Almost three-quarters of Earth’s surface is covered by water, in which live about 21,700 known species of fishes. From their earliest jawless ancestors in the Silurian period, which began about 438 million years ago, fishes have diversified to inhabit widely differing aquatic habitats. Sleek, fast-swimming tuna live in the surface waters of the oceans, while the dark abyssal regions are inhabited by lantern fish and other deep-sea forms. Fresh water contains fishes such as trout able to survive in the violence of mountain torrents—and, at the other extreme, lungfish, which inhabit temporary pools and can withstand several years of drought. Some fishes have even more specialized habitats, such as the lightless caves of the blind cave characin.

Despite their similar basic design, fish forms show considerable variety. One of the smallest fishes the pygmy goby of the Philippines, which grows to less than half an inch (13 millimeters) long is also among the smallest vertebrates. The basking shark grows up to 40 feet (12 meters) long and weighs up to four tons (3.6 metric tons), and the whale shark can reach more than 60 feet (18 meters) in length.

Coloration also varies, from the dazzling multicolored fishes of coral reefs to the colorless cave fishes, and the almost totally transparent glass catfish.

Fishes are vertebrates, with a backbone of either bone or cartilage. Among their distinguishing characteristics is the fact that they have fins, mostly in paired sets, and that their bodies are covered in scales. Most fishes breathe using internal gills, to which blood is pumped by a two-chambered heart—mammalian hearts, by contrast, have four chambers. Fish are poikilothermal animals that is, they cannot regulate their body temperature, which varies according to the temperature of the water. The blood of tropical fishes, therefore, is not cold at all. Most fishes are oviparous, or egg-laying, though some species are ovoviviparous that is, the eggs hatch inside the female’s body. A few species, such as the Atlantic manta ray, are viviparous, and the developing embryos are nourished by internal secretions from the female before being born alive.

Fishes are divided into two groups: jawed and jawless. The only jawless species are lampreys and hagfish. Jawed fish are further divided into two groups according to the composition of their skeletons: bony fishes (class Osteichthyes) and cartilaginous fishes (class Chondrichthyes).

A manta ray feeds on plankton, which it funnels into its mouth with the two “horns” that are extensions of its snout. Its huge pectoral fins propel it through the water by flapping like wings. This photograph also shows several remoras attached by their suckers to the manta’s large gill openings, where they feed on plankton that the manta leaves behind, and on parasites from the manta’s skin.
The streamlined shape and the segmented body muscles of fishes allow them to use sinuous side-to-side movements of the body and tail to swim through water.


Because of its higher density and viscosity, water offers more resistance to movement than air. To minimize drag, therefore, the basic fish shape is streamlined. Backward-pointing scales cover the body, and the skin secretes slippery mucus to cut down water resistance further.

Most fish move mainly by using the caudal, or tail, fin. Muscles on either side of the backbone contract alternately, thus causing the body to bend and the tail to flex from side to side. The caudal fin forces the water backward and this propels the fish forward. Other fins control the direction of movement and help the fish stop. Pectoral fins on the fish’s side act as elevators, adjusting the vertical pitch of the fish as it moves. Dorsal and anal fins on the upper- and underside of the fish, which are unpaired, help the fish remain upright. The paired pelvic fins on the underside often act like rudders in changing direction.

In some species, the tail has acquired other functions apart from providing propulsion. The sea horse, for instance, uses its tail to cling to weeds; it swims by lateral undulations of its dorsal fin.


Cartilaginous fishes are denser than water and so tend to sink. This is described as negative buoyancy. To overcome this tendency they use their pectoral fins, the front of their head, and their tail fin to produce lift as they swim. When stationary, however, they still tend to sink.

In contrast, most bony fishes can give themselves the same density as water that is, they achieve neutral buoyancy and are therefore able to maintain their position in the water even when not swimming. They do this using their swim bladder, a gas-filled, lunglike sac, which can be inflated or deflated to alter the buoyancy of the fish, allowing it to rise or descend in the water. Predators, such as the pike, exhibit this ability well, remaining stationary as they wait for passing prey, using only small fin movements to maintain their position.

A fish relies on its fins for movement, balance, and position. The dorsal fins on the fish’s back help to keep it upright and traveling straight, though some lateral “weaving” inevitably occurs. The pectoral fins at the sides control its horizontal position.

Sense organs

Most fishes living in clear water have good eyesight. The archerfish, for example, is able to aim a jet of water at insects above the surface and even can adjust for the bending of the light rays as they pass from air to water.

