Fish cold-blooded, limbless, completely aquatic vertebrates, having gills, commonly fins, and typically elongated torpedo-shaped body mostly covered with scales. Fish are usually grouped into four classes, three living (Agnatha, Chondrichthyes, and Osteichthyes) and one extinct (Placodermi):
1. Agnatha, the primitive jawless fishes, including the cyclostomes (lampreys and hagfishes) and extinct armoured fishes known as ostracoderms, none of which has gill arches (bony frames for the gills);
2. Placodermi, extinct armoured fishes with jaws;
3. Chondrichthyes, the cartilaginous fishes, including sharks, rays and their allies, all of which have a predominantly cartilaginous skeleton; and
4. Osteichthyes, the bony fishes, including the great majority of food and game fishes. A number of aquatic invertebrate animals and groups have common names compounded with the term fish, which are unrelated to true fish. For example cuttlefish (cephalopod mollusks, relatives of the octopus), jellyfish (coelenterates), starfish (echinoderms), and shellfish (mollusks such as the oyster and clam), and the crustacean arthropods such as the crayfish. The bony fishes are divided into two groups: the fleshy-finned fish (Crossopterygii) and the ray-finned fish (Actinopterygii). The latter group includes over 95% of all living fish species. The earliest bony fishes were fleshy-finned. They evolved during a period of widespread drought and stagnation and gave rise to the amphibians (the first terrestrial vertebrates) on the one hand, and to the ray-finned fish on the other. The only surviving fleshy-finned fishes are the lungfishes and one species of Coelacanth. These fishes retain some of the traits of ancestral bony fishes: fleshy fins with supporting bones (precursors of the limbs of land vertebrates), internal nostrils, and lungs. Ray-finned fishes are now predominant in both fresh and marine waters. There are about 22,000 species of fishes worldwide, in about 450 families, and almost 40% of the species live in fresh water. In Bangladesh there are 442 species of marine fishes, in 18 orders, 123 families. Of these 442 species, 56 are cartilaginous fishes (class Chondrichthyes), in 3 orders and 15 families; 386 species are bony fishes (class Osteichthyes), in 15 orders and 108 families. There are 266 species of inland fishes (in freshwaters and brackish waters) in 61 families and 14 orders. Among the inland fishes, the family Cyprinidae (order Cypriniformes) includes the largest number of species: 61 species under 25 genera; these include carps (Rui, Catla, Mrigal, Kalibaus, etc); barbs (Punti, Mahashol, etc); minnows (Darkina, Chela, Mola,etc). About 55 species of catfishes (Tengra, Aid, Shingi, Magur, etc), are found in the freshwaters of Bangladesh. Loaches (Rani, Gutum, Puiya, Panga, etc) are the least explored fish species (about 12 species). Once abundant in the wetlands, the snakehead fishes (Shol, Taki, Gajar etc) are now becoming rare. Of the five species of family Channidae three are threatened: Barca snakehead (Pipla shol), Channa barca, is critically endangered, Giant snakehead (Gajar), Channa marulius, is endangered, and Asiatic snakehead (Telo Taki),Channa orientalis, is vulnerable. Of the eels (usually with two lateral gill-openings), the Gangetic Mudeel (kuichya), Monopterus cuchia, is unique in possessing a single gill-opening on the ventral side. Once abundant, it is now a vulnerable species. Another beautiful eel, the one-stripe spinyeel (Tara Baim), Macrognathus aral, is also now vulnerable. The snake-eels (2 species), Pisodonophis spp., are not usually eaten by the local people, and face no threats at the moment. The largest eel is the Indian Longfin Eel (Bamosh/Bamchara/Bao Baim/Telkoma), Anguilla bengalensis, found in the estuaries and freshwaters; the largest specimen caught from Netrokona was 118 cm long and weighed over 6 kg; the fish is known to be catadromous, and migrates down the rivers to the...
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