fish are vertebrate animals that live in almost every part of the ocean, from the surface to the bottom of the deepest trenches, and even at the very edge of the sea. By 2004 over 20,000 species, including the 200 or so edible species, had been described, Since about 100 new species are being described each year, the total number of fishes may exceed 30,000 species.

About 60% of all vertebrate animals (animals with backbones) are fishes. About half the described species are marine, and approximately 75% of them live in shallow coastal waters. The types range from hagfish (Myxine spp.) to lungfish (Dipnoi), but here the discussion will centre on the bony fishes, or teleosts, which are by far the most diverse and species-rich group of fishes. They are distinct from the cartilaginous fishes, i.e. sharks and their relatives, by having bony rather than cartilaginous skeletons, gills covered with a flap, and mouths that are usually on the front of the head. The mouths of some fishes are armed with teeth that are used to rasp soft tissue, grind up molluscs, or scrape algae off rocks. Others, such as the seahorses, have no teeth and have tubular-shaped mouths adapted to suck up individual plankton.

The rich diversity of fishes is partly the result of their having adopted a great variety of ways of feeding. Their basic body form is spindle shaped, with dorsal fins on the back, two pairs of lateral fins, the pectorals and the pelvics, a ventral anal fin, and a large tail, or caudal, fin that is usually symmetrical. The fins are composed of fine bones, or fin rays that normally are webbed. These fin rays can be developed into long sensory structures or hard spiny structures that may be armed with poison glands. In remoras (family Echeneidae) the dorsal fin has been modified into a sucker. The spindle shape gives a good hydrodynamic shape that slides through the water with minimum resistance when the posterior region of the body, and the tail, beats from side to side. However, this basic body shape has been greatly modified in different families of fish, especially in those families that live around the seabed. Eels have lost most of their fins and developed long sinuous bodies and a serpentine mode of movement, ideal for moving in and out of crannies in, reefs, but less effective for swimming in midwater. Flatfishes (order Pleuronectiformes) have flattened bodies, with either the left side or the right side becoming the lower surface, and during development the head rotates 90°, whereas scorpion fishes (order Scorpaeniformes) have bodies that are flattened dorsoventrally. Boxfishes (family Ostraciidae) have inflexible armoured bodies, and so their mode of swimming is by sculling with their fins. Mudskippers (Periophthalmus spp.) that inhabit the fringes of mangrove swamps use their pectoral fins to climb up out of the water.

The coloration of fishes is almost as diverse is their habits. Some, like flatfishes, can change their colour at will, either to blend in with different backgrounds or to flash warnings to would-be predators. In many shallow coral reef species, males display bright colours to guard their territories, but generally fishes use their coloration as camouflage. The commonest colour pattern in the fishes that swim in midwater is a counter-shading, with dark backs and pale bellies and flanks that may be banded with a disruptive pattern, often silvery or, in deep-sea species, lined with light organs. The brightness of light changes with depth, and its colour also changes—red light being absorbed very quickly—so the range of colours used by fish is restricted. Also, fish see only monochromatic blue-green light, which is the colour of light that penetrates furthest in water. In very deep water many fish species do not have functional eyes, since that there is almost nothing to see in the permanent darkness below about 1,000 metres (3,250 ft).

Another important sensory feature of fishes is their lateral line system. This is a chain of sense organs, similar to those in our ears, that can either be open to the water or semi-enclosed. With this system the fishes feel the water, sensing currents and the low-frequency vibrations transmitted by the movements of other animals. Many deep-sea fishes have long filamentous tails that, by extending the length of the lateral line organ, enable them to feel the direction from which any movements are coming.

Fishes' blood is about half as salty as sea water—which is why thirsty shipwrecked mariners can safely drink it to slake their thirst—so fish continually have to get rid of salt from their bodies across their gills. Although this helps to make fishes less dense, many still have systems for adjusting the density of their bodies to be much the same as the sea water, so when they stop swimming they do not sink. Above the gut of many fishes is a swim-bladder that is filled with gas. In some fishes this is filled by gulping in air at the surface, but the vast majority never approach the sea surface, so the swim-bladder is filled by a special gland that extracts dissolved gases from the blood. Some fishes that swim continuously, like tunas and marlin, have no swim-bladder. Instead, they have rigid pectoral fins that act like hydroplanes to generate lift as they swim. Swim-bladders do not occur in many deep-sea fishes, because the greater the hydrostatic pressure (i.e. depth) the greater the energy required to fill them, so at depths below about 500 metres (1,625 ft) it becomes physiologically too expensive. In these fishes either the swim-bladder is filled with oily fats, or it disappears. Deep-sea fishes tend to have very watery tissues, and their bones contain very little calcium, which reduces their density. However, some still retain gas-filled swim-bladders in very deep water which are connected by bones to sensory organs, and function as hearing organs. They also often have drumming muscles attached to them for the fish to produce sounds for communication. During the Cold War when hydrophones were deployed in deep water to listen for the movements of submarines, it was found that the deep ocean is quite noisy, especially during the breeding season for deep-sea fishes and when whales migrate.