In most fish, the olfactory organs (organs of smell) consist of two pouches, one on each side of the snout. Sharks are renowned for their ability to detect blood in the water from as far away as one-third of a mile (.5 kilometer). Many bottom dwellers, such as the catfish, possess whiskerlike barbels around the mouth, which give them the sense of taste and touch.

Two other sensory systems are found exclusively in aquatic animals, and both relate to water’s conducting properties. These are the pressure sense provided by the lateral line system, and an electric sense possessed by some species. In a few species, the latter is also associated with an ability to generate an electrical pulse.

The lateral line system can detect pressure changes caused by disturbances in the water around the fish. In experiments, blinded fishes have been trained to locate a moving glass rod by this method. In the wild, it enables a fish to detect moving food items, approaching predators, or other members of a shoal.

Many fishes that live in muddy water or are active at night also have an electric sense. Electric receptors can be identified in sharks, dogfish, catfish, and mormyrids. Of those species that also possess electric organs capable of generating electrical pulses, some, such as the gymnarchid, can detect irregularities in the electric field and thus locate objects in the cloudiest water. The electric eel has the well-known ability to produce much more powerful pulses (up to 500 volts) when alarmed. It uses such electrical pulses to stun prey.

The basic vertebrate shape of a fish is apparent even in a flatfish, where the body appears to have turned onto its side. This computer-colored X ray of a plaice shows the fish’s head, skeleton, internal organs, and muscles.


The internal gills of fishes have a large surface area over which gaseous exchange between the water and the blood can occur. Gill structure is related to the life style of the fish. Active species need more oxygen and so have large gills. Sluggish forms, like the bottom-dwelling toadfish, have smaller gill areas in proportion to their body size. The mouth and gill cavities form a mechanism that pumps water over the gills. A one-way flow is ensured by flaps of skin in the mouth. The gills are so arranged that the blood in the gill filaments and the water-carrying oxygen flow in opposite directions, and this countercurrent system allows for the most efficient exchange of gases. In some sharks, the respiratory current from the gill slits is strong enough to propel the shark forward.
Certain cells in the gills also help fishes maintain their internal water and salt balance. In freshwater fishes, these cells absorb salts from the water to keep up the body’s salt level. In marine bony fishes, other cells excrete salt to compensate for the high levels of salt swallowed in sea water.

A typical bony fish (A) has unpaired dorsal, anal, and caudal fins—used for balance and propulsion—and paired pectoral and pelvic fins that help control its direction of movement. Internal organs (B) include the heart, liver, and—in advanced species—a swim bladder. The gills (C) absorb oxygen from water that is taken in through the mouth and pumped out through the gill filaments

Cartilaginous fishes

Because they lack bony skeletons yet possess gill slits, cartilaginous fishes (class Chondrich-thyes) were originally considered a primitive group. They do not appear before bony fish in the fossil record, however, and zoologists now believe that their ancestors had bony skeletons. Cartilaginous fishes are divided into three orders: the chimaeras (Chimaeriformes), the rays (Rajiformes), and the sharks (Squali-formes).

About 600 species of cartilaginous fishes have been identified. Most are marine and carnivorous, although the largest representatives are actually harmless filter feeders.
All cartilaginous fishes reproduce by means of internal fertilization. The male cartilaginous fish has a pair of claspers formed from the pelvic fins, which are inserted into the female to transfer the sperm. But after fertilization, cartilaginous fishes show the complete range of different reproductive strategies. Egg-layers lay large, yolky eggs the familiar “mermaid’s purse” is the horny egg case of the dogfish. Offshore sharks are often ovoviviparous the newly hatched sharks feeding on unfertilized eggs within the female before birth. Some sharks, however, and some rays are viviparous.

There are about 30 species of chimaeras, or ratfish. There are short-, long-, and elephant-nosed species. The upper jaw carries tooth plates and is fixed to the brain case, unlike the mobile jaws of sharks. Chimaeras have long, thin tails and swim by flapping their large pectoral fins. They are mediumsized fish with large eyes, and they live near the ocean bottom.

