...uring only 1 cm 0.4 in long.Fishes are generally streamlined ith a pointed snout and pointed posterior and a broadpropulsive tail. Unlike the shape of a human body, a fishs body shape is ideal for speeding through the ater ithout creating excess resistance. This torpedo-shaped body is typical of the fastest-simming fishes, the billfish and the tunas. One billfish, the sailfish, can sim in bursts of over 110 kmh 70 mph. Tunas are built for long-distance endurance as ell as speed, simming as fast as 50 kmh 30 mph and migrating as far as 12,500 km 7700 mi in only four months. Other fishes come in a ide variety of shapes. The snakelike eels, flat halibuts, and boxy puffers are all sloer simmers that have evolved distinctive bodies best adapted to their specific habitats. Unlike fishes that sim through the open ater, these fishes have adapted to life in caves, on the ocean floor, and among coral reefs here speed is less important than camouflage or maneuverability.Fishes are an important source of protein for millions of people orldide. 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 orlds population, second only to beef.IITYPES OF FISH Fishes may be divided into to distinct groups, jaless fish and jaed fish. The jaless fish are represented by to families of distantly related eel-like fish, the hagfish and the lampreys. Both fishes have tongues equipped ith numerous small teeth and lack paired fins and a bony skeleton. Although these to families include only a handful of living species, the fossil record shos they ere once a highly diverse group that also included fish hose head and trunk ere covered ith 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 aay tissue ith their toothed tongue.The jaed fish may also be separated into to major groups bony fish, hich have skeletons made of rigid bone, and cartilaginous fish, hich 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 shallo ocean aters and occasionally freshater, hile chimaeras are found only in the ocean, mostly in deep ater. Sharks have an age-old reputation for savagery, but only a fe of the approximately 370 species deserve this reputation. Most sharks, like the spiny dogfish, are predators of small fish and invertebrates, hile the largest, such as megamouths, hale sharks, and basking sharks, feed by filtering tiny invertebrates from the ater. 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, ith about 24,000 species inhabiting nearly every body of ater on the earth. They are divided into to groupsthe 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 to major groups, the primitive sturgeons and paddlefish, and the more evolved ne-finned fishes. Most of the common and ell-knon fish species are ne-finned fish, including the herrings, hich support one of the largest fisheries in the orld, and the eels, hich are found in nearly all marine habitats. Other ne-finned fishes include the ostariophysansminnos, characins, and catfishhich inhabit the freshaters of the tropics and surrounding areas. Salmon have adapted to the coasts of northern oceans by living part of their lives in freshater 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 aters. Closely related to the perches are the flatfish, hich look and sim like normal fish hen young, only to lay on one side of their body as adults after one eye migrates to the top side.IIIHABITATS OF FISH Fishes may be classified as either freshater or saltater species. Although freshater lakes and rivers comprise less than 0.001 percent of the volume of ater on earth, 40 percent of fish species are found there. Most of the rest are found in the salty oceans, hile only 2 to 3 percent are found in somehat salty, or brackish aters. Similarly, most marine fishes are found associated ith the seafloor or ith 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 hile the relatively uniform habitat of open aters has not. Only 13 percent of fish species live primarily in the open ocean.Fish are not randomly distributed in the orlds aters. For example, the continental shelves, shallo areas of the ocean typically 200 m 650 ft deep or less, ith abundant light from the sun and nutrients from the continents, contain most of the oceans 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 arm, ell-lit aters near coral reefs also promote a rich diversity of fish species. In freshaters, the greatest diversity of fish species occurs in the arm tropics of South America, Africa, and southeast Asia. There are many species of tropical minnos, characins, and cichlids that are yet to be identified.IVBODIES OF FISH Most fishes are mobile underater predators and their bodies have adapted accordingly. For most fishes, this means a streamlined body that can move siftly through the ater. A typical fish has a fusiform shape, pointed to penetrate the ater in front and tapered to the rear, finished ith a broadly expanded tail fin that provides propulsive force. Additional fins on the bodys 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 forard thrust, or, together ith 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 underater ith the help of special respiratory organs called gills. Gills are made of a series of thin sheets or filaments through hich blood circulates. As ater moves into a fishs mouth and passes over the gills, dissolved oxygen passes across the thin gill membranes into the bl...
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