Fish, diverse group of animals that live and breathe in water. All fishes are vertebrates (animals with backbones) with gills for breathing. Most fish have fins for swimming, scales for protection, and a streamlined body for moving easily through the water.Fishes live in nearly every underwater habitat, from near-freezing Arctic waters to hot desert springs; from mud in dried-up tropical ponds to the deepest ocean abyss. Special antifreeze chemicals in the blood of Antarctic icefish enable them to survive in water below 0° C (32° F). Desert pupfish found in hot springs of western North America live in temperatures higher than 40° C (100° F). Killifish release their eggs, or spawn, as the dry season begins in the tropics of South America and Africa, leaving their eggs to dry in the ground until the rains return six months later. In the deep ocean, where sunlight never reaches, many fishes cooperate with glowing bacteria to create their own light for communication and to attract mates and prey. With approximately 25,000 recognized species, fishes make up the most diverse vertebrate group, comprising about half of all known vertebrate species. New fishes continue to be discovered and named at the rate of 200 to 300 species per year. With this vast number of different fishes comes a diversity of sizes and shapes, from huge whale sharks that reach 12 m (40 ft) in length to the smallest vertebrate, a tiny goby, measuring only 1 cm (0.4 in) long.Fishes are generally streamlined with a pointed snout and pointed posterior and a broadpropulsive tail. Unlike the shape of a human body, a fish’s body shape is ideal for speeding through the water without creating excess resistance. This torpedo-shaped body is typical of the fastest-swimming fishes, the billfish and the tunas. One billfish, the sailfish, can swim in bursts of over 110 km/h (70 mph). Tunas are built for long-distance endurance as well as speed, swimming as fast as 50 km/h (30 mph) and migrating as far as 12,500 km (7700 mi) in only four months. Other fishes come in a wide variety of shapes. The snakelike eels, flat halibuts, and boxy puffers are all slower swimmers that have evolved distinctive bodies best adapted to their specific habitats. Unlike fishes that swim through the open water, these fishes have adapted to life in caves, on the ocean floor, and among coral reefs where speed is less important than camouflage or maneuverability.Fishes are an important source of protein for millions of people worldwide. Since the early1970s, 70 to 100 million metric tons of fish are caught each year for food (see Fisheries). People consume about 70 percent of fish caught, and nearly 30 percent are used as animal feed that helps produce other forms of protein. Fish protein represents about 25 percent of the total animal protein consumed by the world’s population, second only to beef.
II TYPES OF FISH
Fishes may be divided into two distinct groups, jawless fish and jawed fish. The jawless fish are represented by two families of distantly related eel-like fish, the hagfish and the lampreys. Both fishes have tongues equipped with numerous small teeth and lack paired fins and a bony skeleton. Although these two families include only a handful of living species, the fossil record shows they were once a highly diverse group that also included fish whose head and trunk were covered with a hard bony shell. Hagfish are the vultures of the abyss, feeding on carcasses of dead fish and other animals. Lampreys, in contrast, feed on live fish by attaching their sucking disk to their host and rasping away tissue with their toothed tongue.The jawed fish may also be separated into two major groups: bony fish, which have skeletons made of rigid bone, and cartilaginous fish, which have skeletons made of elastic cartilage. There are nearly 1000 species of cartilaginous fish, including sharks, rays, and chimaeras, or ratfish. Sharks and rays live in relatively shallow ocean waters and occasionally freshwater, while chimaeras are found only in the ocean, mostly in deep water. Sharks have an age-old reputation for savagery, but only a few of the approximately 370 species deserve this reputation. Most sharks, like the spiny dogfish, are predators of small fish and invertebrates, while the largest, such as megamouths, whale sharks, and basking sharks, feed by filtering tiny invertebrates from the water. The nearly 200 species of rays are essentially sharks flattened like a pancake that have adapted to life on the ocean floor.The bony fishes encompass by far the largest diversity of fish, with about 24,000 species inhabiting nearly every body of water on the earth. They are divided into two groups—the lobe-finned fish and the ray-finned fish. Lobe-finned fishes include the lungfish, a small group of primitive air-breathing fish, and the coelacanth, the single living species of a group long thought to be extinct.The ray-finned fishes are divided into two major groups, the primitive sturgeons and paddlefish, and the more evolved new-finned fishes. Most of the common and well-known fish species are new-finned fish, including the herrings, which support one of the largest fisheries in the world, and the eels, which are found in nearly all marine habitats. Other new-finned fishes include the ostariophysans—minnows, characins, and catfish—which inhabit the freshwaters of the tropics and surrounding areas. Salmon have adapted to the coasts of northern oceans by living part of their lives in freshwater and part in the ocean. There are over 9000 species of perch, including tunas, jacks, billfishes, sunfishes, and darters, making it the largest vertebrate order. Perches and their relatives are the dominant fishes in tropical marine waters. Closely related to the perches are the flatfish, which look and swim like normal fish when young, only to lay on one side of their body as adults after one eye migrates to the “top” side.
