Fish: What Do We Really Know?

Fish are animals that live and breathe in water. Some fish live in the fresh water of lakes and streams. Some fish live in the salt water of oceans. A few fish can live in both kinds of water. All fishes are vertebrates, that is, animals with backbones. Most fish have a covering of scales to protect their bodies.

Fun Fact: Antarctic icefish can live in water below 0° Celsius. Desert pupfish can live in water that is hotter than 40° Celsius.

Kinds of Fish

• There are about 25,000 species of fishes.
• Fish come in all sizes and shapes. The enormous whale shark can be 40 feet long. The tiny goby is only 0.4 inch long.
• Most fish are called jawed fish. Hagfish and lampreys are jawless fish. They are more like eels. Eels are fish that look like snakes. Jawless fish do not have fins, scales, or jaws with teeth.

All other fish have jaws. There are bony jawed fish and cartilaginous jawed fish. Bony fish have skeletons made of bone. Cartilaginous fish have a softer material called cartilage instead of bone. Sharks, rays, and chimaeras are cartilaginous fish.

How Do Fishes Swim?

Most fish use their side, back, and tail fins for swimming. Fins are movable, fanlike parts. A fish moves through the water by swinging its tail fins back and forth. A fish uses its other fins to steer and to stop. Sometimes the other fins also help it go fast through the water.

Most fishes have a streamlined body. Their bodies are pointed at the head and at the tail. This streamlined body helps fish speed through water. Sailfish can swim as fast as 110 kilometers per hour.

Some other fishes have unusual shapes. Manta rays have flat, disc-shaped bodies. Large fins like wings go out from a ray’s head. To swim, rays flap the fins.

Most fish also have swim bladders. They fill their swim bladders with air. Their air-filled swim bladders act like life jackets. They keep the fish from sinking to the bottom.

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How Do Fishes Breathe?

Most fishes have gills for breathing. Gills have thin tubes that carry blood. As water goes into a fish’s mouth it passes over the gills. The gills take oxygen out of the water. All animals must have oxygen to live. Blood in the gills picks up the oxygen and takes it everywhere in the fish’s body. A waste gas carbon dioxide goes out of the fish’s body through the gills.

What Do Fish Eat?

Most fish eat meat. Some eat tiny plants. Sharks and other big fish eat smaller fish. Some sharks eat seals.

Some fish have large, sharp teeth. Sharks and other big fish grab their prey and bite it. Other fish suck prey into their mouths and use smaller teeth to grind them up.

Some fish do not have teeth. Their mouths sift tiny plants and animals from the water.

Fun Fact: In the deep ocean where sunlight never reaches, the bodies of some fishes have chemicals that make a glowing light to attract prey.

How Do Fishes Reproduce?

Female fish lay eggs. Most kinds of female fish lay their eggs right in the water. Males fertilize the eggs. Some fish lay their eggs in nests on the bottom of a lake or ocean and guard the nests.

Some male and female fish mate. Female guppies and sharks keep the eggs in their bodies until the baby fish hatch. Female pipefish and sea horses give the eggs to the male to take care of.

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