See also fisheries.

Bibliography

Bone, Q.,, Marshal, N. B.,, and and Blaxter, J. H. S. , The Biology of Fishes (1995).www.fishbase.org/home.htm

M. V. Angel

Fish

At least 20,000 species of fishes live in the world's lakes, streams, estuaries , and oceans. Adult fish may be very small, such as the guppies that many people keep in their home aquariums, or extremely large, such as whale sharks that can weigh over 15 metric tons and reach 12 meters (about 40 feet).

The study of fishes is called ichthyology, and scientists who study fish anatomy, physiology, and taxonomy are known as ichthyologists. Fisheries scientists and fishery biologists are scientists who are interested in the population structure, reproduction, growth, and behavior of fishes that are economically important: that is, those species associated with recreational and commercial fisheries.

Overview of Characteristics

Fish are biologically and behaviorally well suited for their specific habitat. Body shape, feeding adaptations, and swimming behavior are examples of the characteristics unique to species.

Salinity and Habitat.

While one species of fish may be restricted to fresh water (e.g., largemouth bass), another may be found only in the open ocean (e.g., halibut). Such species are called stenohaline, meaning that they cannot tolerate much change in salinity . Many fishes, on the other hand, are found in estuaries where salinity is constantly changing. Examples of these euryhaline species are summer flounder, red drum, and striped bass.

Fish that reproduce in fresh water and have offspring that migrate to the ocean where they spend their adult lives are called anadromous (e.g., Atlantic and Pacific salmon). In contrast, fish that reproduce in the ocean and whose young migrate to fresh water to grow into adulthood are known as catadromous (e.g., American and European eels). Species that do not move into and out of fresh water at certain stages of their lives vary greatly with respect to their movements. Some fish (e.g., moray eels) develop very small home ranges, whereas others (e.g., tuna) may cover thousands of kilometers as they wander the world's oceans.

Physical Characteristics.

Fish may have skeletons made of bone, as found in most species, or cartilage, as found in sharks, skates, and rays. While the majority of fishes have scales—sometimes very tiny ones—there are some that do not have any scales (e.g., catfish). Scales may give the fish a silvery appearance, or they may contain pigment that helps the fish blend in with its environment.

All fishes have mouths, which may or may not contain well-developed teeth. All fishes also have intestines of some form, but not all species have stomachs. The majority are carnivores or omnivores , although a few species are herbivores . For example, grass carp eat aquatic plants, and some reef fish scrape up algae as food.

Body shape is highly variable, though the majority of fishes are essentially torpedo-shaped. There are many variations, from short and fat to long and very slender. Flatfish (halibut and flounders), as the name implies, have very thin bodies, and instead of swimming upright, they swim and lie on the bottom on one side. Both eyes are located on the same side of the head (i.e., the upper side). Flatfish have the remarkable ability to change the color and pattern on their upper surface to blend in with the type of bottom with which they are associated. Some fish look like lumps of coral or rock (e.g., stonefish), another form of camouflage.

Locomotion, Protection, and Feeding.

Most fish have a well-developed swimming ability and may be able to move very rapidly. Their tails are used to provide the thrust needed for high-speed swimming. Salmon, for example, are able to leap over low waterfalls on their upstream spawning migrations. Not all fish are strong swimmers, however. The ocean sunfish, for example, can be found floating at the surface of the water in the Pacific Ocean. A large fish (up to a few hundred kilograms), it has small fins and limited mobility. Moray eels hide in holes in rocks or coral reefs and spring out to grab prey that swim by.

To avoid being eaten, prey species often swim in schools that number many thousands of individuals. A predator seeing such a school may avoid it on the basis that the school could, in fact, be a fish larger than itself. If the predator does attack, it will be able to take only a small fraction of the total number of fish available.

Many fish do not swim in schools but depend on camouflage, swimming ability, or the tendency to seek refuge in crevices to avoid predation. Anemone fish can live among the stinging tentacles of sea anemones. Those fish are immune to the stinging cells, so they are protected from predators.

Most fish seek out their food by sight and actively search for it by swimming around. A few, on the other hand, wait for food to come to them. Anglerfish have a fleshy protrusion on their heads that acts like a fishing lure. Some anglerfish rest on the bottom, camouflaged to match their surroundings, and wave their lure until an unsuspecting smaller fish investigates. Anglerfish that inhabit the deep ocean, where light does not penetrate, have lures that glow in the dark to attract prey.

Reproduction.