Sharks, dogfish, skates, and rays all have gill slits and, usually in front of these, another opening called the spiracle. The mouth, on the underside of the head, is filled with rows of teeth, which are developed from the toothlike structures that cover the body. In sharks, only one row of teeth is used at a time, but as soon as teeth are lost or worn away, others move forward to replace them.
Skates and rays (order Rajiformes) have flattened bodies with the gill slits opening on the underside. Some sharks also have flattened bodies, but the gill slits are always on the side of the head. Many skates and rays are bottom dwellers, often lying buried in sand with only the eyes protruding. As with chimaeras, the long thin tail is not used for swimming in most species. Instead they move by flapping or forming ripples in the very large pectoral fins. One of the largest rays is the Atlantic manta, measuring up to 23 feet (7 meters) from the tip of one pectoral fin to the other. Like the largest sharks, it is a filter feeder, straining small fish and crustaceans from the water. Electric rays (suborder Torpedinoidea) use their electric organs capable of delivering a charge of 200 volts to stun their prey, or as a means of defense. The stingrays, sluggish bottom dwellers, have a different defensive strategy a poisonous spine at the base of the tail.

Most sharks and dogfish (order Selachii) have a streamlined, torpedo shape. Probably the most feared and the most likely to be dangerous to humans is the great white shark, which can grow up to 21 feet (6.4 meters) in length. Stomach content analyses of the white shark suggest their normal diet is fish, dolphins, sea lions, and seals. They are also known to attack bathers, but they do not generally eat them.

The smallest members of this group are the dogfishes (family Scyliorhinidae), many of which are smaller than 3 feet (1 meter) in length. Dogfishes live in shallow coastal waters, where they feed on mollusks, worms, and other invertebrates.

Electric fishes can not only sense electric currents in the water but can also generate an electric field, and some species can produce strong electric pulses. The organs responsible consist of rows of electroplates that work neurochemically, intensifying what is essentially a nervous impulse. In the electric eel, these organs are located on either side of the body, and produce an electric field that resembles the magnetic field of a bar magnet.

Bony fishes

About 95 per cent of living fishes belong to the class Osteichthyes, or bony fishes, making this the largest vertebrate class. As their name suggests, at least part of the skeleton is formed from bone, although cartilage often also occurs. Bony fishes can be readily distinguished from cartilaginous fish by the presence of a bony gill cover called an operculum. Also, the mouth of bony fishes is usually at the very front of the head, instead of on the underside, as in cartilaginous fishes.

Reproductive strategies also differ from those of cartilaginous fishes. In contrast to the large yolky eggs of the latter, bony fishes tend to lay numerous small eggs. The number of eggs, which can be millions, varies according to the hazards of a particular fish’s life style. Fish that lay eggs in areas more vulnerable to predators tend to lay great numbers of eggs. Because male bony fishes do not possess claspers to transfer sperm, fertilization takes place externally.

Another typical feature of bony fishes is their air sac, which functions as a lung in the more primitive forms of fishes, and as a swim bladder in the rest. Bony fishes are usually subdivided into three groups: lungfishes (subclass Dipnoi), lobefinned fishes (subclass Crossopterygii), and ray-finned fishes (subclass Actinopterygii).

A shark’s teeth grow in rows (lower inset). The illustration shows the teeth of a sand tiger shark, which are typical of many shark species. A shark s placoid scales (upper inset) have a structure similar to its teeth; this is apparent in the cross section that shows the hard toothlike covering and the soft core of pulp.

Lungfishes and lobefins

Zoologists have studied lungfishes and lobefins with particular interest since it was realized that similar species probably gave rise to the first land vertebrates. Lungfishes possess choanae (nostrils that connect the mouth cavity to the outside air) as in modern air-breathing amphibians. They and lobefins have paired fins containing bony supports and muscles in their bases, from which the weight-bearing limbs of land vertebrates could have developed. Since modern lungfishes have highly specialized jaws with tooth plates, however, they are unlikely to have been the ancestors of four-legged animals. Most zoologists believe that land vertebrates arose from a group of freshwater lobefins. Today, lobefins are represented by a single species, the coelacanth, although more than 50 fossil species are known. This “living fossil” has altered very little since it first appeared in the Devonian period, 400 million years ago.

The lungfishes are also survivors from an earlier period. They were the most numerous fishes in the Devonian period, but there are now only six living species, distributed in Africa, Australia, and South America. All have functional lungs, which allow them to survive in poorly oxygenated waters that occasionally dry up.

Camouflage is the only means of defense for some fishes. The mottled skin and the flat shape of a plaice enable it to blend with the rocks and gravel of the seabed. Plaice may also try to bury themselves under the sand for even greater concealment.