III HABITATS OF FISH
Fishes may be classified as either freshwater or saltwater species. Although freshwater lakes and rivers comprise less than 0.001 percent of the volume of water on earth, 40 percent of fish species are found there. Most of the rest are found in the salty oceans, while only 2 to 3 percent are found in somewhat salty, or brackish waters. Similarly, most marine fishes are found associated with the seafloor or with other natural or artificial features, such as reefs or docks. These structures offer them protection from predators or serve as focal points for feeding and social interactions. The variety of seafloor habitat has enabled fish to diversify while the relatively uniform habitat of open waters has not. Only 13 percent of fish species live primarily in the open ocean.Fish are not randomly distributed in the world’s waters. For example, the continental shelves, shallow areas of the ocean typically 200 m (650 ft) deep or less, with abundant light from the sun and nutrients from the continents, contain most of the ocean’s fishes. This habitat promotes large populations of tiny invertebrate animals that are in turn eaten by fishes. Similarly, the upper 200 m (650 ft) of the ocean holds 78 percent of marine fish species. The warm, well-lit waters near coral reefs also promote a rich diversity of fish species. In freshwaters, the greatest diversity of fish species occurs in the warm tropics of South America, Africa, and southeast Asia. There are many species of tropical minnows, characins, and cichlids that are yet to be identified.
IV BODIES OF FISH
Most fishes are mobile underwater predators and their bodies have adapted accordingly. For most fishes, this means a streamlined body that can move swiftly through the water. A typical fish has a fusiform shape, pointed to penetrate the water in front and tapered to the rear, finished with a broadly expanded tail fin that provides propulsive force. Additional fins on the body’s midline, the dorsal and anal fins, and paired pelvic fins act as stabilizers to prevent rolling from side to side. Paired pectoral fins provide fine movements, add forward thrust, or, together with the pelvic fins, serve as brakes. Typically, fins consist of a thin membrane stretched over a fanlike series of thin rods called spines or rays.Most fish breathe underwater with the help of special respiratory organs called gills. Gills are made of a series of thin sheets or filaments through which blood circulates. As water moves into a fish’s mouth and passes over the gills, dissolved oxygen passes across the thin gill membranes into the blood and carbon dioxide passes out into the water. Some species, such as the lungfish, lack gills but can breathe air by means of lungs.A critical event in the evolution of bony fishes was the development of an air-filled organ called a swim bladder. The swim bladder enables fish to float at a desired depth without spending extra energy to swim in place. There are two types of swim bladders in bony fish: One exchanges gas through the fish’s mouth and anus, as in herrings and minnows, and the other exchanges gas through a complex system of blood vessels, as in perches and sea bass.To avoid other predators and survive in the aquatic world, fish have evolved scales, spines, and an extraordinary sensitivity to changing water pressure, odors, and tastes. Scales developed from the massive bony armor protecting early fishes and the smaller scales found on fish today permit more flexibility. Spines probably originated simply to support the fins, but later developed a secondary function for protection.Unlike the senses of land animals, fish have senses that exploit water’s ability to carry and maintain pressure waves and chemicals. Using a special sense organ called the lateral line, a fish can feel the nearness of a predator or obstacle before it can see the object. Especially in water of low visibility, fish that swim in tight-knit groups, called schools, use their lateral lines to sense and coordinate sudden turns. Highly sensitive olfactory, or smell, organs permit fish to sense chemicals in the water, which helps identify food or other fish. Some fish, such as minnows, are also sensitive to chemicals in the skin of other members of their species. When