My Science School

Fish, mammals, and birds all have something in common - a backbone. They belong to a group of animals called vertebrates. But some of the most fantastic creatures in the sea are animals that have no backbones. They belong to a group of animals called invertebrates.

Some invertebrates are huge, like the giant squid. Others are so tiny that you need a microscope to see them.

Some invertebrates, like crabs, clams, and sea spiders, have hard shells. The shells protect their soft bodies. Other invertebrates, like starfish or sea urchins, have spikes to protect them. Jellyfish and other invertebrates are soft all over. Jellyfish look like see-through blobs. They float gracefully through the sea. Their stinging tentacles are their protection!

Whether they have shells, spines, or stinging tentacles, invertebrates are some of the most interesting creatures in the sea!

There are thousands of kinds of fish. If you tried to choose the most fantastic fish, you’d have a hard time.

The mudskipper has a head like a frog and a body like a fish. It often crawls onto land. It can jump up and catch flying insects in its mouth.

The archerfish shoots its food down with water. The archerfish swims at the top of the water until a beetle comes buzzing past. Then the fish shoots a stream of water from its mouth. The water hits the beetle with such force that it falls down into the water. The archerfish gobbles it up.

The elephant-nose mormyrid is well named. Its long nose looks Re an elephant’s trunk.

Leafy sea dragons are odd-looking, too. Their spiky fins make them look like pieces of seaweed.

A gulper eel looks just like a huge mouth with a long, tapering tail attached to it. It can open its mouth wide enough to swallow a fish much larger than itself.

A flounder has both eyes on one side of its body. And let’s not forget the glass catfish. You can see its insides.

These are just a few of the strange fish that live in the world. Which do you think is the most fantastic?

Many people fear sharks. But most sharks are harmless. In fact, the biggest sharks of all, the whale shark and the basking shark, eat only small fish and plankton.

Great white sharks and tiger sharks are more dangerous. They eat almost anything, from rubbish to big fish, seals, and sea turtles. Another dangerous shark is the hammerhead. It has a wide head shaped like a hammer, with an eye at each end. These sharks have been known to attack people.

Sharks are different from most other fish in several ways. Their skeletons are made of a tough, rubbery substance called cartilage, not bone. And the scales of sharks are not smooth like those of other fish. They are like millions of tiny, rough teeth that protect the animal. Shark’s gills are open, not covered like those of other fish. They look like slits on each side of the shark’s body. Sharks don’t have an air bladder like other fish, either. Instead, their livers are filled with oil, which is lighter than water. Also, gulping air helps them stay afloat. Even so, if a shark stops swimming, it will sink!

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Grunt, croak, snore, squeak, click, and roar - the ocean is a noisy place! Many of these sounds are made by fish.

Some fish are named after the sound they make. One kind of fish rubs its teeth together to make a grunting noise. That fish is called a grunt. Another fish is called a croaker. Can you guess why?

Pollock, haddock, angelfish, grouper, and many other fish also make grunting noises. They grunt by vibrating some of the muscles against the swim bladder, an air-filled sac inside their bodies. The seahorse makes a clicking noise by hitting a bone on its head against a bone on its back.

Sharks sometimes make a roaring sound. But they aren’t really roaring – they’re burping! Many, sharks swallow air to help them stay close to the surface. When they want to dive deeper in the water, they have to burp up the air. The burp sounds like a roar.

Scientists study fish sounds to see if they mean anything. They say that many sounds seem to be made by male fish calling to female fish. Others are made by fish getting ready to fight.

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Most fish eggs never hatch. Many wash ashore and dry up, and many are eaten by other fish. So, in the dangerous world of the sea, some male fish give their eggs extra care.

A male smallmouth black bass cares for its young even before they’re hatched. He makes a wide, saucer-shaped hole in the sand where the female bass lays her eggs. The eggs are sticky. They stick to the sand and do not float away.

After the female lays the eggs, the male guards them all by himself. He also fans them with his tail. This keeps the water around the eggs fresh and helps them hatch.

