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Fishing is the activity of trying to catch fish. Fish are normally caught in the wild. Techniques for catching fish include hand gathering, spearing, netting, angling.
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For a fuller treatment of this taxonomy, see the vertebrate article. The position of hagfish in the phylum Chordata is not settled. Phylogenetic research in and supported the idea that the hagfish and the lampreys form a natural group, the Cyclostomata , that is a sister group of the Gnathostomata. The various fish groups account for more than half of vertebrate species.

There are almost 28, known extant species, of which almost 27, are bony fish, with sharks, rays, and chimeras and about hagfish and lampreys. About 64 families are monotypic , containing only one species. The final total of extant species may grow to exceed 32, The term "fish" most precisely describes any non- tetrapod craniate i.

As paraphyletic groups are no longer recognised in modern systematic biology , the use of the term "fish" as a biological group must be avoided. Many types of aquatic animals commonly referred to as "fish" are not fish in the sense given above; examples include shellfish , cuttlefish , starfish , crayfish and jellyfish. In some contexts, especially in aquaculture , the true fish are referred to as finfish or fin fish to distinguish them from these other animals. A typical fish is ectothermic , has a streamlined body for rapid swimming, extracts oxygen from water using gills or uses an accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two rarely three dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with scales , and lays eggs.

Each criterion has exceptions.

Lungfish have paired lungs similar to those of tetrapods, gouramis have a structure called the labyrinth organ that performs a similar function, while many catfish, such as Corydoras extract oxygen via the intestine or stomach. Similarly, the surface of the skin may be naked as in moray eels , or covered with scales of a variety of different types usually defined as placoid typical of sharks and rays , cosmoid fossil lungfish and coelacanths , ganoid various fossil fish but also living gars and bichirs , cycloid , and ctenoid these last two are found on most bony fish.

Fish species diversity is roughly divided equally between marine oceanic and freshwater ecosystems.

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Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests , especially the Amazon , Congo , and Mekong basins. Most fish exchange gases using gills on either side of the pharynx. Gills consist of threadlike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills.

In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys , possess multiple gill openings. However, bony fish have a single gill opening on each side.

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This opening is hidden beneath a protective bony cover called an operculum. Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians. Fish from multiple groups can live out of the water for extended periods. Amphibious fish such as the mudskipper can live and move about on land for up to several days, [ dubious — discuss ] or live in stagnant or otherwise oxygen depleted water.

Many such fish can breathe air via a variety of mechanisms. The skin of anguillid eels may absorb oxygen directly. The buccal cavity of the electric eel may breathe air. Catfish of the families Loricariidae , Callichthyidae , and Scoloplacidae absorb air through their digestive tracts. Gar and bowfin have a vascularized swim bladder that functions in the same way. Loaches , trahiras , and many catfish breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin similar to frogs.

Good Fish Guide | Marine Conservation Society

A number of fish have evolved so-called accessory breathing organs that extract oxygen from the air. Labyrinth fish such as gouramis and bettas have a labyrinth organ above the gills that performs this function. A few other fish have structures resembling labyrinth organs in form and function, most notably snakeheads , pikeheads , and the Clariidae catfish family.

Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and cichlids , quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of aestivation summertime hibernation until water returns.

Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish , must breathe air periodically or they suffocate.


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Facultative air breathers, such as the catfish Hypostomus plecostomus , only breathe air if they need to and will otherwise rely on their gills for oxygen. 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 a closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body.

In most fish, the heart consists of four parts, including two chambers and an entrance and exit. The atrium serves as a one-way antechamber, sends blood to the third part, ventricle. The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part, bulbus arteriosus , a large tube, and then out of the heart. The bulbus arteriosus connects to the aorta , through which blood flows to the gills for oxygenation.

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Jaws allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the esophagus. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called pyloric caeca , which secrete digestive enzymes and absorb nutrients. Organs such as the liver and pancreas add enzymes and various chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption. As with many aquatic animals, most fish release their nitrogenous wastes as ammonia.

Some of the wastes diffuse through the gills. Blood wastes are filtered by the kidneys. Saltwater fish tend to lose water because of osmosis. Their kidneys return water to the body. The reverse happens in freshwater fish : they tend to gain water osmotically. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater.

The scales of fish originate from the mesoderm skin ; they may be similar in structure to teeth. Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. Fish brains are divided into several regions. At the front are the olfactory lobes , a pair of structures that receive and process signals from the nostrils via the two olfactory nerves. Behind the olfactory lobes is the two-lobed telencephalon , the structural equivalent to the cerebrum in higher vertebrates. In fish the telencephalon is concerned mostly with olfaction.

Connecting the forebrain to the midbrain is the diencephalon in the diagram, this structure is below the optic lobes and consequently not visible. The diencephalon performs functions associated with hormones and homeostasis. This structure detects light, maintains circadian rhythms, and controls color changes. The midbrain or mesencephalon contains the two optic lobes.

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These are very large in species that hunt by sight, such as rainbow trout and cichlids. The hindbrain or metencephalon is particularly involved in swimming and balance. The brain stem or myelencephalon is the brain's posterior. Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human's see vision in fishes. Many fish also have chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears, many fish may not hear very well.

Most fish have sensitive receptors that form the lateral line system , which detects gentle currents and vibrations, and senses the motion of nearby fish and prey. Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols.

Fish behavior in mazes reveals that they possess spatial memory and visual discrimination.

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Vision is an important sensory system for most species of fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rods and cones for scotopic and photopic vision , and most species have colour vision.

Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish , the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. Hearing is an important sensory system for most species of fish. Fish sense sound using their lateral lines and their ears.