Movement in Water

Many kinds of animals move in water. Some animals stay on the surface of the water and use their limbs to propel them. Other animals move underwater. Water is more difficult to move through than air. This is because it is thicker than air (scientists describe water as being more dense than air). Because it is a thicker medium than air it offers more resistance to something which moves through it. We all know how tiring it can be to walk through water that reaches our waists! For animals to move through water efficiently they need to have certain adaptations which either allow them to push against it forcefully or limit the resistance of the water they are moving through.

Fish

Fish live in water all of the time. In fact they cannot breathe air and they die very quickly if they are out of water. They have special structures called fins which help them to move through the water.

The body of a fish is long and thin and pointed at both ends. This is called a streamlined shape. A streamlined shape moves faster through water than other shapes. This is because it can cut through the water more efficiently than other shapes.
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The tail (caudal) fin helps the fish to move forward quickly. the back (dorsal) fin keeps the fish in a straight line as it moves forward. The tail of the fish is mostly made of muscles. These muscles contract to move the tail from side to side. The pectoral and pelvic fins are in pairs. These are not used for fast swimming and are held close to the body. They are used when the fish is turning or is moving slowly. they help the fish to keep its balance.

Amphibians

Amphibians spend the first part of their lives in water. Tadpoles eat water plants. The adult frogs feed on land but they always stay near water and they swim very well.

Amphibians, such as the frog, have back legs with webbed feet. Webbed feet have skin joining the toes. This helps to push against the water.

Reptiles

Sea turtles spend their whole lives in water. Their limbs are adapted to allow them to paddle underwater.

A few reptiles hunt in water. Alligators and crocodiles are carnivores. When they make a kill they do not eat their prey all at once. The bury it at the bottom of the river and continue to eat it later when they get hungry.

When they are underwater their nostrils close and they have earlids which also close. They are able to keep their eyes open because they have special transparent eyelids which close over their eyes. Sea snakes, coral snakes and grass snakes are also good swimmers.

Mammals

Many mammals spend the whole or much of their lives in water. Unlike fish, however, they breathe air.

Dolphins eat fish which they chase through the water. Other mammals live much of the time on land but feed in water. Seals and sea lions also catch fish which they swallow whole. Small fish are eaten under water, but the seal and the sea lion come up to the surface in order to eat larger prey.

Dolphins are streamlined and their limbs have adapted into flippers. Flippers are very efficient paddles to help the dolphin steer through the water. There are no hind (back) limbs which gives the dolphin a more streamlined shape. At the end of the tail there is one large fluke. Seals and sea lions have similar adaptations. They cannot move very well or quickly on land but they swim very fast indeed.

Birds

Many birds swim well. The penguin cannot fly and seems to find it difficult to walk on land. In the water it can swim very fast to catch fish. Ducks swim on the surface of the water and they are excellent divers. They dive to the bottom of the pond and use their sensitive beaks to find insects which live in the mud. Many other birds dive to find or catch their food.

Ducks and other water birds have legs with webbed feet. Webbed feet have skin joining the toes. This helps to push against the water. These water birds also have a special oil which does not allow water to penetrate their feathers.

Invertebrates

Many invertebrates swim. The great diving beetle is a carnivore which feeds on such things as insect larvae and tadpoles. The water boatman is also a carnivore which swims on its back under water. The great diving beetle and the water boatman have back legs which are shaped like paddles.

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