body and protection provided by dermal armour arose in response to streamflow in the freshwater environment and to the need to escape from and resist the clawed invertebrate eurypterids that lived in the same waters. Because of the marine distribution of the surviving primitive chordates, many paleontologists doubt that the vertebrates arose in fresh water. Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the Silurian, about 400,000,000 years ago, European heterostracan remains are found in what appear to be delta or coastal deposits. In the Late Silurian of the Baltic area, lagoon or freshwater deposits yield jawless fishes of the order Osteostraci. Somewhat later in the Silurian from the same region, layers contain fragments of jawed acanthodians, the earliest group of jawed vertebrates, and of jawless fishes. These layers lie between marine beds but appear to be washed out from fresh waters of a coastal region. It is evident, therefore, that by the end of the Silurian both jawed and jawless vertebrates were well established and already must have had a long history of development. Yet paleontologists have remains only of specialized forms that cannot have been the ancestors of the placoderms and bony fishes that appear in the next period, the Devonian. No fossils are known of the more primitive ancestors of the agnaths and acanthodians. The extensive marine beds of the Silurian and those of the Ordovician are essentially void of vertebrate history. It is believed that the ancestors of fishlike vertebrates evolved in upland fresh waters, where whatever few and relatively small fossil beds were made probably have been long since eroded away. Remains of the earliest vertebrates may never be found. By the close of the Silurian, all five known orders of jawless vertebrates had evolved, except perhaps the modern cyclostomes, which are without the hard parts that ordinarily are preserved as fossils. Cyclostomes were unknown as fossils until 1968, when a lamprey of modern body structure was reported from the Middle Pennsylvanian of Illinois, in deposits almost 300,000,000 years old. Fossil evidence of the four orders of armoured jawless vertebrates is absent from deposits later than the Devonian. Presumably they became extinct at that time, being replaced by the more efficient and probably more aggressive placoderms, acanthodians, selachians (sharks and relatives), and by early bony fishes. Cyclostomes survived probably because they early evolved from anaspid agnaths and developed a rasping tonguelike structure and a sucking mouth, enabling them to prey on other fishes. With this way of life they apparently had no competition from other fish groups. Early jawless vertebrates probably fed on tiny organisms by filter feeding, as do the larvae of their descendants, the modern lampreys. The gill cavity of the early agnaths was large. It is thought that small organisms taken from the bottom by a nibbling action of the mouth, or more certainly by a sucking action through the mouth, were passed into the gill cavity along with water for breathing. Small organisms then were strained out by the gill apparatus and directed to the food canal. The gill apparatus thus evolved as a feeding, as well as a breathing, structure. The head and gills in the agnaths were protected by a heavy dermal armour; the tail region was free, allowing motion for swimming. Most important for the evolution of fishes and vertebrates in general was the early appearance of bone, cartilage, and enamel-like substance. These materials became modified in later fishes, enabling them to adapt to many aquatic environments and finally even to land. Other basic organs and tissues of the vertebrates such as the central nervous system, heart, liver, digestive tract, kidney, and circulatory system undoubtedly were present in the ancestors of the Agnatha. In many ways, bone, both external and internal, was the key to vertebrate evolution. Form and function Body plan The basic structure and function of the fish body is similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the organs and organ systems parallel those of other vertebrates. The typical fish body is streamlined and spindle-shaped, with an anterior head, gill apparatus, and heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today. The skeleton forms an integral part of the fish's locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone. The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the sections below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, almost always without fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin. The skin The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnaths. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish's body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess. Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosmine-like layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid scales (the latter distinguished by serrations at the edges), lack enameloid and dentine layers. Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface. Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by expansion and contraction of the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed near iridocytes or leucophores (bearing the silvery or white pigment guanine) melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.