They live in water and are found throughout the world in nearly every type of water, and there are 32,000 species of them. They are fish. A fish is a fish for several reasons.
The first being that they live in water such as lakes, oceans, rivers, ponds, streams, or even in a tank. They are cold-blooded, meaning their body temperature is the same as the environment where they live. Next, they are vertebrates, which means they have backbones, just like humans, and all fish have fins and gills to help them swim and breathe.
Humans have backbones or are vertebrates too, but people are not a fish because they do not live in the water, have fins or gills, and humans are warm-blooded, meaning the body temperature stays the same in all environments.
A fish must meet all the above conditions to be a fish. They live their entire lives in water, but all fish are not the same. A fish can be found in a wide variety of colors, sizes, and more. Some are found in freshwater only, like a lake, or just in saltwater, like an ocean. In addition, there are three different classes or types of fish: jawless, cartilaginous, and bony.
Jawless fish usually have long bodies and look like eels and they have no scales, and their skeletons are made of cartilage and not bones. Bony fish have skeletons made of bone, includes scales, a single pair of gill openings, and is the largest class of fish with over 20,000 species. Finally, cartilaginous fish have skeletons made of cartilage, not bone; no scales but they are covered with a tough outer skin and includes between 5 to 7 gill slits.
Though there are some organisms people sometimes confuse as fish because they live in water, scientifically, they are not fish. For example, whales, dolphins, octopus, starfish, and jellyfish are not fish because they do not meet all the reasons a fish is a fish.
A whale is a mammal because they have lungs and breathe air, starfish do not have fins or gills, a jellyfish and octopus have no backbones or vertebrates, and a dolphin is a mammal because they are warm-blooded, and they cannot breathe in water using gills like fish do.
In addition, other characteristics of fish include gills allowing fish to breathe water. Fish do not have lungs and get their oxygen from the water, not the air. Fish have fins allowing them to move and swim through the water, and some live at the bottom of a body of water and others live near the top.
And just like all animals, fish need to eat. Some eat plant life in the water, or algae scraped off rocks. Other fish are predators when they eat other fish and animals. Of course, fish kept at home in a tank are usually given flakes of special fish food to eat. The shape of the mouth is a clue as to what they can eat. The larger the mouth, the bigger the prey it can eat. The shape of the mouth can also tell you if they eat from the bottom or top of a body of water.
Many fish travel in a group called a school because when they swim together in a large group, they are harder to catch, and a predator can become confused. Most fish have scales too, which help protect them from predators. The scales cover the outside of the body and protect the skin from attacks and help them swim faster through the water. Fish have a sharp sense of smell and can detect odors in the water.
Fish have existed on Earth for over 500 million years and were the first animals on Earth with backbones. About half of the fish live in freshwater and the rest in the saltwater of oceans. Many people in the world catch fish for food and their lives depend on fish found in waters everywhere.
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