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Learn about fish

Updated on August 18, 2013

What is a fish

Fish are animals that live in the water. Most fish have scales and fins. They live in a wide variety of waterways. Some live in the salty oceans, others in the purest mountain streams. Most fish have long, narrow bodies. This makes them excellent swimmers.

How Do Fish Breathe Under Water?

Most fish have specialized organs called gills. Gills are used to exchange gases much the same way our lungs do.

When a fish opens its mouth it draws in a mouthful of water. It then pulls the sides of its throat together, forcing the water past the gills and back outside the body. Gills are made of threadlike structures called filaments. Each filament has tiny blood vessels that remove oxygen from the water and send it into the fish’s bloodstream. After the blood has traveled through the fish and returns to the gills it releases carbon dioxide from the blood back into the water.

Gills are hidden beneath a protective bony cover called an operculum or gill cover. Some fish, like, sharks, have multiple gill openings. Most sharks can’t move water over their gills with their mouth. They need to be swimming in order to breathe effectively.



Almost all fish have a special protective layer of scales. They are small and thin and look like clear plastic. Scales overlap and are like a suit of armor that helps fish avoid injury.

Some fish have big scales, others have scales that are so small you can’t see them without a microscope. Larger scales offer better protection, but make it harder for the fish to swim. Fish that swim quickly, or that live in fast moving waters (Trout, Tuna etc) tend to have small scales. Fish that swim slower, like carp, tend to have larger scales.

True scales continue to grow throughout a fish’s life. If you look at a scale under a microscope, you can see rings. Each ring represents one year of growth. A fish with four rings is four years old.

Sharks have a different kind of scale. They are more like a tooth. These scales don’t grow with the shark. Instead, as the shark gets bigger the space between scales grows and new scales grow in the empty spot. Hagfish and lampreys are fish, but they don’t have any scales.

All fish secrete a protective slime that covers their body. This is why fish feel slimy. The slime coat contains enzymes and antibodies that fight infection.

When a fish is handled or caught in a net the slime coat is disturbed and makes the fish vulnerable to disease or parasites.

Large and small

The largest fish is the whale shark. These fish live on microscopic plants and animals that it filters from the water.

The largest verified specimen was nearly 42 feet long.

The worlds smallest known fish has a name that's longer than it is. The Paedocypris progenetica lives in the forest swamps of Sumatra and is less than 8 millimeters long. It belongs to the carp family and is the smallest vertebrate animal in the world.

Whale shark




Underwater oddities

Seahorses are shaped like a horse and are one of the few fishes that mate for life. They are found in shallow tropical waters throughout the world, and range in size from 0.6 inches to 14 inches long.

Male seahorses have a brood pouch on their stomach. When mating, the female deposits her eggs into his pouch. He carries the eggs in his pouch until they hatch, then releases fully formed, miniature seahorses into the water.

They swim by using a small fin on their back that flutters up to 35 times per second. Even smaller pectoral fins located near the back of the head are used for steering.


Mantas might look scary, but they eat tiny sea creatures called plankton. They are the biggest of the ray family. Unlike most fish that swim by wiggling side to side, rays swim by flapping their “wings.”

Their flat shape makes it easy for rays to hide on the bottom of the ocean. They are related to sharks, but are usually friendly. Some rays have stingers on their tails that can cause a nasty injury that is very likely to get infected.

Something fishy

These animals might look like fish, but they are not. The killer whale (left), sperm whale (center), and the bottle-nosed dolphin (right), are not fish, they are mammals. They can’t breathe under water, but must come to the surface to breathe air. They have babies that are born alive and drink milk from their mothers.


What do fish eat?

The answer is almost everything. Some fish eat microscopic plankton, some eat only plants. many are voracious predators.

Most people think of sharks when they think of fish that eat other fish. Although sharks do eat fish, along with lots of other things, there are thousands of fish that live by eating other fish.

Gamefish are notorious for being aggressive eaters. That’s one reason they are easier to catch than fish that live on plants and small prey.

The largemouth bass is named after its mouth that is as big around as it’s body. Largemouths will try to eat anything that fits in their mouth. The video below shows bass being caught on a lure that is almost as long as the bass.

Fast forward to :40 seconds

Many fishing lures are designed to look like fish, crawdads, worms, or other types of food that fish regularly eat.

Carnivorous (meat eating) fish also eat insects, crabs, frogs, mice, and almost anything else that moves and is small enough to swallow.

Sharks have been found with old shoes and even license plates in their stomachs.


Activity for the kids

Activity - Cup of Fish

You will need the following for each person: One clear plastic cup, several edible “gummy fish,” blue colored jell-o type dessert.

Mix the jell-o type dessert as directed. Pour some of the liquid into each cup. Place the cups in a refrigerator and let the blue “water” start to set up. Before the “water” is completely set, place several different colored fish in each cup. Allow the “water” to completely set up. After you’ve examined your mini ocean, eat it.

Fish have a sixth sense

Fish have a special tool that helps them be aware of what is going on around them. It’s called the lateral line and it is used to detect movement and vibration in the surrounding water.

Lateral lines are usually visible as faint lines running front to back down each side of the fish. It goes from behind the gill cover to the tail. In the picture below you can see the white line in the middle of this brook trout’s body. This is his lateral line. It helps him avoid danger and locate food.


Fish are cold blooded

Almost all fish are exothermic. This means that their body heat comes from an outside source, usually the temperature of the water in which they live. During the winter, when many lakes are covered with a layer of ice and the water is very cold, fish will become less active. As the water warms up they will move around more, but when the water gets too hot they have no way of cooling down, so they again become inactive.

Some tuna, swordfish, and sharks are able to heat themselves above the temperature of the water they live in.

Activity: How fast is that fish?

In the swim of things

Fish come in many different sizes and shapes. Fish that are more streamlined can swim faster than those that are flat or squatty.

Salmon and tuna are fast swimmers. They can swim more than ten times their body length in a second.

Eels and rays are much slower. They can swim about half their body length in a second.

1. If you were a salmon one foot long, how far could you swim in ten seconds?

2. If you were an eel one foot long, how far could you swim in ten seconds?

Measure how tall you are and figure out how far you could swim in ten seconds if you were a salmon or an eel.

For the salmon multiply your height by ten. For the eel divide it in half.


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