Ray-finned fishes

Most freshwater fishes and commercially exploited marine fishes are ray-finned. As their name suggests, they have fins supported by bony rays. They have two pairs of nostrils, but these do not penetrate to the mouth. Ray-finned fishes fall into four subdivisions: Polypteri, which includes birchirs and reedfish; Chondrostei, which includes sturgeons and paddlefish; and Neopterygii, which includes all other fish. The first two are considered primitive forms.

Birchirs and reedfishes occur in African inland waters. They possess a pair of lungs, so like the lungfishes they can survive in oxygendeficient swamps. Sturgeons include the world’s largest freshwater fish, the beluga, which can grow to a length of 14 feet (4 meters) and a weight of over 2,800 pounds (1,300 kilograms). Sturgeons are also the sought-after source of caviar, which is salted sturgeon roe (eggs). Their skeletons are made mainly from cartilage, and they have several apparently primitive features, such as spiracles and an asymmetrical tail fin. Sturgeons do not have a swim bladder.

The North American gar and bowfin are both primitive neopterygians. They have a lunglike swim bladder and can breathe air when necessary. With sharp teeth, both are voracious carnivores, preying on other fishes and often causing damage to commercial nets in the process.

In stickleback courtship, the male acquires a red belly as he establishes his territory and prepares the nest The colors intensify as he identifies a female by her egg-swollen shape, then encourages her to lay her eggs in the nest. Once fertilized, the eggs are tended by the male, who continues to protect the young when they have hatched.

Teleost fishes

By far the most numerous and diverse group of fishes are the teleosts, which belong to the subdivision Teleostei. Teleost comes from two Greek words meaning complete and bone. Teleosts are divided into about 30 different orders, of which the most important are eels; salmon, trout, and pike; carps, characins, and catfishes; codfishes; spiny-finned fishes; and flatfishes.
The success of the teleosts can perhaps be attributed to their mobility, achieved by the development of the swim bladder, highly mobile fins, and reduced scales.

The swim bladder allows for buoyancy control, and within the group various stages of swim bladder development can be seen. In the carp, the swim bladder is still connected to the gut by a duct and is filled by swallowing air. In the perch and other advanced teleosts, however, this duct has been lost, and the bladder is filled by gas drawn from the blood at a special “gas gland.”
The fins of teleosts also enhance their mobility. The tail fin is symmetrical, giving effective forward thrust. Another feature that can be observed in this group is the tendency of the paired pelvic fins to move forward to underlie the pectoral fins in more “advanced” species. In the salmon, which is considered a less advanced group, the paired fins are well separated, whereas in the more modern perch the pelvic fins have moved forward. Forward pelvic fins appear to aid the mobility of fishes, because stopping and turning are easier with this arrangement.

Other teleost features include a reduction of the bone in the scales to a very thin layer. Two types of teleost scale occur cycloid scales, as in the carp, and ctenoid scales, as in the perch. Both types are considerably lighter than the thick ganoid scales of primitive bony fish. Scales are of considerable interest to fisheries biologists in temperate regions, because the age of the fish can be determined by counting the growth rings on its scales.


Characterized by their snakelike bodies and smooth, slimy, often scaleless skin, eels (order Anguilliformes) are easily recognized. They have no pelvic fins, and their anal and dorsal fins have fused to form a fin seam, which is used in swimming.

Eels have an unusual transparent larval form called a leptocephalus (“leaf-head”), which bears little resemblance to the adult, and was originally considered a completely different fish. Freshwater European eels have an extraordinary reproductive cycle. Adult eels change from freshwater to saltwater fishes, then migrate from Europe to spawning grounds in the Sargasso Sea, an area of the Atlantic Ocean northeast of the West Indies, more than 3,350 miles (5,400 kilometers) away. Once hatched, the larvae make the return journey, aided by the Gulf Stream. Eventually possibly after as much as three years they enter European rivers, where they grow into freshwater adult forms.

Most eels are marine, however, like the moray and conger eels, both of which are voracious fish eaters. Garden eels, in contrast, live in tubes and filter tiny zooplankton for food. Despite its name and shape, the electric eel is not a true eel and is more closely related to the carps.

Salmon, trout, and pike

The order Salmoniformes is typified by salmon and trout, both migratory fishes from northern temperate areas, which often use offshore feeding grounds and return to fresh water to spawn. This group also includes the char, grayling, smelt, and whitefish. All can be recognized by their large mouths and the presence of a small, rayless adipose fin between the dorsal and tail fins. All are popular food fishes.

