LIFE CYCLES: Band 16/Sapphire

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Browse the complete Collins catalogue at

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An information book

LIFE CYCLES

Follow the amazing journey of the salmon, from the rivers they're born in, out to the oceans and back again, as you find out all about their and other animals' life cycles.

What is a life cycle?

An animal’s life cycle starts when it’s born.

The young animal grows and gets larger until it becomes an adult. Then the animal reproduces to create more animals. This marks the beginning of the next life cycle.

But larger animals, such as mammals, have much longer life cycles that take many years to complete.

For example, a female elephant may not be ready to reproduce until she’s 20 or more years old.

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life cycles that are completed within a few weeks.

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Some animals, especially insects, have very short

Mosquitoes have one of the shortest life cycles – about three days.

Some animals, such as the salmon, only reproduce once and then die. Others may reproduce many times during their lives.

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Many animals start life as an egg, for example goldfish, blackbirds

Insects, such as butterflies, have a more complicated life cycle.

and crocodiles. The eggs hatch into young animals that grow to

The butterfly life cycle starts with an egg that hatches into a larva

become adults. Other animals, such as lions, give birth to live young.

called a caterpillar. This is the growing stage of the life cycle.

However, caterpillars look nothing like adult butterflies. This means their bodies have to go through a stage where they’re completely rearranged to become adults. This amazing change is called metamorphosis.

caterpillar

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butterfly

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Salmon Salmon are large, powerful fish that are designed for speed.

The shape of their body is a bit like a torpedo – it’s long and gets narrower towards the tail. This streamlined shape slips effortlessly through the water.

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One of the largest salmon ever found was a chinook.

It weighed 38 kilograms.

All fish have gills for breathing in water. They use them to pick up oxygen in the water. The gills look a bit like feathers, and they’re red because they contain lots of blood. The gills are found behind the head and they’re protected by a flap.

Water enters the mouth and flows through the gills, where the oxygen is picked up by the blood. Then the water passes

Like all fish, a salmon has fins which help it to swim and stay upright in

water gill

out under the flap.

the water. There are two sets of paired fins, called the pectoral and pelvic fins, which help the fish to steer. There is a dorsal fin on the back, which

Salmon are unusual fish as they’re born in fresh water,

stops the fish from rolling over in the water. One of the most important fins

but spend their adult lives in salt water. There are

is the tail fin. This helps the fish to push through the water.

two main groups of salmon. Atlantic salmon, found in the Atlantic Ocean, and Pacific salmon, found in

tail fin

adipose fin

dorsal fin

the Pacific Ocean. There are different species, or types, lateral line

of Pacific salmon, including the cherry, chinook, chum, gill flap

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coho, pink and sockeye salmon.

mouth anal fin

pelvic fin

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pectoral fin

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Salmon lay their eggs in fresh water, that is, water we can drink and which

her side on the gravel and uses her powerful tail to dig her nest, which is

does not have much salt in it, unlike salty sea water. The female salmon

called a redd. With each beat of her tail, she removes a handful of

look for streams and shallow rivers where water flows quickly over gravel.

small stones. Then she lays hundreds of small, round eggs in the nest.

These places are called spawning grounds. Fast-flowing water carries more

The eggs are immediately fertilised by a male who is waiting close by.

oxygen than slow-flowing water, which is important as the eggs need

Then the female moves upstream a little and does this again. As she

a steady supply of oxygen, otherwise they die.

digs out another nest, the gravel is tossed over the eggs lying in

search for the perfect place to build their nests. Then, the female lies on

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The salmon arrive at the spawning grounds in the autumn and the females

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Laying eggs

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A large female salmon lays as many as 7,000 eggs.

the first nest. The gravel hides the eggs from predators. Each female builds several nests and lays thousands of eggs.

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Hidden in the gravel

eggs showing the eyes of the unborn salmon

When the tiny fish inside the egg are ready to

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hatch they move around and push out through The eggs lie hidden in the gravel for

the soft shell. They’re barely two centimetres

about three months. They have to survive

long and have a large yolk sac attached to

the icy water and at times they may be

the underside of their body. These tiny fish

buried under a thick layer of snow and

are called alevins and they remain hidden

ice. The eggs are pale pinky-orange with

amongst the gravel. For the next week or

a partly see-through soft shell. They’re just

two, they feed on the yolk which contains

a few millimetres in diameter. Inside, there’s

all the nutrients they need to grow.

a large egg yolk that’s rich in nutrients.

Eventually the yolk sac disappears.

After about four weeks, a large black dot appears – Now the alevins are ready to move away,

the eye of the unborn salmon.

but they have a problem. They cannot float Although the eggs are hidden amongst the gravel,

in the water yet. So, they beat their tails

only about ten in every 100 eggs survive and hatch.

and push themselves to the surface to gulp

Some die from disease, but most are eaten by predators

some air. The air is used to fill their swim

such as other fish, birds and raccoons. This is why

bladder, a balloon-like structure in their body

the female salmon lays so many eggs, in the hope that

that allows them to swim up and down in

a few will make it through the winter.

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the water. These small salmon are now called fry. raccoon

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Shark eggs Not all fish lay small eggs. Many sharks

baby dogfish swimming out of its mermaid’s purse

Other egg layers Fish are not the only animals

lay a small number of very large eggs.

that lay eggs. Other egg layers

Each egg is protected by a tough

include insects, amphibians,

rubbery case. The egg of the dogfish

reptiles and birds. Amphibians,

is called a mermaid’s purse. There are

such as frogs, lay eggs that are

four tendrils that wrap around

protected by a thick coat of jelly.

seaweed so that the egg

The jelly stops the eggs from drying

does not get carried

out and makes it difficult for

away.

predators to eat them. The common frog lays a mass of eggs called frogspawn. It takes less than two weeks for the eggs to develop adult dogfish

into tadpoles, which wiggle their way out of the jelly.

Insects, such as moths, also lay eggs.

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