Reproduction in fish usually involves laying eggs that are externally fertilized, though a few give birth to live young. Eggs may be dispersed into the water column, laid in nests hollowed out in the bottom sediments, incubated in the mouth of the adult, or attached to rocks or plants in a gelatinous mass. There may be no parental care, or one or both parents may provide close attention during incubation, hatching, and even the first few weeks of life.

see also Ecology, Fresh-Water; Ecology, Marine; Fisheries, Fresh-Water; Fisheries, Marine; Fishes, Cartilaginous; Food from the Sea; Life in Water.

Robert R. Stickney

Bibliography

Bond, Carl E. Biology of Fishes, 2nd ed. Philadelphia, PA: W. B. Saunders, 1997.

Moyers, Peter B., and Joseph J. Cech, Jr. Fishes: An Introduction to Ichthyology, 4th ed.Englewood Cliffs, NJ: Prentice Hall, 1999.

FISH (German Geheimschreiber Cipher Machine)

█ ADRIENNE WILMOTH LERNER

As late as the World War I era, cryptology depended on highly trained people at both ends of a communication to cipher and decipher a message. Codes were often kept in books that were vulnerable to enemy capture. The capturing of German code books by British military intelligence in World War I gave the Allies a significant tactical advantage. Soon after the war, technological advances in communication were applied to the sending and receiving of complexly coded text. Skilled cipherers and and codebooks were replaced by cipher machines. Modern cryptographers, therefore, not only had to break enemy codes, but also determine how foreign cipher machines operated and generated codes. Cipher machines produced more mathematically intricate and random codes that were difficult to break. Because many cipher machine codes were dependent upon both the sender and the receiver machines, the caputre of coded teleprinters did not dictate that a code could be broken.

In the 1930s, the German government comissioned the Seimans Company to create a cipher machine teleprinter that could produce, send, and receive plain and coded text. The idea behind the teleprinter was to randomize codes to make them more difficult to break, and to increase code information security. Seimans developed their first cipher teleprinter, the Geheimschreiber, with two encription features, overlaying of code and transposition of pulses. Long pre-dating digital technology, both the basic encription functions and the receipt of transpositioned pulses depended on mechanical circuts, namely various code wheels for text and charged capacators and their corresponding relays for the pulse. The machine's ten code wheels had periods corresponding with prime numbers between 47 and 73. Thus, the wheels combined to form 893,622,318,929,520,960 permutations, or steps. Eight basic patterns with over two billion variations were possible in regards to pulse transposition. These combined encryption mechanisms led the German government to assume that the Geheimschreiber was nearly random and unbreakable; however, the mathematical patterns used by the machines proved to be more systematic than they perceived.

Teleprinters utilized the 32-character Baudot code. The code output consisted of five channels, represented as holes or no holes in varying orders, to produce each character. The German cipher machines relied on the Vernam cipher system, a mathematical code based on the principle of binary addition. That is, two coded characters were added together to produce the ciphered text. Code breakers knew of both the Baudot code and Vernam system, but the obscuring factors of the German Geheimschreiber made deciphering the code difficult.

The German cipher machines were supposed to change starting positions with every message, notifying the receiving end of a given transmission in plain text of the starting steps on the code wheels. Thus, the obscuring sequence of each code was supposedly unique. Code breakers in Sweden worked to break the Geheimschreiber code mathematically, and did so with measurable success in 1942. However, the work was tedious and by the time they had produced several decoding machines, the highest levels of the German command had begun to use the newer Lorenz cipher machine. Swedish cryptologists were unable to decipher any wire traffic after February, 1944.

British intelligence cryptologists at Bletchley Park thought the best hope of readily deciphering German teleprinters was to intercept a depth, or two messages that utilized the same starting position. While codebreakers had some success mathmatically decoding Fish ciphered German transmissions, on August 30, 1941, British intelligence intercepted a 4,000-character-long depth. The Lorenz code was broken soon afterward by John Tiltman and Bill Tutte. Working out long code sequences by hand, the two uncovered the logical structure of the German cipher. With this knowledge, several "Tunny," now the code name for Lorenz transmissions, machines were constructed to facilitate decoding of intercepts. However, the start position settings of each message still had to be discovered by hand.

In 1943, British mathematician Max Newman and British engineer Tommy Flowers designed and built Colossus, a machine that not only simplified the process of deciphering German teleprinter intercepts, but that could be used with Geheimschreiber, Lorenz, and radio transmissions. Colossus' greatest contribution to codebreaking however was its ability to electronically decode the start position of each ciphered intercept, eliminating the need for painstaking hand calculations. The system was instrumental in the planning and execution of the allied D-Day invasion.

█ FURTHER READING:

BOOKS:

Goldreich, Oded. Foundations of Cryptography: Basic Tools. Cambridge: Cambridge University Press, 2001.

Hinsley, F. H. British Intelligence in the Second World War. Cambridge: Cambridge University Press, 1988.

Hinsley, F. H. and Alan Stripp, eds. Codebreakers: The Inside Story of Bletchley Park. Oxford: Oxford University Press, 2001.