The pike is also a member of this order. A large mouth filled with sharp teeth enables a pike to feed on other fish and even small mammals and birds taken at the water’s edge or surface.

Carps, characins, and catfish

The majority of the world’s freshwater fishes, more than 3,500 species, belong to the same group as carps and catfishes (order Cyprini-formes). All possess a swim bladder connected to the gut, cycloid scales, and well-separated pectoral and pelvic fins.

The 2,000 species of carplike fishes include most European coarse fishes, such as carp, roach, bream, dace, and tench, plus the nonEuropean goldfish. Carp lack jaw teeth, but possess teeth on the pharyngeal (throat) bones. In the common carp, these crush the plant material and invertebrates on which it feeds. The mouth can also be protruded, which allows carp to suck up food particles effectively. Originally from central Asia, the common carp has been widely distributed and farmed. In 1963, another species, the herbivorous grass carp, was brought to the United States from Asia to help control the overgrowth of certain water plants.

Unlike the carps, characins—which include the brightly colored piranha and tiger fish-have well-developed teeth. Piranha and tiger fish are native to South America and Africa, respectively. Many species of characins, however, like the tetra, have been distributed worldwide to tropical fish enthusiasts, who prize them for their colorful markings.

Catfishes (order Siluriformes), in contrast, are not colorful. They are mainly nocturnal bottom-dwelling fishes. Consequently, their eyes are small, but they bear up to four pairs of conspicuous barbels around the mouth that are sensitive to touch and taste.

All of this group of carps, characins, and catfishes (superorder Ostariophysi) share two interesting features. The first is that they possess a series of three bones (the Weberian ossicles), which connect the swim bladder to the inner ear. The gas-filled bladder picks up and amplifies sound vibrations in the water, which are then transmitted to the ear, giving these fish excellent hearing. To match this, some catfish can also produce sounds by drumming on the swim bladder or scraping the spines of the pectoral fins.

The groups’ second common feature is that they all have a similar “fright” reaction. When one fish is injured, its skin cells release a chemical into the water. Other individuals react to this by either fleeing or hiding, so that if a predator attacks one member of a shoal, the others can escape.

Moray eels are found in subtropical and tropical seas, where they hide among rocks and in crevices, except when hunting for food. Most morays are aggressive. All have sharp teeth, and some species have glands in the mouth that pour poison into the wound caused by the moray’s bite, to disable or kill the prey.


Another commercially important group, including the Atlantic cod, haddock, and hake, are known collectively as codfishes or gadids (order Gadiformes). They are essentially cool sea fishes, living mainly in the northern waters of the Atlantic and Pacific oceans. Hake, however, also live in deep water, and have invaded subtropical areas. At night, they rise to the surface to feed on other fish, such as herring. Growing up to 1 foot (30 centimeters) in length, hake now form the main catch of fishing industries in Europe, South Africa, and South America.

Spiny-finned fishes

The most advanced teleosts have spiny rays in the dorsal, pelvic, and anal fins. Advanced features of teleosts include ctenoid scales, a swim bladder not connected to the gut, and pelvic and pectoral fins placed close together.

There are more than 7,000 species of perch-like fish of the order Perciformes, which include the angelfish, butterfly fish, mackerel, and tuna. They are active, often brightly colored fishes, with good eyesight and color vision.

The European perch is an active predator, feeding on other fish—as is the much larger Nile perch, but within this group all manner of feeding specializations are found. One family, for example, Cichlidae, or the cichlids, are particularly diverse. In southeastern Africa’s Lake Malawi alone, 200 different species of cichlids coexist, feeding according to their separate tastes on mollusks, insects, fishes, eggs, plankton, algae, or even the scales of other fishes. In each case, the pharyngeal teeth appear specially adapted to the diet.


Many teleosts are flattened from side to side, but this is taken to extremes in the flatfish (order Pleuronectiformes).

The larvae of flatfish are conventional in appearance, but as they grow they gradually become compressed and turn over on one side. One eye begins to move closer to the eye on the opposite side of the head, and the mouth becomes twisted. The underside becomes white, and the upper surface darkens.

This shape is ideal for the flatfish habitat. Most live on the seabed in coastal waters, and some, such as the plaice, possess the ability to alter their coloration to match their surroundings. Commonly eaten flatfish, such as sole, are small, although others, such as halibut, reach weights of 400 pounds (180 kilograms).

Eels migrate vast distances to spawn (right), and their larvae return to European rivers, where they spend their adult lives. Herrings migrate over short distances. The inset map shows North Sea shoals.