Fish: A Comprehensive Guide to Anatomy, Evolution, and Diversity

Fish Browse the article Fish

Introduction to Fish

Fish, a water-dwelling animal that breathes through gills and has a backbone, fins, and a two-chambered heart. Certain other animals that live in water—such as frogs in the tadpole stage—also have gills and backbones, but fish are the only backboned animals with two-chambered hearts.

Jawless fish were the first of all living vertebrates. There are more species of jawed fish—between 15,000 and 17,000—than of any other group of vertebrates.

Exceptions can be found to almost everything that can be said about fish in general. Some fish have no bones, some lack eyes, others have no jaws, and some have no scales. Some fish are brilliantly colored, some have luminous spots, and some are drab. Of those fish that have lungs and can breathe air, some drown when kept under water. Some species can live a few hours out of water. Although the overwhelming majority of fish are cold-blooded (their body temperature is the same as the temperature of the surrounding water), a few are warmblooded (have a body temperature higher than the temperature of their surroundings).

The saddleback butterflyfish flaunts brilliant colors and patterns.

Many fish are edible and are an important food for humans. The nutritive value of fish is equal to that of meats. Fish is an excellent source of protein. Its fat content ranges from .1 per cent to 26 per cent. Fish is a good source of minerals, especially iodine and phosphorus. Fish-liver oils provide vitamins A and D, and fish is also a source of thiamine, niacin, riboflavin, omega-3 fatty acids, and three of the eight amino acids essential to human nutrition. The belief that fish is “brain food” is without basis.

Catching fish is a popular sport as well as an important industry. Many people keep tropical fish in home aquariums. Fish of many kinds are exhibited in public aquariums and aquariums maintained by educational institutions.

The branch of science concerned with the study of fish is called ichthyology (from the Greek for “fish” and “discourse”). Scientists who study fish are called ichthyologists.

The Body of the Fish

The body of a typical fish tapers from the middle toward both ends and is slightly compressed, or flattened, from side to side. Eels, however, look like snakes. Sunfish are considerably compressed from side to side, while flounders are greatly compressed from top to bottom. There are many other variations from the shape of the typical fish.

Guppy females are gray whereas males have colorful splashes, spots, or stripes.Scales

Most fish are covered with horny scales that overlap like shingles and help to protect the body from injury. The placoid scales of sharks and related fish do not overlap. They are embedded in the skin, and each bears a tiny toothlike projection, or denticle. Bowfins and others of their group have hard, bony ganoid scales that meet edge to edge. Some catfishes and some other species have no scales.

As a fish ages, its scales grow in size but not in number. The scales of many fish bear annual rings similar to those of a tree trunk. From these rings the age of the fish can be determined. A thin layer of skin covers the scales. This skin secretes mucus, a slimy substance that helps protect the fish from bacterial and fungal infections.

Fins

Most fish have both paired and unpaired fins. The paired fins are the pectoral fins, which grow on each side just behind the head; and the pelvic fins, which grow on each side farther back on the body. The paired fins are usually used in steering and braking, but in some fish, such as the seahorses, they are important swimming organs.

The unpaired fins consist of one or more dorsal fins, which grow along the back; an anal fin, at the rear of the anus; and a caudal fin, which grows at the end of the tail. Dorsal and anal fins serve as keels, to keep the fish upright. The caudal fin is used as an aid in swimming.

Some fish, such as eels, swim by making a series of muscular contractions which alternate on each side of the body. Most other fish swim by lashing their tails from side to side. When the tail is whipped in one direction, the front part of the body tends to turn the opposite way. Water pressure resists the turning movement and changes it into a forward motion.

Gills

Fish breathe through four or more pairs of gills, organs that are situated on each side of the body just behind the head.

Eyes

Almost all fish focus their eyes by using their eye muscles to change the position of the lens rather than by changing the shape of the lens or cornea, as do the higher vertebrates. However, some sharks and rays are capable of slight changes in the shape of the lens.

Most fish are nearsighted. Sharks are farsighted. Some fish have eyes mounted on tubes that can be moved to and fro. Some fish can see colors.

The eyes of the four-eyed fish, which habitually swims on the surface with its eyes half submerged, have divided pupils. The upper half of each eye is adapted to seeing in air; the lower half can see in water. Archerfish and some other species can see well in air. Many fish that live in caves and in the depths of the ocean have no eyes.