Stinson, Douglas. Cryptography: Theory and Practice, second edition. Chapman and Hall, 2002.

SEE ALSO

Bletchley Park
Cipher Machines
Colossus I

Fish

Ocean saltwater covers more than three-quarters of Earth's surface; lakes, rivers, streams, ponds, canals, swamps, marshes, and other forms of freshwater cover vast expanses of the planet's surface as well. One of the most successful groups of animals that have evolved to fill all these habitats are fish.

There are two types of fish: a small group with skeletons made of cartilage (a bonelike elastic tissue less rigid than true bone) and an enormous group with skeletons made of bone (like that found in humans). Cartilaginous fish include the sharks, skates, rays, and dogfish. The remainder—more than 25,000 species (more than all other species of vertebrates combined)—are known as bony fish.

All fish are cold-blooded, meaning they do not have a constant body temperature but take on the temperature of the surrounding water. The majority of fish species have bodies that are streamlined; their bodies are covered with tiny, smooth scales that offer no resistance to a fish's movement through water. The scales themselves are covered with a slimy coating that further reduces friction. Additionally, a fish's external appendages (fins) have been reduced to produce minimal resistance to the water as they propel the fish through it. Fins fall into two categories: vertical fins, which occur individually, and paired fins. Examples of the vertical fins are a dorsal fin that runs down the middle of a fish's back and the anal fin that runs along its underside. Examples of paired fins are those that appear on either side of a fish's upper body, below and behind its eyes.

The form, size, and number of fins varies considerably according to an individual species' habitat and requirements. In fast-swimming species, such as the tuna or mackerel, the dorsal and anal fins have thin, sharp shapes that reduce friction. In puffer or porcupine fish, by contrast, the fins are greatly reduced—for use in short paddling movements. Other species, such as eels, have lost almost all traces of external fins and swim instead by rhythmic movements of their muscular bodies.

Another important adaptation made by fish is their swim bladder. The swim bladder is a chamber filled with air that allows a fish to remain at the same level in water while expending very little energy.

Fish breathe through structures known as gills. When a fish takes in water through its mouth, the flaps that cover its gills are closed. When the fish closes its mouth, the flaps open and water is expelled through the gills. In this process, oxygen dissolved in the water is absorbed into the fish's bloodstream.

Bony fish are either carnivorous (meat-eating), herbivorous (planteating), or both. And fish are, of course, one of the world's most popular foods. In island nations and countries with long coastlines, fish are a major part of the diet. They are also a healthful food since they are high in protein and low in fat content.

Cartilaginous fish

The cartilaginous fish—whose skeletons are made of cartilage—include both sharks and rays. An intriguing characteristic of sharks is the presence of tiny primitive teeth on their skin. These denticles are similar in some ways to human teeth, although much smaller in size. Thus the texture of a shark's skin is similar to that of fine sandpaper. Human swimmers can be badly cut by coming into contact with the skin of a shark. The skin of a ray, on the other hand, is entirely smooth except for the back or upper tail surface, where denticles have developed into large, strong spines.

The jaw teeth of both sharks and rays are, in fact, modified denticles. These teeth are lost when they become worn and are replaced by rows of new teeth from the space behind them. In some species of sharks, the jaw looks like an assembly line, with new teeth filling spaces immediately.

Like bony fish, both sharks and rays breathe through gills. They also have an opening called a spiracle on both sides of the head behind the eye. The spiracle allows water to flow through the gills without taking in large amounts of mud and sand. This adaptation is especially useful for rays, which often bury in the sand, and for sharks, which often rest on the ocean bottom. Unlike the bony fish, sharks and rays do not possess a swim bladder.

Cartilaginous fish are predatory: they feed on other animals, from zooplankton to shellfish to whales. And they themselves are sought after by humans as a food source. Shark meat, once marketed under the pseudonyms of flake and steakfish, is now popular worldwide. Shark fins have long been popular in Asia. Rays are considered delicacies in Great Britain and France, and thornback rays and flapper skates are often sold as sea trout.

[See also Coelacanth ]

