Any cold-blooded aquatic vertebrate without legs, but typically possessing paired lateral fins as well as median fins. There is a 2-chambered heart, a series of respiratory gills present throughout life in the sides of the pharynx, and a body usually bearing scales and terminating in a caudal (tail) fin. As a subgroup of the Vertebrata, the fishes are sometimes referred to collectively as Pisces. The primitive jawless fishes, the hagfishes and lampreys, are the only living members of a formerly large group, the Agnatha, which have an abundant fossil record. Members of another major group, the Chondrichthyes (800 living species), containing the sharks, rays, and ratfishes, are characterized by a cartilaginous skeleton, and are commonly referred to as the cartilaginous fishes. By far the largest extant group are the bony fishes (Osteichthyes; 20 000 living species), exhibiting a rich diversity and found in all aquatic habitats - freshwater, estuarine, and marine, from the tropics to polar latitudes, and from high altitude streams to the ocean abyss. Although many fish have the familiar elongate shape, body form shows great variety - strongly compressed from side to side or (in bottom-living species) from top to bottom, asymmetrical with both eyes on the same side (as in the true flatfishes), extremely slender or heavily robust, armoured, spinose, or with reduced tails and huge heads (as in some of the bizarre deep-sea species). Body length ranges from as small as a few centimetres to over 18 m/60 ft in the massive whale shark. Many species have bright coloration, others well-developed camouflage patterns. Light organs are common in those forms living in the darkness of deep oceanic waters. Several species that inhabit turbid shallow waters have well-developed electric organs used for navigation, as defence against predators or for stunning prey. Some fish that live in shallow waters low in oxygen have evolved a capacity for air-breathing, using lungs or other accessory respiratory organs. A few, such as the mudhopper, are well-adapted to living out of water on coastal mud flats. Fish are of immense importance as a source of food, and to the angler and aquarist in pursuit of leisure.
There are over 29,000 species of fish, making them the most diverse group of vertebrates. a common division is into the jawless fish (class Agnatha, 75 species including lampreys and hagfish), the cartilaginous fish (class Chondrichthyes, 800 species including sharks and rays), with the remainder classed as bony fish (class Osteichthyes).Fish come in different sizes, from the 16 m (51 ft) whale shark to a 8 mm (just over ¼ of an inch) long stout infantfish. Many types of aquatic animals named "fish" are not true fish, and in the case of animals such as jellyfish and cuttlefish, are not even vertebrates.
Although most fish are exclusively aquatic and cold-blooded, there are exceptions to both cases. Fish from a number of different groups have evolved the capacity to live out of the water for extended periods of time. Also, certain species of fish maintain elevated body temperatures to varying degrees. Endothermic teleosts (bony fishes) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus).
Fish are an important source of food in many cultures. Other water-dwelling animals such as mollusks, crustaceans, and shellfish are often called "fish" when used as food.
Classification
Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods, which are not fish.
Fish are classified into the following major groups:
Hyperoartia Petromyzontidae (lampreys) Pteraspidomorphi (early jawless fish) Thelodonti Anaspida Cephalaspidomorphi (early jawless fish) Galeaspida Pituriaspida Osteostraci Gnathostomata (jawed vertebrates) Placodermi Chondrichthyes (cartilaginous fish) Acanthodii Osteichthyes (bony fish) Actinopterygii (ray-finned fish) Sarcopterygii (lobe-finned fish) Actinistia (coelacanths) Dipnoi (lungfish)Some palaeontologists consider that Conodonta are chordates, and so regard them as primitive fish.
Fish anatomy
Digestive system
The advent of jaws allowed fish eat a much wider variety of food, including plants and other organisms. When it enters the stomach, the food is further broken down and, in many fish, further processed in fingerlike pouches called pyloric ceca.