Mouth and Teeth

The mouth is usually large in flesh-eating fish and small in plant eaters. In skates and most sharks and rays, the mouth is located under the body. In suckers and sturgeons the mouth is also under the body, and can be protruded to pick up substances from the bottom.

Various fish species have teeth adapted to piercing, cutting, crushing, and chopping. Some fish have no mouth teeth. Several, however, have grinding teeth at the top of the gullet, the tube through which food passes to the stomach.

Elephant-nose fish have specially adapted mouths to root out food from murky river bottoms.Hearing Organs

Fish have inner ears but lack external and middle ears. They can hear sounds made in water, but they hear only faintly sounds made in air. In some fish, the gas bladder serves as a resonator that aids in hearing.

A fish's sense of hearing is supplemented by an organ called the lateral line, which senses vibrations in the water. The position of this organ is indicated in many species by a line extending along each side of the body. The lateral line also branches over the head, though the branches are usually not visible.

The lateral line is a canal just under the surface of the skin. It is connected to the surface by numerous pores, each of which is located over a group of cells sensitive to pressure changes. The cells are connected to a nerve, deeper in the body, that leads to the acoustico-lateralis area of the brain.

Gas Bladder

Within the belly of many fish is a gas bladder (also called an air, swim, or sound bladder) containing gases absorbed from the blood. In most fish the gas bladder absorbs or gives off gases to enable the fish to float at any depth of water without effort. In some fish, such as the lungfish, the gas bladder has evolved into a lung.

Smelling and Tasting Organs

It is difficult to distinguish between the senses of smell and taste in fish, and they may be identical. Certainly fish have keen smell-taste senses, for a large area of a fish's brain is given over to these senses. Fish have sensory nerves within their nostrils, which in most species consist of a pair of double sacs that open only outward. Other sensory nerves are found within the mouth and around the snout. Still other sensory nerves associated with smell and taste are distributed over the body of a fish. Catfish and some other species have additional taste organs in their barbels (fleshy whiskers that trail from their chins or snouts).

Reproduction and Growth

Reproduction

Fish reproduce from eggs. In herring, mackerel, and many other species, spawning (the act of reproduction) is a mass enterprise. Gathered in huge schools, the females expel eggs and the males discharge milt (sperm) into the water. In some species the eggs float; in others they sink. In all species thus spawned, vast numbers of eggs and young are eaten by fish and birds or are otherwise destroyed. However, each female produces thousands, even millions, of eggs at a spawning.

Many fish lay their eggs in nests. The nests may be depressions in the beds of streams and lakes, such as those made by salmon and bass. The stickleback's nest is made of plant material. Fighting fish use floating bubbles as nests.

The eggs and young of nesting fish are commonly guarded by one of the parents, usually the male. The males of certain ocean catfishes carry the eggs, and later the young, in their mouths for as long as six weeks, fasting until the young are large enough to fend for themselves. Male sea horses and pipefish carry the eggs in abdominal pouches similar to those of kangaroos.

In some species, especially shark species, the females give birth to living young from eggs that are fertilized and hatched within their bodies.

Guppies give birth to live young called fry.Growth

Unlike most other vertebrates, fish grow as long as they live and eat, though they grow more slowly as they age. The rate of growth varies greatly, being most rapid where food is most abundant. Growth is commonly quicker in warm regions than in cold.

Fish have a wider range of size than any other group of vertebrates. The smallest fish known, the pygmy goby of the Philippines, averages less than one-half of an inch (13 mm) at maturity. The largest is the whale shark, which is estimated to reach a length of 60 feet (18 m) or more. (The largest of all sea-dwelling animals, the blue whale, is not a fish but a mammal.)

The normal life span of fish varies greatly. Some die after one year. Certain large fish such as sturgeon, lake trout, and Atlantic halibut may live for decades.

Where Fish Live

Fish have adapted to many different habitats throughout the world. They live in mountain lakes at high altitudes as well as at depths of about seven miles (11 km) beneath the surface of the ocean. Polar species live in temperatures of 29° F. (-2° C.), while some fish, such as those found in Death Valley, live in temperatures of 104° F. (40° C.).

