India is rich with inland freshwater fish, with about 940 species known from its rivers, lakes and estuaries. Of these about 500 species are primary freshwater fish with around 65% endemic, cloistered in the two hot spots of India, the Western Ghats and the North East.
Fishes are member of paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Most fishes are ectothermic "cold-blooded", allowing their body temperatures to vary as ambient temperatures change.
Fish is ectothermic, has streamlined body for rapid swimming, extracts oxygen from water using gills or uses accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with scales, and lays eggs.
Fish can be found in nearly all aquatic environments, from high mountain streams to the abyssal and even hadal depths of the deepest oceans.
Fish are an important resource for humans worldwide, especially as food.
They lacked true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts.
Unlike groupings such as birds or mammals, fish are not a single clade but paraphyletic collection of taxa including hagfishes, lampreys, sharks and rays, ray-finned fish, coelacanths, and lungfish.
Fish species diversity is divided equally between marine (oceanic) and freshwater ecosystems.
Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. The gills push the oxygen-poor water out through openings in the sides of the pharynx.
Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators.
Fish have closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body.
Jaws allow fish to eat wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the esophagus.
Fish release their nitrogenous wastes as ammonia. Some of the wastes diffuse through the gills
The scales of fish originate from the mesoderm (skin); they may be similar in structure to teeth.
Fish have quite small brains relative to body size compared with other vertebrates.
Most fish possess highly developed sense organs. All daylight fish have colour vision that is good as a human. Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish sense sound using their lateral lines and their ears.
Fish move by alternately contracting paired sets of muscles on either side of the backbone. The streamlined body of the fish decreases the amount of friction from the water.
Fish reproductive organs include testes and ovaries. Over 97% of all known fish are oviparous that is, the eggs develop outside the mother's body.
The newly hatched young of oviparous fish are called larvae. The larval period in oviparous fish is relatively short (only several weeks), and larvae rapidly grow and change appearance and structure to become juveniles.
Fish suffer from diseases and parasites. To prevent disease they have variety of defences.
Freshwater fish seem particularly threatened because they live in relatively small water bodies. Overfishing is major threat to edible fish such as cod and tuna.
Threats to these fauna are aplenty with urbanization, deforestation, habitat loss, pollution, over-harvesting, and culture of exotics.
The only way for a fish enthusiast to save the fish and its habitats is creating awareness among the people.
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