Respiratory system
Most fish exchange gases by using gills that are located on either side of the pharynx. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gill filaments. Some fishes, like sharks and lampreys, possess multiple gill openings. However, most fishes have a single gill opening on each side of the body. Some fishes, such as lungfish, have developed an adaptation known as a labyrinth that allows them to survive in oxygen-poor areas or places where bodies of water constantly dry up. These species of fish possess specialized organs that serve as lungs.
Circulatory system
Fish have a closed circulatory system with a heart that pumps the blood in a single loop throughout the body. The sinus venosus is a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the atrium, which is a large muscular chamber. At the front end, the bulbus arteriosus connects to a large blood vessel called the aorta, through which blood flows to the fish's gills.
Excretory system
As with many aquatic animals, most fishes release their nitrogenous wastes as ammonia. Kidneys help fishes control the amount of ammonia in their bodies. Saltwater fish tend to lose water because of osmosis. In saltwater fish, the kidneys concentrate wastes and return as much water as possible back to the body. The kidneys of freshwater fish are specially adapted to pump out large amounts of dilute urine. Some fish have specially adapted kidneys that change their function, allowing them to move from freshwater to saltwater.
Sensory and nervous system
Fish have well-developed nervous systems that organize around a central brain, that is divided into different parts. Unlike most vertebrates, the cerebrum of the fish primarily processes the sense of smell rather than being responsible for all voluntary actions. Most fishes possess highly developed sense organs. Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. 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 catfish and sharks, have organs that detect low levels electric current.
Muscular system
Fish locomotion
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. The streamlined body of the fish decreases the amount of friction as they move through water. Since body tissue is more dense than water, fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a swim bladder that adjust their buoyancy through manipulation of gases.
Reproductive system
The eggs of fish are fertilized either externally or internally, depending on species. These kind of fish are called oviparous fish. Oviparous fish develop by obtaining food from the yolk in the egg.
Ovoviviparous fish keep the eggs inside of the mother's body after internal fertilization.
Some species of fish, such as various sharks, are viviparous. Viviparous fish allow their embryos to stay in the mother's body like ovoviviparous fish. However, the embryos of viviparous fish obtain needed substances from the mother's body, not through material in the egg.
Immune system
Types of immune organs vary between different types of fish. In the jawless fish (lampreys and hagfishes), true lymphoid organs are absent. Instead, these fish rely on regions of lymphoid tissue within other organs to produce their immune cells. Cartilaginous fish (sharks and rays) have a more advanced immune system than the jawless fish. Chondrostean fish (sturgeons, paddlefish and birchirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system) and their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), where many different immune cells are housed. In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. Recently, a lymphatic system similar to that described in mammals was described in one species of teleost fish, the zebrafish.
Evolution
The early fossil record on fish is not very clear.
The formation of the hinged jaw appears to be what resulted in the later proliferation of fish because un-jawed fish left very few ancestors.
Some speculate that fish may have evolved from a creature similar to a coral-like Sea squirt, whose larvae resemble primitive fish in some key ways.
Fish disease
Fish are susceptible to disease as any other organism.
Fish diseases can be refered to as etiology:
Bacterial Disorders Fungal Disorders Parasitic Disorders Viral Disorders Metabolic Disorders Water conditions Malnutritionor the organ system most affected
Neurological Disorders Body Cavity Eye Disorders Fecal Disorders Fin Disorders Gallbladder Gill Disorders Intestinal Disorders Kidney Disorders Liver Disorders Locomotor Disorders Skin Disorders & "fishes""Fishes" is the proper English plural form of "fish" that biologists use when speaking about two or more fish species, as in "There are over 25,000 fishes in the world" (meaning that there are over 25,000 fish species in the world). When speaking of two or more individual fish organisms, then the word "fish" is used, as in "There are several million fish of the species Gadus morhua" (meaning that G. To see both in action, consider the statement "There are twelve fish in this aquarium, representing five fishes" (meaning that the aquarium contains twelve individuals, some of the same species and some of different species, for a total of five species).
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