The Watery World of Fish

Fish is the common name for 29,000 species of cold-blooded, backboned animals that live in freshwater and ocean environments. Fish breathe using gills. Most have a body covering of scales and move using fins. Many fish hunt other fish for food. Fish are also an important food for other animals such as humans, birds, seals, and whales. Overfishing by humans has now made some fish endangered.

The Big Three

Living fish are grouped into one of three categories:

Jawless fish – These are long snake-like fish with no scales. They have a suction mouth with teeth they use to hold on to prey. They live in freshwater and seawater. Jawless fish first appeared on Earth over 500 million years ago. Only two groups still survive: the hagfish and the lampreys.

Jawless fish ©Benjamin Cummings

Cartilaginous fish – These include sharks, rays, and skates. All have a skeleton made of cartilage (not bone, like mammals), specialized rows of teeth, and scales that look like tiny pointed teeth. They live in freshwater and seawater.

Cartilaginous Fish©J.Stafford-Deitsch/

Bony fish - Most of the world’s fish species are bony fish. Bony fish like goldfish, tuna, tilapia, and salmon have long tapered bodies that are excellent for powerful swimming through ocean waters, lakes, and in rivers and streams.

Bony Fish ©G.Ellis/

Some Fish Really Get Around

Fish are pretty amazing. They can live almost anywhere where there is water. Some live in the ocean or in freshwater habitats. A few species can live in both. There are even some species, like the mudskipper, that can spend short periods of time on the land before they must return to the water.

Fish play a different role in each environment they live in. In the Amazon, the large river floods each year, rising over 15m. During flooding, the waters of the river flow through the trees of the tropical rainforest. For the fish in the river, it means they can swim in the treetops. Some fish, like the Pacu (PAH-koo), use flood waters as a chance to eat fruit and seeds. As they eat and swim, they spread new seeds everywhere. This helps the tropical rainforest stay healthy.

Fish have adapted to every environment, including the icy waters of Antarctica. Fish that live there have substances in their blood that work like antifreeze. This keeps the fish from freezing solid in water that is close to 0ºC!

Prehistoric Fish

Scientists believe the first fish with jaws and teeth evolved over 400 million years ago. These first fish were simple in design. But they had many features similar to the sharks and rays living today.

Fish have been around for millions of years. They have evolved into many different species that are able to survive in different aquatic environments. Scientists look at fish fossils to figure out what ancient fish looked like and where they lived.


Just Right for Water

Fish require water for survival. Everything from breathing air in water to swimming requires fish to have features and organs different from those of animals that live on land.

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©W.Meiderts/Foto Natura/Minden Pictures

Born to Swim

Fish have many different methods for having young, or spawning. Some lay eggs. Others give birth to live fish that are ready to swim. Some newborn fish swim off with their egg sac still attached. The sac supplies the new fish with nutrients in the early days of life. After a few days, the fish’s body absorbs the egg sac. The little fish, called a fry, must now find its own food.

Survival by the Numbers

Most fish survive by producing thousands or millions of eggs. The female spreads her eggs and the male follows behind to fertilize them in the open water. The eggs all hatch about the same time. Baby fish fill the water. Predators such as other fish, frogs, and birds eat many of them. Because there are so many, a few escape the predators and survive to grow up.

Seahorse Fathers

Seahorse fathers carry eggs in a pouch while they are developing. After hatching, the baby seahorses may hang out around dad for a day or two before swimming off on their own.


A Little Help From Mom and Dad

Most fish must survive on their own after hatching. A few baby fish get some help from their parents. Seahorses’ eggs develop in a pouch on the father’s abdomen. After hatching, the baby seahorses exit through an opening. They often spend a few days clinging to their father before swimming off on their own. Some species of cichlids (SIK-lids), a type of fish found mainly in African lakes, are mouthbreeders. The parents of these fish carry eggs inside a pouch in their mouth until they hatch. For a short time after hatching, the parents let the fry hide in their mouths for safety.

The All-Day Buffet

Fish spend all day searching for food. The various species of fish find and eat food in many different ways. Some use their teeth to catch other fish and animals. Then they swallow their meal whole. Sharks usually tear off bites and swallow. Seahorses use their mouths like a straw to suck up little shrimp and fish. Some fish surprise prey with a giant gulp of their huge mouth that sucks the prey in instantly.

A Free Cleaning With Every Meal

Cleaner fish (wrasses) hop on larger fish that come to their “cleaning station.” The cleaner fish move all over the larger fish - even inside the gills and mouth. The cleaner fish use their sharp teeth to bite off and eat dead skin and parasites on the larger fish.

Luring Them Right In

Anglerfish are fish with their own built-in “fishing poles.” A piece of skin hanging from the front of their heads serves as a fleshy fishing lure. When a smaller fish gets close to the lure thinking it has found something to eat, the anglerfish lunges forward and swallows it.

Don’t Go Toward the Light!

Deep in the ocean where there is little or no light, some fish have tiny lights in their bodies that attract other fish. These deep-sea fish often have sharp, needle-like teeth and huge mouths. When the prey comes close to investigate the light, they are gobbled up.

Fish as Food for People

For centuries, fish have been an important food source for people in many countries. Fish live in nearly every aquatic environment. People fish the oceans, rivers, and lakes to catch fish they can eat.

People even raise fish like tilapia (ti-LAH-pee-uh) in small pools. This is called aquaculture. It helps supply fresh food to many people around the world. As the population of the world increases, many people believe aquaculture will be one of the only ways everyone will be able to have a supply of fresh meat. Some globally important food fish are tuna, salmon, sardines, cod, snapper, tilapia, catfish, and perch.

Too Much Fishing

Anemone fish

Anemone fish use anemones for protection from predators. They also build their nests in an anemone. Doing this protects the eggs while they are developing into little anemone fish.


Many people make their living by fishing. In 2004, the total catch of fish from the world’s oceans was over 75 million metric tons. People are becoming concerned that we are taking too many fish from the sea. Scientists believe that 70% of the fish caught in oceans are close to becoming overfished. So many fish are being caught that the fish cannot reproduce fast enough to replace the numbers caught. This has caused fish populations to decrease.

Another problem is by-catch. By-catch is made up of fish and other animals that a fishing crew did not intend to catch. Usually, by-catch is caught when fishermen use giant nets. These huge nets help catch more fish faster, but they sometimes catch endangered species, such as sea turtles. They also catch other fish and animals like dolphins. The fishing crew throws these fish and other animals back into the ocean, but most do not survive.

Keep the Fish Around


Overfishing is now one of the greatest threats to fish species that are caught for food. Overfishing occurs when so many fish are caught that the fish cannot reproduce fast enough to replace the numbers caught. Overfishing causes fish populations to decline.


In many places, fish are a critical part of the food chain. Without fish, many other species - in the water and on land - would struggle to survive. The role fish play in the health of oceans, rivers, and lakes around the world is still not well explored.

Scientists do know that some fish populations are decreasing rapidly. Many nations have placed limits on the numbers of certain fish species that can be caught to help address the issue of overfishing. Some fishing crews now use fishing methods that cut down on by-catch. But much more needs to be done. Scientists and environmentalists are urging nations to work together to help fish populations recover and to develop new conservation measures.