released to the water during a predator attack, these chemicals stimulate a fright response that warns other minnows to escape.Fishes feed in a variety of ways. Primitive fishes feed much like mammals by opening their mouth and biting down on food to either bite off manageable sections or to chew. Predatory sharks, for example, use large serrated teeth that line the edges of their mouths to shear off pieces of flesh. Most fishes also have teeth inside their mouth and farther back just before the esophagus. More advanced fishes draw the water containing the prey into their mouths by suction feeding—a complex network of interconnected bones pulled by several muscles create a large force of suction directed at prey and the surrounding water. Pipefish and sea horses provide the best example of this type of feeding.There are fewer fishes on the dark ocean floor to hunt and catch for food. Some deep-sea fishes attract prey with specialized light organs. The deep-sea whipnose angler, for example, has evolved a modified, elongated dorsal fin spine that acts like a fishing pole with a light organ at its tip that serves as a lure. It uses its very long pole as a fly fisher would, throwing the lure out again and again, each time pulling it, and occasionally an unsuspecting fish, into its mouth.
V REPRODUCTION
Fish exhibit a wide array of reproductive strategies. Sharks, for example, produce only a few eggs at a time while cods may produce several million. Some species simply release their eggs into the open water while others carefully place individual eggs on a surface and care for them for days. Typically, a male will fertilize the eggs by moving alongside a female and releasing sperm as the female releases her eggs. In some species, such as guppies, rockfish, and sharks, females retain the eggs in their bodies and accept sperm from males. The young hatch within the mother’s body and are then released into the water. In pipefish and sea horses, females transfer their eggs to the male, who then carries and incubates them. Once fertilized, fish eggs can take anywhere from one day to hatch in some warm water species, to several months in some cold water species.To signal readiness and prepare their surroundings for spawning, fishes have evolved an amazing diversity of behaviors. In nest builders, males build a nest and advertise to females their interest in mating by dancing in front of the nest. Three-spine sticklebacks build nests of vegetation glued together by a cement secreted by their kidneys, and gouramis build bubble nests held together by mouth secretions. The splash tetra of South America lays and fertilizes its eggs by leaping out of the water and attaching eggs and sperm onto leaves above the water surface. The eggs are kept moist by the male who splashes them with his tail until the young hatch.Deep-sea fishes use light to attract mates as well as prey. Lanternfish likely use the specific patterns of light organs along the sides of their bodies to signal other members of their own species. The bright colors in some nonschooling fishes are specific to individual species and may serve to attract appropriate mates. VI EVOLUTION OF FISH

The first vertebrates evolved from sedentary vase-shaped marine animals called sea squirts (see Tunicate) about 500 million years ago. The larvae of modern sea squirts are strikingly similar to young fish and have a primitive backbone, called a notochord. The first fish were jawless and probably fed by filtering tiny particles from the water. The fossil record is not clear because only the teeth remain, but these early fish probably lacked scales. Later fish evolved armor plates and scales for protection from large predatory arthropods.The diversity of fish species exploded during the Devonian period (410 million to 360 million years ago) in what is known as the Age of Fish. Following hagfish and lampreys, which originated about 400 million years ago, cartilaginous fish (sharks, rays, and chimaeras) evolved and became diverse. True bony fish also arose about 400 million years ago, forming three major lineages of modern fish: the lungfish, coelacanths, and ray-finned fish, which encompass all the remaining living fish. An ancestral lungfish ultimately gave rise to all other vertebrates.