fish¹ / fish/ • n. (pl. same or fish·es ) a limbless cold-blooded vertebrate animal with gills and fins and living wholly in water: the sea is thick with fish. ∎  the flesh of such animals as food: hot crab appetizers stuffed with fish. ∎  (the Fish or Fishes) the zodiacal sign or constellation Pisces. ∎  used in names of invertebrate animals living wholly in water, e.g., cuttlefish, shellfish, jellyfish. ∎  inf. a person who is strange in a specified way: he is generally thought to be a bit of a cold fish. ∎  inf. a torpedo. • v. [intr.] catch or try to catch fish, typically by using a net or hook and line: he was fishing for bluefish I've told the girls we've gone fishing. ∎  [tr.] catch or try to catch fish in (a particular body of water): they did fish the mountain streams when game grew scarce. ∎  search, typically by groping or feeling for something concealed: he fished for his registration certificate and held it up to the policeman's flashlight. ∎  try subtly or deviously to elicit a response or some information from someone: I was not fishing for compliments. ∎  [tr.] (fish something out) pull or take something out of water or a container: the body of a woman had been fished out of the river. PHRASES: a big fish an important or influential person: he became a big fish in the world of politics. a big fish in a small (or little) pond a person seen as important and influential only within the limited scope of a small organization or group. drink like a fish drink excessive amounts of alcohol.fish or cut bait see bait. a fish out of water a person in a completely unsuitable environment or situation.fished out depleted of fish: the grayling here have hardly been fished out. have other (or bigger) fish to fry have other (or more important) matters to attend to. like shooting fish in a barrel extremely easy: picking cultivated berries is like shooting fish in a barrel. neither fish nor fowl (nor good red herring) of indefinite character and difficult to identify or classify. there are plenty more fish in the sea used to console someone whose romantic relationship has ended by pointing out that there are many other people with whom they may have a successful relationship in the future.DERIVATIVES: fish·like adj. fish² / fish/ • n. a flat plate of metal, wood, or another material that is fixed on a beam or across a joint in order to give additional strength, esp. on a ship's damaged mast or spar as a temporary repair. • v. [tr.] mend or strengthen (a beam, joint, mast, etc.) with a fish. ∎  join (rails in a railroad track) with a fishplate.

fish in Christian art, a fish is a symbol of Christ, and sometimes also of the newly baptized and of the Eucharist; it is often found in paintings in the catacombs. The reason for the symbolism is not wholly clear, although it may derive from the Greek letters of ‘Jesus Christ, Son of God, Saviour’ read as ikhthus ‘fish’.

A fish is an emblem of St Anthony of Padua and other saints.
The Fish or The Fishes are names given to the zodiacal sign or constellation Pisces.
all is fish that comes to the net everything can be used to advantage; saying recorded from the early 16th century. A similar thought to all is grist that comes to the mill.
a big fish in a small pond a person who is important only within the limited scope of a small field or group.
the fish always stinks from the head downwards as the freshness of a dead fish can be judged from the condition of its head, any corruption in a country or organization will be manifested first in its leaders. Recorded in English from the late 16th century. The saying ‘a fish begins to stink from the head’ is found in Greek.
fish and guests smell after three days one should not outstay one's welcome. The saying is recorded from the late 16th century, but the same idea is found in the Miles Gloriosus of the Roman comic dramatist Plautus (c.250–184 bc), ‘no host can be hospitable enough to prevent a friend who has descended on him from becoming tiresome after three days.’
fish in troubled waters take advantage of disturbance or trouble to gain one's end; recorded from the mid 16th century.
a fish out of water a person in a completely unsuitable environment or situation.
have other (or bigger) fish to fry have other (or more important) matters to attend to.
neither fish nor fowl nor good red herring of indefinite character and difficult to identify or classify; with original reference to dietary restrictions in times of fasting and abstinence.
there are as good fish in the sea as ever came out of it now often used as a consolation to rejected lovers in the form there are plenty more fish in the sea. The saying is recorded from the late 16th century.

see also better are small fish than an empty dish, big fish eat little fish, the cat would eat fish but would not wet her feet, keep your own fish-guts for your own sea-maws, kettle of fish, little fish are sweet, loaves and fishes.

Fish family long prominent in New York politics.

Nicholas Fish, 1758-1833, b. New York City. He studied law before serving ably as a major in a New York regiment throughout the American Revolution. A New York City alderman (1806-17), he was a leading Federalist and a close friend of Alexander Hamilton. He also served (1824-32) as chairman of the board of trustees of Columbia College, a post later held by his son, Hamilton Fish (1808-93), the most illustrious member of the clan (see separate articles for Hamilton Fish , 1808-93, and for his youngest son, Stuyvesant Fish ). Nicholas Fish, 1848-1902, b. New York City, was Hamilton's eldest son. He entered (1871) the U.S. diplomatic service and was minister to Belgium (1882-86).

A third son, Hamilton Fish, 1849-1936, b. Albany, N.Y., studied law and was admitted to the bar in 1873. He was a member of the New York state assembly (1874-96), serving as speaker in 1895-96, and was long Republican boss of Putnam co. On appointment by President Theodore Roosevelt, he was Assistant Treasurer of the United States in New York City (1903-8). He also served (1909-11) as a U.S. Representative.

The family's third Hamilton Fish, 1888-1991, son of the foregoing, b. Garrison, N.Y., was a football player at Harvard. A lawyer, Fish served in the New York state assembly (1914-16), distinguished himself in World War I as captain of an African-American infantry company, and from 1920 to 1945 was a U.S. Representative. A leading isolationist and vigorous anti-Communist, once accused of having connections with the Bundists and with other Axis supporters, he was opposed for renomination in 1944 by Gov. Thomas E. Dewey and other Republican leaders. Fish nevertheless won the primary but was defeated for reelection in November.

