Kingdom: Animalia... Phylum: Chordata... Class: Vertebrata... Superclasses: see text ...

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Fish are aquatic vertebrates that are typically ectothermic aka cold-blooded. They are covered with scales, and equipped with two sets of paired fins and several unpaired fins. Fish are abundant in the sea and in fresh water, with species being known from mountain streams as well as from the deepest depths of the ocean. They are of tremendous importance as food for people around the world, either collected from the wild or farmed in aquaculture, much the same way as chickens are. Fish are also exploited for recreation and are commonly exhibited in public aquariums. Fish have an important role in many cultures through the ages, ranging as widely as deities and religious symbols to subjects of books and popular movies.
The term "fish" is most precisely used to describe any non-tetrapod chordate, i.e., an animal with a backbone that has gills throughout life and has limbs, if any, in the shape of fins. Fish are not a single clade but a paraphyletic collection of taxa like…lampreys, sharks and rays. Fish is used either as singular noun or to describe a group of specimens from a single species. Fishes describes a group containing more than one species.



Legends of half-human, half-fish mermaids have featured in stories. A few deities were said to take the form of a fish. The astrological symbol Pisces is based on a constellation of the same name. Large fish, particularly sharks, have frequently been the subject of horror movies. The golden fish for many buddhists symbolises the auspiciousness of all living beings in a state of fearlessness.

A random assemblage of fishes merely using some localised resource such as food or nesting sites is known simply as an aggregation. When fish come together in an interactive, social grouping, then they may be forming either a shoal or a school depending on the degree of organisation. A shoal is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. Schools of fish are much more tightly organised, synchronising their swimming so that all fish move at the same speed and in the same direction.

Fish are classified into the following major groups:

  • Subclass Pteraspidomorphi… Class: Thelodonti… Class: Anaspida
  • Cephalaspidomorphi …Hyperoartia…Petromyzontidae…  Class: Galeaspida…Class: Pituriaspida…Class: Osteostraci
  • Infraphylum… Gnathostomata… Class: Placodermi… Class: Chondrichthyes … Class: Acanthodii … Superclass: Osteichthyes … Class: Actinopterygii… Class: Sarcopterygii

The various fish groups taken together account for more than half of the known vertebrates. There are almost 28,000 known extant species of fish. It is predicted that the eventual number of total extant species will be near 32,500 if ever completed.



There are even fishes that spend most of their time out of water. Mudskippers feed and interact with one another on mudflats and are only underwater when hiding in their burrows. Fish range in size from the 16 m for the whale shark, to 8 mm for the long stout infantfish.
Some species show some warm-blooded adaptations, and are able to raise their body temperature significantly above that of the ambient water surrounding them. Streamlining and swimming performance varies from highly. Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as seahorses, pufferfish, anglerfish, and gulpers. Similarly, the surface of the skin may be naked or covered with scales of a variety of different types usually defined as placoid, cosmoid, ganoid and ctenoid.
Once in fish, food is ingested through the mouth and then broken down in the esophagus. When it enters the stomach, the food is further broken down. Then it`s the secretion of digestive enzymes and absorbption nutrients from the digested food. Organs such as the liver and pancreas add enzymes and various digestive chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption

Most fish exchange gases by using gills that are located on either side of the pharynx. Gills are made up of threadlike structures called filaments. Each filament contains a network of capillaries that allow a large surface area for the exchange of oxygen and carbon dioxide. This gill`s opening is hidden beneath a protective bony cover called an operculum.

Many fish can breathe air. Being able to breathe air is primarily of use to fish that inhabit shallow, seasonally variable waters where the oxygen concentration in the water may decline at certain times of the year. At such times, fishes dependent solely on the oxygen in the water, such as perch and cichlids, will quickly suffocate, but air-breathing fish can survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some of these air-breathing fish are able to survive in damp burrows for weeks after the water has otherwise completely dried up, until the water returns.

Fish have a closed circulatory system with a heart that pumps the blood in a single loop throughout the body. The blood goes from the heart to gills, from the gills to the rest of the body, and then back to the heart. In most fish, the heart consists of four parts: the sinus venosus, the atrium, the ventricle, and the bulbus arteriosus. Despite consisting of four parts, the fish heart is still a two-chambered heart.
Fish typically have quite small brains relative to body size when compared with other vertebrates, typically one-fifteenth the mass of the brain from a similarly sized bird or mammal. However, some fishes have relatively large brains, most notably sharks, which have brains of about as massive relative to body weight as birds do.

Most fish possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least as good as a human's. Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears in their heads, many fish may not hear sounds very well. However, most fishes have sensitive receptors that form the lateral line system. The lateral line system allows for many fish to detect gentle currents and vibrations, as well as to sense the motion of other nearby fish and prey. Some fishes such as catfishes and sharks, have organs that detect low levels electric current.

Most fish move by contracting paired sets of muscles on either side of the backbone alternately. These contractions form S-shaped curves that move down the body of the fish. As each curve reaches the back fin, backward force is created. This backward force, in conjunction with the fins, moves the fish forward. The fish's fins are used like an airplane's stabilizers. Fins also increase the surface area of the tail, allowing for an extra boost in speed. The streamlined body of the fish decreases the amount of friction as they move through water. Many bony fishes have an internal organ called a swim bladder that adjust their buoyancy through manipulation of gases.

Over 97% of all known fishes are oviparous that means, the eggs develop outside the mother's body. Examples of oviparous fishes include salmon, goldfish, eels. In the majority of these species, fertilisation takes place outside the mother's body. However, a few oviparous fishes practise internal fertilisation, with the male using some sort of intromittent organ to deliver sperm into the genital opening of the female. The newly-hatched young of oviparous fish are called larvae. They are usually poorly formed, carry a large yolk sac and are very different in appearance to juvenile and adult specimens of their adult species. The larval period in oviparous fish is relatively short and larvae rapidly grow and change appearance and structure through a metamorphosis. 

Some species of fish are viviparous. In such species the mother retains the eggs, as in ovoviviparous fishes, but the embryos receive nutrition from the mother in a variety of different ways. Typically, viviparous fishes have a structure analogous to the placenta seen in mammals connecting the mother's blood supply with the that of the embryo.

As of 2006, 1,173 species of fish were being threatened with extinction. In the case of edible fishes such as cod and tuna, the major threat is overfishing.
A key stress on both freshwater and marine ecosystems is habitat degradation including water pollution, the building of dams, removal of water for use by humans. 



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English: Fish
Fish range in size from 8 mm to 16 m

Español : Pescado
Uno Pescado puedes mesurar de 8mm a 16 m

Français: Poisson
Un poisson peut mesurer de 8 mm à 16 m

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