When the young hatch, the male watches over them as they learn to swim. He fights anything that comes near. Later, the male guards them while they find food.

A male jawfish keeps eggs in his mouth. After they hatch, he holds the young in his mouth until they’re big enough to live on their own. Then he spits them out into the water, and away they swim.

If an animal or plant lives in the water, chances are it is food for a hungry fish.

In the ocean, most fish eat only other fish. Large ocean fish, such as cod, hake, tarpon, and tuna, dine on smaller fish - herrings, sardines, and anchovies. Of course, sometimes the bigger fish are food in turn for sharks!

In rivers and lakes, fish eat fish, too. But some add other tasty things to their diet. Trout jump out of the water to snap at flying insects. Big hungry bass, pike, and bowfins gobble up frogs, baby ducks, and even baby muskrats.

Some kinds of fish eat plants. Carp and suckermouth catfish swim along the bottom of rivers and ponds. With tiny teeth, they chew up bits of plants that grow in the mud.

Some fish eat plants, animals, and other organisms. Parrotfish eat plant-like organisms called algae and tiny worms that live in coral. The ocean sunfish eats small shrimp, baby fish, jellyfish, and algae.

And the largest fish of all eat the smallest food. Whale sharks, giant manta rays, and basking sharks eat only plankton.

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At the beach, some tiny brown minnows swim near your legs. They come closer and closer and then they turn, all at once. If just one fish senses danger, it darts away. Then the fish next to it turns in a flash. The fish copy each other so quickly that they seem to be moving at the same time.

Some kinds of fish swim in groups called schools or shoals. There might be thousands of fish in the school, but they act as one. They all swim the same way, at the same speed. Fish swim in schools to stay safe. One fish swimming alone is an easy meal for a bigger creature or an easy target for somebody’s fishing net. But a large group can confuse an enemy.

It is also easier for fish in schools to find food, because thousands of pairs of eyes are on the lookout. If one fish spots food and turns towards it, the whole school follows.

Not all fish live in schools. Fish that hunt prey, such as sharks, usually live by themselves. Other fish form schools only when they eat, rest, or lay eggs, or when they are young.

An angelfish swims in an aquarium. Its mouth opens and shuts, opens and shuts. This is how it breathes. Water goes in through its mouth and out through the gills at each side of its head. The gills take oxygen from the water, and the oxygen passes into the fish’s blood. Like all animals, a fish needs oxygen to live.

As the angelfish swims, it moves its tail from side to side. This helps it to move forwards. The fish uses its fins to swim, steer, and keep its balance. An air bag called a bladder helps keep it upright.

The angelfish seems to stare at you with wide eyes. Most fish have very good eyesight. With an eye on each side of its head, a fish can see in almost all directions at the same time.

The aquarium is kept in a place that is not too hot or too cold. The angelfish, like all fish, is cold-blooded. Its body is as warm or cold as the water around it. If the aquarium were in a cold place, the water would get too cold and the fish would die.

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Frogs and toads look much alike. But take a close look, and you’ll see some differences. Toads are fatter than frogs and have shorter back legs. A toad’s skin is rough and dry. A frog’s skin is smooth and moist. Most toads have bumps on their skin that look like warts. But you won’t catch warts if you touch a toad.

Frogs and toads have no tails. The amphibians called salamanders and newts all have tails. There are many kinds of newts and salamanders. Pygmy salamanders, which live in the U.S.A., are no longer than one of your fingers. Giant salamanders live in Japan and are longer than you are tall.

Some of the strangest of all the amphibians are the caecilians. Caecilians live in tropical lands, and they look like big, fat worms. Sometimes they are as big around as a person’s thumb and as long as a person’s leg - or even longer!

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Dolphins and seahorses live in water. A dolphin looks like a fish, but it isn’t. A seahorse doesn’t look like a fish. But it is.

How can you tell if an animal is a fish? All fish have a backbone, and all fish have gills. Gills are openings on a fish’s head that are used for breathing.