His son Hamilton Fish, 1926-96, b. Washington, D.C., continued the family's involvement in Republican politics. Admitted to the bar in 1957, he was elected to the U.S. House of Representatives from New York in 1968, where he consistently supported civil-rights legislation. He retired from Congress in 1995.

fish limbless aquatic vertebrate animal with fins and internal gills. There are three living classes of fish: the primitive jawless fishes, or Agnatha; the cartilaginous (sharklike) fishes, or Chondrichthyes; and the bony fishes, or Osteichthyes. These groups, although quite different from one another anatomically, have certain common features related to their common evolutionary origins or to their aquatic way of life. Fish were the earliest vertebrates and presumably evolved from a group of aquatic lower chordates (see Chordata ); the terrestrial vertebrates evolved from fishes.

There are over 20,000 living species of fish. They range in size from the .31-in. (7.9-mm) Paedocypris that lives in tropical swamps in Sumatra to the 45-ft (14-m) whale shark. Many are brightly colored, and many have shapes and patterns that serve as camouflage. They are found in all marine, fresh, and brackish waters throughout the world and at all depths. Members of different species of fish tolerate water temperatures ranging from freezing to over 100°F (38°C). Most are confined either to saltwater or to freshwater, but some are physiologically adapted to moving from one to the other. A number of fishes that are born in freshwater spend their adult lives in the ocean, returning to their birthplace to spawn; the reverse of this migration occurs in some fishes born in the ocean. Many fishes stay in tightly organized groups, called schools; others are solitary and congregate only for feeding and spawning. Fish may be carnivorous, herbivorous, or omnivorous. Some fish are scavengers on lake or ocean bottoms. Fish are a major source of human food as well as of oil, fertilizer, and feed for domestic animals (see fishing ).

A number of aquatic invertebrate animals and groups have common names that include the term fish (for example, crayfish and shellfish), but these do not resemble and are not related to true fishes. Furthermore, there are members of the terrestrial vertebrate classes, such as whales and sea snakes , that have adopted an aquatic way of life; these may superficially resemble fishes and are sometimes erroneously called fishes, but they are air-breathers, and their anatomical structure reveals their relationship to land animals.

Characteristic Anatomical Features

A typical fish is torpedo-shaped, with a head containing a brain and sensory organs, a trunk with a muscular wall surrounding a cavity containing the internal organs, and a muscular post-anal tail. Most fish propel themselves through the water by weaving movements of their bodies and control their direction by means of the fins . All have skins covered with slimy glandular secretions that decrease friction with the water; in addition, nearly all have scales , which together with the secretions form a nearly waterproof coating. All fishes have a lateral line system of sensory organs for detecting pressure changes in the water. All have water-breathing organs called gills located in passages leading from the throat, or pharynx, to the exterior; a few fishes also have air-breathing lungs as an additional means of respiration. In all but the most primitive class, the gill passages are supported by skeletal structures called gill arches. Plankton-feeding fish have structures called gill rakers attached to the gill arches; these strain minute organisms from the water as it passes out of the pharynx. Fish breathe by taking water into the mouth and forcing it out through the gill passages; as the water passes over the thin-walled gills, dissolved oxygen diffuses into the gill capillaries and carbon dioxide diffuses out. The circulatory system is closed, and the heart is two-chambered; the blood is red. With few exceptions, fish are cold-blooded; that is, they cannot regulate their body temperature, which is the same as that of the environment.

Reproduction

Methods of reproduction are varied. Sharks have internal fertilization, and most give birth to live young. Those that lay eggs produce large ones with tough shells. Since embryonic development is well-protected in these fish, they produce a relatively small number of young, only seven or eight at a time in some species. A few of the bony fishes, including some aquarium species, are live bearers, but most lay small, unprotected eggs that are fertilized after deposition in water. In most marine species the eggs float freely in the currents, where they are eaten by other animals. An enormous number of eggs is therefore necessary to ensure the maturation of a few; in many species a female produces as many as 5 million eggs in one spawn. The eggs of most marine fishes contain oil droplets that buoy them up, while those of most freshwater fishes are heavy, with sticky surfaces that adhere to objects in the water. Most freshwater species build nests for the protection of the eggs, and in some the adults guard the nests.

Types of Fish

The Jawless Fishes

The primitive fishes of the class Agnatha lack jaws and the paired pelvic and pectoral fins characteristic of more advanced fishes. This largely extinct class includes two living groups, the bloodsucking lampreys and the scavenging hagfishes . Fishes of the extinct class Placodermi were the first vertebrates to develop jaws and paired fins. These fish had bony skeletons and were covered with bony armor. A branch of this group probably gave rise to the two main modern classes of fish, the cartilaginous fish and the bony fish.

