Structure and Function. Biology article by D G Mackean (2022)

For illustrations to accompany this article see Fish, an Introduction

Fish are vertebrate animals, that is, they all have a vertebral column or ‘spine’. There are two main groups of fish, bony fish (Teleosts) and cartilaginous fish (Elasmobranchs). As the common names imply, the skeletons of teleosts are made of bone while the elasmobranchs have cartilaginous skeletons. The elasmobranchs comprise sharks, rays and dogfish which differ from teleosts in many respects. The teleosts are far more numerous, with a greater diversity of species than the elasmobranchs.

All fish are aquatic and breathe by absorbing dissolved oxygen in the water using their gills. The bodies of both teleosts and elasmobranchs are covered with scales but those of elasmobranchs are spiky and project through the skin. This makes the skin feel very rough, like coarse sandpaper. The scales of the teleosts have a flattened, discoid shape and are covered by a thin layer of skin and mucus which probably reduces friction between the body and the surrounding water and makes them very slippery.

Fish are commonly described as ‘cold-blooded’ but this is misleading. Their temperature varies with the temperature of their surroundings. Fish living in warm seas will have correspondingly warm blood

External features

Nostrils. The nostrils of fish do not open into the back of the mouth as do those of mammals, and are not, therefore, for breathing. They lead into organs of smell which are as a rule, very sensitive, so that a fish can detect the presence of food in the water at considerable distances.

Eyes. The eyes of a fish have large round pupils which do not vary in size.

Hearing. Although fish have no ears visible externally they can hear by transmission of vibrations through the body to sensitive regions of the inner ear.

Mouth. The mouth serves for taking in food; also for the breathing current of water. Some fish have a wide gape, and filter microscopic plants and animals out of the surface waters as they swim along, trapping them in gill rakers before the water is expelled from the operculum.

The operculum is a bony structure covering and protecting the gills in teleosts; it plays an important part in the breathing mechanism. Elasmobranchs do not have an operculum but there are separate gill slits for each gill.

The lateral line is a jelly-filled tube or canal just below the skin. It opens to the water outside by a series of tiny pores. Its function is to detect movements in the water. A disturbance set up, for example, by a person's hand moving in the water, will cause the jelly in the tube to vibrate. The canal is lined with nerve endings which are stimulated by vibrations and send impulses to the brain. In this way the fish is made aware of the direction and intensity of water movements. The sensitivity of this system makes even a blind fish very difficult to catch by hand.

Fins give stability, and control the direction of movement during swimming, as explained later.


The vertebral column consists of a series of vertebrae held together by ligaments, but not so tightly as to prevent slight sideways movement between each pair of vertebrae. The whole spine is, therefore, flexible. The muscles on each side of the spine contract in a series from head to tail and down each side alternately, causing a wave-like movement to pass down the body. Such a movement may be very pronounced in fish such as eels, and hardly perceptible in others, e.g. mackerel. The frequency of the waves varies from about 50 /min in the dogfish to 170 /min in the mackerel.

The sideways and backward thrust of the head and body against the water results in the resistance of the water pushing the fish sideways and forwards in a direction opposed to the thrust. When the corresponding set of muscles on the other side contracts, the fish experiences a similar force from the water on that side. The two sideways forces are equal and opposite, unless the fish is making a turn, so they cancel out, leaving the sum of the two forward forces.

The swimming speed of fish is not so fast as one would expect from watching their rapid movements in aquaria or ponds. Tuna seem to be the fastest at 44 mph, trout are recorded as doing 23 mph, pike 20 mph for short bursts and roach about 10 mph, while the majority of small fish probably do not exceed 2 or 3 mph.

Function of the fins in swimming

It must be emphasized that the swimming movements are produced by the whole of the muscular body, and in only a few fish do the fins contribute any propulsive force. Their main function is to control the stability and direction of the fish.

The tail fin, in its final lash, may contribute as much as 40 per cent of the forward thrust.

The median fins, that is, the dorsal, anal and ventral fins, control the rolling and yawing movements of the fish by increasing the vertical surface area presented to the water.

The paired fins, pectoral and pelvic, act as hydroplanes and control the pitch of the fish, causing it to swim downwards or upwards according to the angle to the water at which they are held by their muscles. The pectoral fins lie in front of the centre of gravity and, being readily mobile, are chiefly responsible for sending the fish up or down. The paired fins are also the means by which the fish slows down and stops.

Swim bladder

Teleosts have in their body-cavity a long air-filled bladder running just beneath the spinal column. The swim bladder makes a fish buoyant so that, unlike the shark or dogfish, it does not sink when it stops swimming. When the fish swims to a different depth the pressure needs to be regulated.

In some fish the bladder opens into the gut and the air pressure in it may be increased or decreased by gulping or releasing air through the mouth. In others, the bladder has no such opening, and the blood vessels surrounding it secrete or absorb air and so control the pressure in it.


Oxygen dissolved in the water is absorbed by the gills. The movements of the mouth floor and operculum are co-ordinated to produce a stream of water, in through the mouth, over the gills and out of the operculum.

There are usually four gills on each side consisting of a curved bony gill-bar bearing many fine filaments. Through the gill-bar run blood vessels which send branches into the gill filaments. The filaments bear smaller filaments down their length which, in turn, divide into smaller branches. So great a number of minute branches provides a very large surface area when the gills are immersed in water. The walls of the gill filaments are very thin, enabling the oxygen to diffuse rapidly into the blood. A convenient way of visualizing the gills is as an orderly system of blood capillaries exposed to the water in such a way as to absorb oxygen.

The mechanism for pumping water over the gills seems to vary in detail according to the type of fish but, in general, the pressure in the mouth cavity is reduced by the floor of the mouth being lowered. Sometimes the bony operculum is moved outwards as well, assisting the increase in volume. In either case the free edge of the operculum acts as a valve, being pressed against the body wall by the higher outside pressure and so preventing water from entering through the opercular opening.

Thus, water enters through the mouth to equalize the pressure. Next, the volume of the mouth cavity is reduced and the pressure increased by raising the floor of the mouth. A valve inside the mouth, formed by an inturned fold of skin, prevents water from leaving the mouth. The increased pressure forces open the operculum and expels the water through the opercular opening, causing it to pass between the gill filaments as it leaves.

Although there is more oxygen in air than in water, a fish will suffocate in air. This is probably because the muscular system of mouth and operculum which can work in water will not function in air. In other words, the valve system which is water-tight is not air-tight. Another important reason is that when a fish is out of water, the surface tension of the water-film covering the gill filaments sticks them together so that the total surface exposed is very much reduced.

Life history

None but the vaguest generalizations about reproduction apply to all fish. Most lay eggs though some are viviparous, that is, the young fish are born as free-swimming individuals and not in egg-cases or membranes. Fertilization is normally external but sometimes internal fertilization occurs. In many species of fish, once the eggs are laid, there is no parental care, but if there is, it is usually carried out by the male. Behaviour patterns between the male and female ensure that sperms are released at the same time as the eggs are laid. Sometimes a crude ‘nest’ is constructed; usually little more than a scrape in the bed of the stream or pond.

For illustrations to accompany this article see Fish, an Introduction

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