Most fish are covered in scales. Scales are little round or diamond-shaped pieces of hard skin.

And nearly every fish has fins on its belly, on its back or sides, or as part of its tail.

If it has gills, scales, and fins and lives in the water, it’s a fish!

In early spring, a female frog lays thousands of eggs in a lake or pond. A few days or even a few weeks later, tiny wiggly tadpoles come out of the eggs.

The tadpoles swim about, nibbling at all plants. They still breathe with gills, like fish. But lungs for breathing air are growing in their bodies.

In time, the tadpoles grow little legs. Many tadpoles are eaten by fish and water insects.

After several months, the tadpoles can leave the water and breathe air with their lungs. They are now young frogs. Their short, tadpole tails shrink away and vanish.

By summer’s end, the frogs are fully grown. During winter they will hibernate at the bottom of the pond. In the spring, the females will lay new eggs.

The life cycle of many frogs takes the animals from egg to tadpole to frog in a matter of months.

How frogs and toads are like lions and tigers? They hunt for living things to eat. But these amphibians use the tongue, not sharp claws and teeth, to catch food.

Frogs and toads eat insects and worms and smaller frogs and toads, too. Big bullfrogs will eat small turtles, snakes, mice, and birds.

Frogs and toads eat only things that move. An insect might be safe right in front of a frog or toad if it didn’t move. But if the insect makes even the tiniest wiggle, the frog or toad will see it and gulp it down.

Many kinds of amphibians, including frogs and toads, use their long, sticky tongues to catch food. If an insect comes near it, the amphibian will slowly move closer and closer until - snap! Its tongue shoots out and pulls the insect into its mouth.

Some frogs are among the most poisonous animals. The brightly coloured arrow poison frog of the Amazon is only about 2.5 centimetres long, but its skin is deadly to predators.

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Can you think of an animal that lives in the water when it is a baby and on land when it grows up? That’s right, a frog!

Frogs belong to a group of animals called amphibians. Like reptiles, amphibians are cold-blooded. But unlike reptiles, an amphibian lays eggs that are soft and have no shells. These eggs dry up easily, so amphibians must lay them in water or wet places. Most baby amphibians are born in the water. They look like baby fish, and they breathe with gills like a fish.

When most amphibians grow up, their gills disappear. Then the amphibians come on land to live. They breathe with lungs as birds, dogs, and people do.

If it is cold-blooded, and lives the first part of its life in the water and the second part on land, it’s an amphibian.

Alligators and crocodiles are large reptiles. The largest crocodiles can reach 7 metres long! Alligators are smaller but still larger than lizards. Alligators and crocodiles usually live in shallow water in warm places.

Both of these reptiles are covered in tough, scaly skin. They have long snouts with many sharp teeth. Their jaws are very strong.

Alligators and crocodiles have a long tail plat pushes them through the water. Their webbed feet help them steer. Their eyes and nostrils are on top of their head. This lets them hide just under the water. An animal that gets too close will be dragged down into the water and eaten.

Both alligators and crocodiles usually eat birds, fish, and turtles, which they swallow whole. Crocodiles sometimes eat large animals, such as wildebeests. Crocodiles and alligators have both attacked and killed people. But attacks on people are rare.

A garter snake wiggles through the grass. Its skin has dried up and a new skin has formed beneath it. So the snake rubs its mouth against a tree trunk. The skin around its lips splits and opens up. But this doesn’t hurt the snake. Now its wrinkled old skin hangs from the tip of its tail.

Soon the snake comes to some rocks. With a twist of its tail, the snake crawls on, but the old skin stays behind. Away crawls the snake in the shiny new skin that grew beneath the old one.

Every few months a snake grows too big for its skin. Each time, it gets rid of its old skin by crawling right out of it.

Lizards cast off their skin, too. Because of their legs, however, they can’t get their old skin off in one piece the way a snake can. Lizards tear off their old skins in bits and pieces.