The Cartilaginous Fishes

The cartilaginous fish ( sharks , rays , and chimaeras ) are distinguished from the bony fish by their cartilage skeletons, by the absence of either a swim bladder or lungs, by the construction of their tail fins, and by the absence in most of a gill covering, or operculum. The skin of members of this group is covered with imbedded toothlike structures called denticles, giving it a rough, sandpapery quality. Sharks are almost exclusively marine in distribution.

The Bony Fishes

The bony fishes are distinguished from other living fishes by their bone skeletons and by the presence of either a swim bladder (which functions as a float) or, in a few fishes, lungs. The bony fishes are divided into two subclasses, the fleshy-finned fish and the ray-finned fish. The latter group includes over 95% of all living fish species.

The earliest bony fishes were fleshy-finned. They evolved during a period of widespread drought and stagnation and gave rise to the amphibians (the first terrestrial vertebrates) on the one hand and to the ray-finned fish on the other. The only surviving fleshy-finned fishes are the lungfishes and one species of coelacanth (see lobefin ). These fishes retain some of the traits of ancestral bony fishes: fleshy fins with supporting bones (precursors of the limbs of land vertebrates), internal nostrils, and lungs.

Ray-finned fishes, now predominant in both fresh and marine waters, represent an advanced adaptation of the bony fishes to strictly aquatic conditions; they are the most highly successful and diverse of the fishes. In nearly all of these fishes the lung has evolved into a hydrostatic organ, the swim bladder. The fins in this group consist of a web of skin supported by horny rays. Each ray is moved by a set of muscles, giving the fin great flexibility. Most ray-finned fish have overlapping scales made of very thin layers of bone. Their skeletal structure is light but strong and most have excellent vision.

Bibliography

See W. S. Hoar and D. J. Randall, Fish Physiology (6 vol., 1969-71); J. E. Webb et al. ed., Guide to Living Fishes (1981).

164. Fish

See also 44. BIOLOGY ; 430. ZOOLOGY

anadromous

a term describing fish that migrate upriver to spawn.

catadromous

a term describing fish that migrate downriver to spawn.

halieutics

1 . the activity of fishing.

2 . a work on fishing. —halieutic , adj.

ichthyism, ichthyismus

a toxic condition caused by toxic fish roe.

ichthyolatry

the worship of fish or of fish-shaped idols.

ichthyology

1 . the branch of zoology that studies fishes.

2 . a zoological treatise on fish. —ichthyologist , n. —ichthyological , adj.

ichthyomancy

a form of divination involving the heads or entrails of fish.

ichthyomania

an abnormal love of fish.

ichthyophagy

the practice of eating or subsisting on fish. —ichthyophagist , n. —ichthyophagous , adj.

ichthyophobia

1 . a ritual avoidance of fish, especially under the pressure of taboo.

2 . an abnormal fear of fish.

ichthyosis

a dermatologie condition in which the skin resembles fish scales. —ichthyotic , adj.

ichthyotomy

the anatomical structure of fishes and its study. —ichthyotomist , n. —ichthyotomic , adj.

piscary

the right of one person to fish in waters belonging to another. See also 239. LAW .

piscatology

Rare. the art or science of fishing.

piscatorian, piscatorialist

an angler or fisherman.

pisciculture

the breeding of fish, as a hobby or for scientific or commercial purposes. —pisciculturist , n. —piscicultural , adj.

planktology, planktonology

the branch of biology that studies plankton, especially as the sustenance of planktivorous fish.

Waltonian

1 . a keen angler or fisherman, after Izaak Walton (1593-1683), English author of The Compleat Angler.

2 . an admirer of the works of Izaak Walton. —Waltonian , adj.

fish Cold-blooded, aquatic vertebrate animal characterized by fins, gills for breathing, a streamlined body almost always covered by scales or bony plates on to which a layer of mucus is secreted, and a two-chambered heart. Fish are the most ancient form of vertebrate life, with a history of about 450 million years. They reproduce sexually, and fertilization may be external or internal. The eggs develop in water or inside the female, according to species. Fish have lateral line organs, which are fluid-filled pits and channels that run under the skin of the body. Sensitive fibres link these channels to the central nervous system and detect changes of pressure in the water and changes of strength and direction in currents. About 75% of all fish live in the sea; the remainder are freshwater species that live in lakes, rivers and streams. A few fish, such as the salmon and eel, divide their lives between salt and freshwater habitats. The classification of fish varies. They are usually divided into three classes: Agnatha, which are jawless fish, including the hagfish and lamprey; Chondrichthyes (cartilaginous fish), which includes shark, skate, ray and chimaera; and the much more numerous Osteichthyes (bony fish), including subclasses of soft-rayed fish (lungfish and lobefin) and the very successful teleost fish, such as salmon and cod. There are more than 22,000 species of bony fish, and they represent c.40% of all living vertebrates. They divide into 34 orders and 48 families.

