Fish are aquatic vertebrates that are typically cold-blooded; 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 (e.g., char and gudgeon) as well as in the deepest depths of the ocean (e.g., gulpers and anglerfish). They are of tremendous importance as food for people around the world, either collected from the wild (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture). Fish are also exploited for recreation, through angling and fishkeeping, and fish are commonly exhibited in public aquaria. Fish have an important role in many cultures through the ages, ranging as wide 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 but lacking four limbs (or having ancestors that had four limbs). Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa including hagfishes, lampreys, sharks, rays, lungfishes and coelacanths, sturgeons, gars, and advanced ray-finned fishes.[1]A typical fish is cold-blooded; has a streamlined body that allows it to swim rapidly; extracts oxygen from the water using gills; has two sets of paired fins, 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 that are fertilised externally. Fish come in many shapes and sizes. This is a sea dragon, a close relative of the seahorse. Their leaf-like appendages enable them to blend in with floating seaweedHowever, to each of these there are exceptions. Tuna and some species of sharks are warm-blooded, and able to raise their body temperature significantly above that of the ambient water surrounding them.[2] Streamlining and swimming performance varies from highly streamlined and rapid swimmers which are able to reach 10-20 body-lengths per second (such as tuna, salmon, and jacks) through to slow but more manoeuvrable species such as eels and rays that reach no more than 0.5 body-lengths per second.[3] Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. 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.[4] 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 (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 lungfishes and coelacanths), ganoid (various fossil fishes but also living gars and bichirs, cycloid, and ctenoid (these last two are found on most bony fish.[5] 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.[6] The catfish Phreatobius cisternarum lives in waterlogged leaf litter[7],[8]The various fish groups taken together account for more than half of the known vertebrates. There are at least 24,600 known species of fish, of which over 23,000 are bony fish, with the remainder being about 850 sharks, rays, and chimeras and about 85 hagfishes and lampreys.[9] They range in size 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 commonly referred to as "fish" are not fish in the sense given above. These include cuttlefish, jellyfish, inkfish, and starfish. Marine invertebrates that are consumed as food are commonly called shellfish. Whales and dolphins have been called fish as well, although they are mammals. This usage is no longer common in English. 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. Of these amphibious fish some such as the mudskipper can live and move about on land for up to several days. 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). All sharks in the family Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are known to have the capacity for endothermy, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of endothermy varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 °C above ambient water temperatures. See also gigantothermy. Endothermy, though metabolically costly, is thought to provide advantages such as increased contractile force of muscles, higher rates of central nervous system processing, and higher rates of digestion. Excretory system As with many aquatic animals, most fishes release their nitrogenous wastes as ammonia. Some of the wastes diffuse through the gills into the surrounding water. Others are removed by the kidneys, excretory organs that filter wastes from the blood. 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 reverse happens in freshwater fish, they tend to gain water continuously. 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. The most anterior, or front, end of the brain are the olfactory bulbs, which are involved in the fish's sense of smell. Unlike most vertebrates, the cerebrum of the fish primarily processes the sense of smell rather than being responsible for all voluntary actions. The optic lobes process information from the eyes. The cerebellum coordinates body movements while the medulla oblongata controls the functions of internal organs. Most fishes possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least 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. In 2003, it was found by Scottish scientists at Edinburgh University performing research on rainbow trout that fish exhibit behaviors often associated with pain. Some fishes, such as catfish and sharks, have organs that detect low levels electric current. Other fishes, like the electric eel, can produce their own electricity. Muscular system Fish locomotion Main article: 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. 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. 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. |