Fish

Fish are gill-bearing aquatic craniate animals that lack limbs with digits. They form a sister group to the tunicates, together forming the olfactores. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered paraphyletic by excluding the tetrapods (i.e., the amphibians, reptiles, birds and mammals
 which all descended from within the same ancestry). Because in this
manner the term "fish" is defined negatively as a paraphyletic group, it
 is not considered a formal taxonomic grouping in systematic biology, unless it is used in the cladistic sense, including tetrapods. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.
The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.
Most fish are ectothermic
 ("cold-blooded"), allowing their body temperatures to vary as ambient
temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.
Fish
 can communicate in their underwater environments through the use of
acoustic communication. Acoustic communication in fish involves the
transmission of acoustic signals from one individual of a species…

Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event.
Early fish from the fossil record are represented by a group of small, jawless, armored fish known as ostracoderms. Jawless fish lineages are mostly extinct. An extant clade, the lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils. The diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.
Fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although perhaps the reverse is the case.
Taxonomy
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.
Traditional classification divides fish into three extant classes, and with extinct forms sometimes classified within the tree, sometimes as their own classes:
• Class Agnatha (jawless fish)
• Class Chondrichthyes (cartilaginous fish)
• Class Placodermi (armoured fish) †
• Class Acanthodii ("spiny sharks", sometimes classified under bony fishes)†

• Agnatha (Pacific hagfish)
• Chondrichthyes (Horn shark)
• Actinopterygii (Brown trout)
• Sarcopterygii (Coelacanth)
The term "fish" most precisely describes any non-tetrapod craniate (i.e. an animal with a skull and in most cases a backbone) that has gills throughout life and whose limbs, if any, are in the shape of fins. Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa, including hagfishes, lampreys, sharks and rays, ray-finned fish, coelacanths, and lungfish. Indeed, lungfish and coelacanths are closer relatives of tetrapods (such as mammals, birds, amphibians, etc.) than of other fish such as ray-finned fish or sharks, so the last common ancestor of all fish is also an ancestor to tetrapods. 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 earlier times, even biologists did not make a distinction – sixteenth century natural historians classified also seals, whales, amphibians, crocodiles, even hippopotamuses, as well as a host of aquatic invertebrates, as fish. However, according to the definition above, all mammals, including cetaceans like whales and dolphins, are not fish. 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 streamlinedbody for rapid swimming, extracts oxygen from water using…

Respiration
Gills
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. 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.
Air breathing
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, 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. Lungfish, with the exception of the Australian lungfish, and bichirs have paired lungs similar to those of tetrapods and must surface to gulp fresh air through the mouth and pass spent air out through the gills. Gar and bowfin have a vascularized swim bladder that functions in the same way. Loaches, trahiras, and many catfishbreathe.

Acoustic communication in fish involves the transmission of acoustic
signals from one individual of a species to another. The production of
sounds as a means of communication among fish is most often used in the
context of feeding, aggression or courtship behaviour. The sounds
emitted can vary depending on the species and stimulus involved. Fish
can produce either stridulatory sounds by moving components of the
skeletal system, or can produce non-stridulatory sounds by manipulating
specialized organs such as the swimbladder.
Stridulatory sound producing mechanisms
There
 are some species of fish that can produce sounds by rubbing or grinding
 their bones together. These noises produced by bone-on-bone
interactions are known as 'stridulatory sounds'.
An example of this is seen in Haemulon flavolineatum,
 a species commonly referred to as the 'French grunt fish', as it
produces a grunting noise by grinding its teeth together. This
behaviour is most pronounced when the H. flavolineatum
 is in distress situations. The grunts produced by this species of
fishes generate a frequency of approximately 700 Hz, and last
approximately 47 milliseconds. The H. flavolineatum
 does not emit sounds with frequencies greater than 1000 Hz, and does
not detect sounds that have frequencies greater than 1050 Hz.
In a study conducted by Oliveira et al. (2014), the longsnout seahorse, Hippocampus reidi, was recorded producing two different categories of sounds; ‘clicks’ and ‘growls’. The sounds emitted by the H. reidiare
 accomplished by rubbing their coronet bone across the grooved section
of their neurocranium. ‘Clicking’ sounds were found to be primarily
produced during courtship and feeding, and the frequencies of clicks
were within the range of 50 Hz-800 Hz. The frequencies were noted to be
 on the higher end of the range…

