Introduction to Fish
It is a 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.
Generalized Anatomy of a Fish
Both the digestive and circulatory systems in most fishes are rather simple. Although a few species can breathe atmospheric air, most fish breathe by means of gills. The swim (or gas) bladder allows fish to maintain a constant buoyancy regardless of the changing water pressure at varying depths.
Fish Make up the Most Diverse Vertebrate Group
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, the stout infantfish (Schindleria brevipinguis), measuring only about 7 mm (0.3 in) long.
The first vertebrates evolved from sedentary vase-shaped marine animals called sea squirts 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.
CLASSIFICATION 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 was 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
i. Class Cephalosphidomorphy
ii. Class Condric-thyes
iii. Class Ostris-thyes
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