All of us have seen fish. They are very elegant in their colouration and many people like to keep them as pets. A fish may be identified as an aquatic vertebrate animal whose body may or may not be covered by scales and bears two sets of paired fins and cadiahoangtuan.com many unpaired fins. fish are cold blooded or ectothermic animals which mean that their body temperature keeps on changing with the temperature of water. fish are distributed in all aquatic habitats ranging from the mountain streams to the deepest oceans. About 32,500 species of fish are known and they together form the largest diversity of the organisms in comparison to other classes of vertebrates. fish are also caught by the fish keepers and are kept in aquaria as a source of decoration. They are also associated with movies, cultures and religions.
The term fish applies to an animal which suggests any non-tetrapod craniate that bears gills throughout its life and has fins instead of limbs. Like other vertebrates fish are of different types and are classified according to their characters. The main types of fish found in the modern world are the hagfish, sharks, rays, lampreys, ray-finned fish, coelacanths and lungfish. A fish possesses streamlined body to swim rapidly in the aquatic environment, has gills or accessory respiratory organs for getting oxygen and have paired as well as unpaired fins. The paired fins include the pectoral and the pelvic fins. The unpaired fins are the dorsal, anal and the caudal fins. Generally the body of the fish is covered with scales but there are some fish also which lack scales and are scale less. The jaws are very well developed in the fish. They are oviparous.
Generally fish are cold blooded vertebrates but exceptions always exist. Tuna, swordfish and some species of sharks have adaptations towards the warm blooded category. The streamlining and the swimming efficiency of the fish also varies for example tuna, salmon and jacks can swim 10-20 body lengths per second while the eels and sharks cannot move more than 0.5 body lengths per second. Some species of fish have the capability of extracting oxygen directly from the air as well as through other substances as they bear some specialized structures for this purpose for example lungfish have paired lungs, gouramis have labyrinth and catfish extract oxygen through the intestine or the stomach. The shape of the body as well as the arrangement of the fins is also variable. The scales are of different types like the placoid, cosmoid, ganoid, cycloid and ctenoid. There are certain fish also which live on land like the mudskippers which live on the mudflats and on being disturbed hide themselves in their underground burrows.
The body size of the fish varies from as small as the stout infant fish which is only 8mm long to as huge as the white shark which is 16meter long. Many animals are often confused with fish as the term fish is associated with them like the starfish, jellyfish, cuttlefish, shellfish. Strictly speaking they are not fish. There are at present 28,000 extant fish of which 27,000 are the bony fish, 970 are the sharks, rays and chimeras and 108 are the hagfish.
About 64 families of fish are monotypic as they contain only one species. The overall total of the fish comes to about 32,500 at present.
fish respire through gills present on either side of the pharynx. The gills are made up of thread-like structure known as filaments. Each filament is supplied by the capillary network which increases the surface area for the easy exchange of oxygen and carbon dioxide. fish draw the oxygen rich water through their mouth and pump it over the gills. In some fish the capillary blood flows in opposite direction through counter current system. The gills pass the water poor in oxygen to the gill openings present on the sides of the pharynx. Sharks and lampreys have multiple gill openings while some fish have single gill opening. The gill openings are covered by a protective covering called as operculum.
The breathing mechanism is also variable in fish. The skin of anguillid eels has the ability to absorb oxygen. The buccal cavity of the electric eel has the potential to absorb oxygen. Some fish like the perches, cichlids are also able to breathe air directly but majority of the fish are dependent on the oxygen dissolved in water. Some air breathing fish also hide themselves under the moist burrows and show a temporary hibernation mechanism and when plenty of water is available they show aestivation by returning to water. fish may be obligate or facultative air breathers. The African lungfish comes under the category of obligate air breathers as it comes to the surface of water for gulping out air otherwise it will die due to suffocation. Facultative air breathers comprise the heavy section of the fish as most of the fish breathe oxygen dissolved in water as they conserve energy by not coming to the surface of water to gulp air. The catfish in absence of dissolved oxygen can rely on oxygen dissolved in gills.