Like other animals, fish suffer from diseases and parasites. To prevent
disease they have a variety of defenses. Non-specific defenses include
the skin and scales, as well as the mucus layer secreted by the epidermis that traps and inhibits the growth of microorganisms. If pathogens breach these defenses, fish can develop an inflammatory response that increases blood flow to the infected region and delivers white blood cells that attempt to destroy pathogens. Specific defenses respond to particular pathogens recognised by the fish's body, i.e., an immune response. In recent years, vaccines have become widely used in aquaculture and also with ornamental fish, for example furunculosis vaccines in farmed salmon and koi herpes virus in koi.
Some species use cleaner fish to remove external parasites. The best known of these are the Bluestreak cleaner wrasses of the genus Labroides found on coral reefs in the Indian and Pacific
 oceans. These small fish maintain so-called "cleaning stations" where
other fish congregate and perform specific movements to attract the
attention of the cleaners. Cleaning behaviors have been observed in a
number of fish groups, including an interesting case between two
cichlids of the same genus, Etroplus maculatus, the cleaner, and the much larger Etroplus suratensis.
Immune system
Immune organs vary by type of fish. In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. These fish rely on regions of lymphoid tissue within other organs to produce 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.) They resemble primitive bone marrow in hagfish. Cartilaginous fish(sharks and rays) have a more advanced immune system. They have three specialize…

The 2006 IUCN Red List names 1,173 fish species that are threatened with extinction. Included are species such as Atlantic cod, Devil's Hole pupfish, coelacanths, and great white sharks.
 Because fish live underwater they are more difficult to study than
terrestrial animals and plants, and information about fish populations
is often lacking. However, freshwater fish seem particularly threatened
because they often live in relatively small water bodies. For example,
the Devil's Hole pupfish occupies only a single 3 by 6 metres (10 by 20 ft) pool.
Overfishing
Overfishing is a major threat to edible fish such as cod and tuna. Overfishing eventually causes population (known as stock)
 collapse because the survivors cannot produce enough young to replace
those removed. Such commercial extinction does not mean that the species
 is extinct, merely that it can no longer sustain a fishery.
One well-studied example of fishery collapse is the Pacific sardine
 Sadinops sagax caerulues fishery off the California coast. From a 1937
peak of 790,000 long tons (800,000 t) the catch steadily declined to
only 24,000 long tons (24,000 t) in 1968, after which the fishery was no
 longer economically viable.
The main tension between fisheries science and the fishing industry
 is that the two groups have different views on the resiliency of
fisheries to intensive fishing. In places such as Scotland,
Newfoundland, and Alaska the fishing industry
 is a major employer, so governments are predisposed to support it. On
 the other hand, scientists and conservationists push for stringent
protection, warning that many stocks could be wiped out within fifty
years.
Habitat destruction

Economic importance
Throughout history, humans have utilized fish as a food source. Historically and today, most fish protein
 has come by means of catching wild fish. However, aquaculture, or fish
farming, which has been practiced since about 3,500 BCE. in China, is
becoming increasingly important in many nations. Overall, about
one-sixth of the world's protein is estimated to be provided by fish.
That proportion is considerably elevated in some developing nations and regions heavily dependent on the sea. In a similar manner, fish have been tied to trade.
Catching fish for the purpose of food or sport is known as fishing, while the organized effort by humans to catch fish is called a fishery.
 Fisheries are a huge global business and provide income for millions of
 people. The annual yield from all fisheries worldwide is about 154
million tons, with popular species including herring, cod, anchovy, tuna, flounder, and salmon. However, the term fishery is broadly applied, and includes more organisms than just fish, such as mollusks and crustaceans, which are often called "fish" when used as food.
Recreation
Fish have been recognized as a source of beauty for almost as long as used for food, appearing in cave art, being raised as ornamental fish in ponds, and displayed in aquariums in homes, offices, or public settings.
Recreational fishing is fishing for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit. The most common form of recreational fishing is done with a rod, reel, line, hooks and any one of a wide range of baits. Angling is a method of fishing, specifically the practice of catching …