fish Broadly speaking, any poikilothermic, legless, aquatic vertebrate that possesses a series of gills on each side of the pharynx, a two-chambered heart, no internal nostrils, and at least a median fin as well as a tail fin. If the Agnatha (lampreys and hagfish) are excluded, the fish (Pisces) would still include the Chondrichthyes (sharks and rays), in which the skeleton is cartilaginous, as well as the Osteichthyes (bony fish). In addition to the criteria mentioned, these two classes also possess well-developed gill arches, a pair of pectoral fins, and a pair of pelvic fins. Some consider, however, that only the bony fish (Osteichthyes) should be classed as real fish.

fish Many species of fish lived in the Sea of Galilee and the Jordan. They were sometimes caught with a hook (Matt. 17: 27) but more often by letting a net into the water in a semicircle and gradually drawing it ashore (Matt. 13: 47–50). There was an industry of fish-curing at Taricheae. Pickled fish were sent up to Jerusalem.

Early Christians used a fish as a cryptogram or symbol for Christ, especially in the Eucharist on account of the fish mentioned at the Feeding of the Multitude (Mark 6: 38). Moreover, additionally, the initial letters in Greek of Jesus Christ, Son of God, Saviour, spelt out the word ichthus (fish).

fish Broadly speaking, any poikilothermic, legless, aquatic vertebrate that possesses a series of gills on each side of the pharynx, a two-chambered heart, no internal nostrils, and at least a median fin as well as a tail fin. If the lampreys and hagfish (Agnatha) are excluded, this definition includes the sharks and rays (Chrondrichthyes), in which the skeleton is cartilaginous, as well as the bony fish (Osteichthyes). Some consider, however, that only the bony fish should be classed as real fish.

fish2 mend (a broken spar, etc.) with a piece of wood (fish or fish-plate). XVII. — (O)F. ficher :- Rom. *fīgicāre, intensive of L. fīgere FIX.
Hence (after F. fiche) sb. (naut.) piece of wood used to strengthen another XVII; plate of iron, etc., to protect or strengthen a beam, rail, etc. XIX.

fish. In Christian art and literature the fish is a symbol of Christ, also sometimes of the newly baptized and of the Eucharist. In modern times some C of E associations willing to help those in need have adopted the symbol of a fish.

From early times fish has taken the place of meat on days of fasting and abstinence.

fish. Although representations of the sea creatures are found on certain Gallo-Roman altars, very few have played important roles in the Celtic imagination. Of these the most notable is the salmon; see also DOLPHIN; EEL. The whale upon which St Brendan and his followers land is Jasconius.

fish1 vertebrate water animal with gills. OE. fisċ = OS., OHG. fisc (Du. visch, G. fisch), ON. fiskr, Goth. fisks :- Gmc. *fiskaz :- IE. *piskos, rel. to L. piscis, Ir. iasc, Gael. iasg.
So fish vb. OE. fisċian, fisher OE. fisċere.

Fish. As a Christian symbol, its use goes back to 2nd-cent. writers. The symbol itself may be derived from the acrostic spelling of ichthus (Gk., ‘fish’) from the Gk. first letters of ‘Jesus Christ, God and Saviour’.

fish An instrument package towed behind a ship so that the measurements are unaffected by the ship and its equipment. A fish commonly contains a magnetometer and side-scan sonar.

fish3 flat piece of bone, etc., used as a counter in games. XVIII. — F. fiche, f. ficher (see prec.), assoc. with FISH1.

FISH

This entry includes three subentries:
Overview
Freshwater Fish
Sea Fish

FISH See fluorescence in situ hybridization.

fish See Chondrichthyes (cartilaginous fish); Osteichthyes (bony fish); Pisces.

fish see early vertebrates and fish

FISH See FIRST IN STILL HERE.

fish n. informal a torpedo.

FISH, see Geheimschreiber.

fish •bish, dish, fish, Frisch, Gish, knish, pish, squish, swish, wish •clayish, greyish (US grayish) •puppyish • babyish •dandyish, sandyish •toadyish • fogeyish • monkeyish •sissyish • Gypsyish • prettyish •heavyish • dryish •lowish, slowish •sallowish • yellowish • narrowish •boyish • tomboyish •bluish, Jewish, newish, shrewish •Pollyannaish • prima donna-ish •nebbish •slobbish, snobbish, yobbish •rubbish • furbish •baddish, caddish, faddish, kaddish, laddish, radish, saddish •blandish, brandish, outlandish, Standish •Cavendish • Netherlandish •horseradish • hardish • reddish •Wendish • old-maidish • Swedish •fiendish • Yiddish • widish •childish, mildish, wildish •cloddish, oddish •baldish • roundish •modish, toadish •coldish, oldish •prudish • goodish • Kurdish