These are poikilothermic, aquatic vertebrates which carry out external fertilization.

They have bodies covered with scales and they move using fins.They breathe by gills.

Fish are streamlined in shape i.e. narrow at both ends and broadest in the middle. The streamlinedness reduces frictional resistance from the water.

Fish are divided into two according to the type of skeleton they possess:

Cartilaginous fish whose skeleton is made of cartilage for example the shark;

Bony fish or teleosts whose skeleton is made of bones for example Tilapia.

A Tilapia

Features on a fish:

Scales

Bony fish are covered with bony plates called scales which overlap one another so that their free ends point backwards.

These scales form a thin exoskeleton which protects the fish and allows it to move fast.

Under the microscope, rings can be seen in the scales and from these rings the age of the fish can be estimated from the distance from one ring to the next.

Mudfish and catfish are bony fish that do not have scales.

Nostrils:

These are not used for breathing as in mammals but are for smelling so that the fish can detect food in water at considerable distances.

Eyes:

The eyes have large round pupils which cannot vary in size. The eyes do not have eyelids and they give the fish a complete range of vision.

Hearing:

There is no visible external ear but only an internal ear with no oscicles or cochlea.

The internal ear is used more for balancing than hearing.

The mouth:

It serves for taking in food and for breathing a current of water.

It opens to show a wide gape. Food particles are prevented from escaping through the opercular opening by the gill rakers found on the gill arches (gill bars).

These gill rakers filter food from the water. Such a type of feeding is called filter feeding.

The gill rakers also protect the gills from damage by food particles.

There are four gills on each side of a fish.

 

The operculum

This is a gill cover which is a bony structure covering and protecting the gills.

It also assists in breathing by opening and closing when necessary.

The lateral line:

This is a series of tiny pores from a fluid-filled tube or canal just below the skin.

 The function of the lateral line is to detect movements in water.

-          So a person's hand moving in water will cause vibrations of the current.

-          These water current vibrations will pass into the pores of the lateral line.

-          So the fluid in the tube or canal will also vibrate.

-          Nerves lining the tube will be stimulated by these vibrations to produce impulses.

-          The impulses will then be sent to the brain so that the fish can detect the direction of water movement.

This helps it to navigate around obstacles or avoid enemies even if vision is impaired in muddy water.

The fins:

The difference between fish and other vertebrates is that fish have fins.

Fins consist of skin with bony rods inside.

There are two types of fins:

Paired fins: These represent the limbs and they include the pectoral fins and the pelvic fins.

Unpaired fins: These consist of one dorsal fin, one caudal fin and one ventral or anal fin.

The dorsal and ventral fins are also called median fins. 

The fish called the Nile perch has got two dorsal fins.

 The swim bladder:

Bony fish have in their body cavity a long air-filled bladder running below the backbone.

This swim bladder opens into the oesophagus.

The size of the swim bladder may be adjusted so that the density of the fish increases or decreases.

If the fish is to go deeper in the water where water pressure is high, it gulps in air into the swim bladder; and so its density will be high.

If the fish is to swim on top where the water pressure is low, it releases air from the swim bladder through the mouth so that the density of the fish is decreased.

So the swim bladder helps the fish adjust to different water pressures and makes it buoyant.

Therefore, unlike the shark (where there is no swim bladder), bony fish do not sink when they stop swimming.

In the lungfish, the swim bladder is used as an extra breathing organ i.e. the swim bladder acts as a lung. This is so because the fish stays in poorly oxygenated water like in swamps.

Fins and Swimming:

Rapid swimming in bony fish is effected by the side to side movements of the tail and the caudal fin.

The tail part can move because the joints of vertebral column allow some movement enough to make the spine flexible.

When the fish is moving fast, the pectoral and the pelvic fins are kept folded against the body.

The pectoral and pelvic fins are for:

• Steering the fish in the right direction;
• Balancing the fish;
• Enabling the fish to slow down then stop;
• Controlling the pitch of the fish i.e. causing it to swim upwards and downwards;
• Slow backward and forward movement when the tail is stiff;

When the fish is resting, it may move the pectoral fins gently forwards and backwards to create a current of water to help it breathe.

The median fins i.e. the dorsal and the ventral fins keep the fish vertical by providing a large surface area; and so they prevent the fish from rolling over.

The dorsal fin also prevents the fish from yawing side-to-side movements.

Breathing in fish:

To push water into the mouth:

      To push out water:

Colour of fish:

Fish are coloured in such a way that it is difficult for their enemies to see them.

Tilapia which swim near the surface of the water are silvery white on the undersurface and darker above.  So when viewed from below they appear like the surface of the water which is silvery white when light shines on it.  The darker upper parts make them difficult to be seen against the dark water.

The development of colour which is similar to the surroundings is called camouflage.

Reproduction in fish:

Eggs and sperms are deposited in water where fertilization takes place i.e. external fertilization.

Many fish do not care for their young ones except Tilapia where the female protects the young and even carries them in her mouth when there is danger.

Otherwise in many fish the young are not cared for and can be eaten by carnivorous animals and may even be eaten by their own parents.

However, some young fish still survive because very many eggs are laid for example the herring lays 30 to 40 million eggs per season.

Migration in Fish:

Many fish move from deep to shallow water before laying eggs so that the young are likely to survive since the water is too shallow for their large enemies to enter.

For example:

The salmon of Europe and America leaves the sea to go and lay eggs upstream in the river.

The eggs hatch and the young stay in water for two years and when mature they go downstream to the sea for one year.

Later at sexual maturity they come back upstream to the river to lay eggs.

Some fish allow seasonal migration for example mud fish travel upstream when the rains start and back downstream when the dry season comes.

When the pool dries up some fish do not migrate for example the lung fish which relies on respiration from the swim bladder.

Bony fish of special interest:

Flying fish: These are found in tropical seas and they have broad wing-like pectoral fins and by swimming fast below the surface of the water they can glide into the air with spread fins for almost 200 metres.  This helps them escape enemies and ships.

But their pectoral fins do not flap like the wings of birds.

The lung fish: These are found in Africa, South America and Australia.  They live in dirty pools of water where there is very little oxygen; in fact where any other fish would die.

They use their swim bladder as lungs.

When there is a drought it buries in mud and later the mud hardens into a protective crust.

So the lung fish is said to be aestivating - a state of dormancy where all living processes including respiration are more or less at a standstill.

When the rains come the mud softens and the lung fish emerges undamaged except that it has lost some weight.

Differences between cartilaginous fish and bony fish:

The scales of cartilaginous fish are stronger than those of bony fish.

The fins of cartilaginous fish have plates of cartilage instead of rods of bones as in bony fish.

Cartilaginous fish do not have an operculum as in bony fish; instead they have gill slits.

Many cartilaginous fish have eyelids whereas bony fish do not have eyelids.