The climbing perch (also called the walking fish) has a supplementary breathing apparatus that enables it to take oxygen from the air. It also has gills, but they are not adequate to sustain life and the fish will drown when kept under water for a few hours. As long as its breathing apparatus is kept damp, it can spend several hours out of water. This fish uses its fins and movable, spiny gill covers as a means of locomotion on land. Certain blennies and other fish also spend much of their time on land. Lungfish hibernate in mud during the dry season. Their skin secretes a substance that keeps the skin moist for up to three years.

The climbing perch uses its fins and spiny gill covers to move on land.

Some Habits of Fish

Feeding

Some fish feed on plants, others on animals, and still others on both plants and animals. Small fish are the prey of larger fish, which in turn are eaten by still larger fish. Fish also feed on crustaceans, mollusks, plankton, and insects. Large sharks occasionally eat seals. The chiasmodon, a fish that inhabits deep ocean waters, is able to swallow fish twice its size.

A number of fish have unusual methods of capturing prey. The archerfish shoots down flying insects by squirting water at them. Electric eels stun their prey with an electric shock. The common angler attracts other fish with a fleshy bait attached to a flexible spine on its head. When a fish nears the bait, the angler opens its huge mouth and engulfs its prey.

Defenses

Most shallow-water fish are protectively colored, dark on the back and light on the belly, thus making it difficult to see them from above or below. Flounders and some other fish can change color to match the background.

Many fishes depend on bursts of speed for protection, but few are able to swim far at high speeds. A sudden spurt is more useful to a fish than sustained speed in escaping enemies. Swordfish are capable of bursts of speed up to 60 miles per hour (97 km/h); bluefin tuna of 55 mph (89 km/h); blue marlin of 40 mph (64 km/h); and the great barracuda of 27 mph (43 km/h).

Some fish, such as sea horses and trunk-fish, are heavily armored. Catfish, stingrays, surgeonfish, and many other species are armed with sharp spines, which are venomous in many species. Electric eels, electric catfish, and torpedo rays defend themselves with electrical discharges.

Sleeping

Some fish sleep on the bottom; others rest on various objects, in crevices, or suspended in the water.

Migrations

A number of species of fish, including shad and salmon, migrate from the sea to fresh water in order to spawn. Others, such as eels, migrate from fresh water to spawn in the sea. Cod, herring, and a number of other species move southward in winter and northward in summer. Many other species migrate from shallow water in summer to deep water in winter. Some of these migrations are made in search of food, others in order to spawn.

Probably the most remarkable of all fish migrations is that of the European eel, which travels about 3,000 miles (4,800 km) to spawn in the Sargasso Sea in the North Atlantic. The eel larvae travel an equal distance to reach Europe. They accomplish the journey in about three years.

Salmon have been caught nearly 2,000 miles (3,200 km) from the rivers from which they entered the sea. The bluefin tuna probably travels farther than any other fish. It is believed to travel constantly throughout its life, seeking warm regions in winter and cool areas in summer.

How Fish Are Classified

There are many systems of classifying fish, some extremely complicated. The classification given here is basically the one adopted by the American Fisheries Society. Fish are grouped in the superclass Pisces, which consists of three classes:

The Agnatha:

lampreys and hagfishes. They have no jaws and have undeveloped backbones. They are the most primitive of all fish. Some biologists do not classify them as fish.

The Chondrichthyes:

sharks, skates, and rays. Fish in this class have no bones. Their skeletons are made of cartilage.

The Osteichthyes:

the bony fishes. They have skeletons composed partly or wholly of bone. This class is divided into two subclasses:

I. Sarcopterygii

fishes with lobed, fleshy fins, such as lungfishes and lobefins.

II. Actinopterygii

the higher bony fishes. This subclass consists of many orders, including:

1. Acipenseriformes.

2. Semionotiformes.

3. Amiiformes.

4. Elopiformes.

5. Anguilliformes.

6. Clupeiformes.

7. Salmoniformes.

8. Myctophiformes.

9. Cypriniformes.

10. Siluriformes.

11. Percopsiformes.

12. Batrachoidiformes.

13. Lophiiformes.

14. Gadiformes.

15. Atheriniformes.

16. Zeiformes.

17. Lampridiformes.

18. Gasterosteiformes.

19. Perciformes.

20. Pleuronectiformes.

21. Tetraodontiformes.