VII FISH AND HUMANS
It is impossible to overstate the importance of fish to human populations around the world. Throughout history, humans have used fish protein as a food source, with wild caught fish providing the bulk of fish protein. Fish have also been farmed in large quantities for more than 2000 years in China. Recent advances in fish farming, especially with some African cichlids (see Tilapia), have alleviated hunger in many parts of the world. In industrialized countries, farm-raised fish provide relief for overfished stocks of wild fish. Fish also have served as a source of recreational pleasure for many people. The catches from sports fisheries (see Fishing) are far larger than commercial catches from most freshwaters and in marine waters close to large population centers. Aquariums provide an intimate acquaintance with the aquatic world. More than 20 million home aquariums are kept in the United States alone. Among the many fish kept in aquariums, the most common are minnows, characins, and cichlids.Some fishes may be dangerous to humans, although in most cases the danger is easily avoided. The stonefish is one of the most venomous vertebrates known, with enough toxin in the sharp dorsal spines to kill an adult human that steps on one (see Rockfish). The toxin of the pufferfish, or fugu, is deadly when eaten. Sharks have perhaps the worst but least deserved reputation for aggressiveness, for only a few of the species have been known to attack humans. Many larger fish when provoked are capable of inflicting wounds on humans. For example, moray eels, as sinister as they appear, do not go out of their way to attack humans, but will bite if provoked.
VIII THREATENED FISH
Humans are far more dangerous to fish than fish are to humans. Sharks kill about 30 people per year, while an estimated 700,000 metric tons of sharks are harvested or killed by people each year. Several species of sharks, including the great white, have been greatly overfished. Sharks are particularly susceptible to overfishing because most species need many years to grow to reproductive maturity.Many stocks of wild fish have been harvested beyond their natural capacity to sustain their populations. Most recently, the collapse of the cod fishery in the Northwest Atlantic has heightened concerns over our ability to responsibly manage natural marine resources. Since the closing of those commercial fishing grounds, evidence is mounting that the fish populations are beginning to recover, although the recovery may take decades. Stocks of fish, like the bluefin tuna, that cross international borders or are found on the high seas are of special concern because they are particularly difficult to manage. Even large marine stocks once thought immune to the effects of overfishing, like the Pacific sardine and the Peruvian anchovy, have declined dramatically. In spite of these problems, fisheries can be successfully managed to reduce the effects of previous overfishing and to prevent further abuse. Proper management requires timely and accurate data on fish populations and harvest, as well as the ability to strictly enforce the protection of vulnerable fish stocks.The most seriously threatened fishes are found in freshwaters of the world, especially in the environmentally sensitive and industrialized areas of the northern hemisphere. Many unique freshwater species are found only in a small area because of the isolation by land barriers. Thus, water pollution or habitat destruction in streams can be devastating to fish populations or even entire species. With increasing human populations, the effects are bound to intensify unless preventive actions are undertaken. Fortunately, significant advances in our knowledge of the effects of pollutants and habitat change have improved habitat restoration and pollution control.Scientific classification: Fishes are classified differently by different zoologists. Some of the classifications are very complex and divide fishes into more than 100 orders and suborders. In the most generally used system, the subphylum Vertebrata is divided into two superclasses: Agnatha, which includes the lamprey and other fishes without jaws; and Gnathostomata, which includes the fishes with hinged jaws. The latter are further divided into the class Chondrichthyes, the cartilaginous fishes such as the sharks, rays, and chimaeras, and the class Osteichthyes, the bony fishes.The bony fishes are made up of the subclass Sarcopterygii, lobe-finned fishes, and the subclass Actinopterygii, ray-finned (or spiny-finned) fishes. The recent ray fins consist of two groups, the Chrondrostei and the Neopterygii, which includes the large division Teleostei or modern bony fishes.