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Fish

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

   A giant grouper at the Georgia Aquarium
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   A giant grouper at the Georgia Aquarium

   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 freshwaters, 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, being either collected from the wild (see fishing) or
   being 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.
   Through the ages, many cultures have featured fish in their legends and
   myths, from the "great fish" that swallowed Jonah the Prophet through
   to the ever-popular half-human, half-fish mermaid around which books
   and movies are still centred (e.g., Splash (film)). Fish have been used
   as symbols in many different ways, from the ichthys used by early
   Christians through to the allegorical use of a marlin in Ernest
   Hemingway's novel, The Old Man and the Sea.

What is a fish?

   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.

   A typical fish is cold-blooded; has a streamlined body that allows them
   to swim rapidly; extracts oxygen from the water using gills; two sets
   of paired fins, one or two dorsal fins, an anal fin, and a tail fin;
   has jaws; has skin that is covered with scales; and lays eggs that are
   fertilised externally. However, 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. Streamling and swimming performance varies from
   highly streamlined and rapid swimmers 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. 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. 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. 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. The catfish Phreatobius cisternarum lives in
   waterlogged leaf litter ,

   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. 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 named "fish" are not in fact fish at all.
   These include cuttlefish, jellyfish, inkfish, and starfish. Marine
   invertebrates that are consumed as food are commonly called shellfish.
   In the past whales and dolphins were sometimes considered fish as well,
   although they are actually mammals.

Classification

   Fish are a paraphyletic group: that is, any clade containing all fish
   also contains the tetrapods, which are not fish. For this reason,
   groups such as the "Class Pisces" seen in older reference works are no
   longer used in formal classifications.

   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)
               o Actinopterygii (ray-finned fish)
               o Sarcopterygii (lobe-finned fish)
                    # Actinistia ( coelacanths)
                    # Dipnoi ( lungfish)

   Some palaeontologists consider that Conodonta are chordates, and so
   regard them as primitive fish.

   For a fuller treatment of classification, see the vertebrate article.

Fish anatomy

Digestive system

   The advent of jaws allowed fish eat a much wider variety of food,
   including plants and other organisms. 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 and, in many fish, further
   processed in fingerlike pouches called pyloric ceca. The pyloric ceca
   secrete digestive enzymes and absorb 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.

Respiratory system

   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. Fish
   exchange gases by pulling oxygen-rich water through their mouths and
   pumping it over their gill filaments. The blood in the capillaries
   flows in the opposite direction to the water, causing counter current
   exchange. They then push the oxygen-poor water out through openings in
   the sides of the pharynx. Some fishes, like sharks and lampreys,
   possess multiple gill openings. However, most fishes have a single gill
   opening on each side of the body. This opening is hidden beneath a
   protective bony cover called an operculum. 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. A tube brings air containing oxygen to this organ
   by way of the fish's mouth. Some kinds of lungfish are so dependent on
   receiving oxygen from the air that they will suffocate if not allowed
   to reach the surface of the water.

Circulatory system

   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 fishes, 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. 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. The atrium serves as a one-way compartment for
   blood to flow into the ventricle. The ventricle is a thick-walled,
   muscular chamber and it does the actual pumping for the heart. It pumps
   blood to a large tube called the bulbus arteriosus. 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.

Homeothermy

   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 colour 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 also found by
   Scottish scientists at Edinburgh University performing research on
   rainbow trout that fish experience 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

   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.

Reproductive system

   The eggs of fish are fertilized either externally or internally,
   depending on species. The female usually lays the eggs, and the embryos
   in the eggs develop and hatch outside her body. These kind of fish are
   called oviparous fish. Oviparous fish develop by obtaining food from
   the yolk in the egg. Salmon, for example, are oviparous.

   Ovoviviparous fish keep the eggs inside of the mother's body after
   internal fertilization. Each embryo develops in its own egg. The young
   are "born alive" like most mammals.

   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. The young of viviparous species are also "born alive".

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. For example, erythrocytes,
   macrophages and plasma cells are produced in the anterior kidney (or
   pronephros) and some areas of the gut (where granulocytes mature)
   resemble primitive bone marrow in hagfish. Cartilaginous fish (sharks
   and rays) have a more advanced immune system than the jawless fish.
   They have three specialized organs that are unique to chondrichthyes;
   the epigonal organs (lymphoid tissue similar to bone marrow of mammals)
   that surround the gonads, the Leydig’s organ within the walls of their
   esophagus, and a spiral valve in their intestine. All these organs
   house typical immune cells (granulocytes, lymphocytes and plasma
   cells). They also possess an identifiable thymus and a well-developed
   spleen (their most important immune organ) where various lymphocytes,
   plasma cells and macrophages develop and are stored. 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. The chondrostean
   kidney is an important hemopoietic organ; where erythrocytes,
   granulocytes, lymphocytes and macrophages develop. 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. in the skin,
   gills, gut and gonads). Much like the mammalian immune system, teleost
   erythrocytes, neutrophils and granulocytes are believed to reside in
   the spleen whereas lymphocytes are the major cell type found in the
   thymus. Recently, a lymphatic system similar to that described in
   mammals was described in one species of teleost fish, the zebrafish.
   Although not confirmed as yet, this system presumably will be where
   naive (unstimulated) T cells will accumulate while waiting to encounter
   an antigen.

Evolution

   The early fossil record on fish is not very clear. It appears it was
   not a successful enough animal early in its evolution to leave many
   fossils. However, this would eventually change over time as it became a
   dominant form of sea life and eventually branching to include land
   vertebrates such as amphibians, reptiles, and mammals.

   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. Lampreys may be a rough representative of pre-jawed fish.
   The first jaws are found in Placodermi fossils. It is unclear if the
   advantage of a hinged jaw is greater biting force, respiratory-related,
   or a combination.

   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. The first ancestors of fish may have kept the larval form into
   adulthood (as some sea squirts do today, see Neoteny), although the
   reversal of this case is also possible. Candidates for early fish
   include Agnatha such as Haikouichthys, Myllokunmingia, and Pikaia.

Fish disease

   Fish are susceptible to disease as any other organism.

   Fish diseases can be referred to as etiology:
     * Bacterial Disorders
     * Fungal Disorders
     * Parasitic Disorders
     * Viral Disorders
     * Metabolic Disorders
     * Water conditions
     * Malnutrition

   or 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 & Changes In Colour
     * Swim Bladder

Note on usage: "fish" vs. "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. morhua
   comprises several million individuals). 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). The usage of the two words is similar to that of the
   words "people" and "peoples". The collective noun for fish is shoal (or
   school).
   Retrieved from " http://en.wikipedia.org/wiki/Fish"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
