The Characteristics of Fish

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

Scientists classify animals based on the characteristics that they share. We are going to look at the characteristics shared by all fish as well as how they use each characteristic.

Is It a Fish in the Water?

You are on a boating adventure with some friends and see something swimming around in the water. You point to it and automatically think it's a fish. That may not necessarily be the case, you find out from your friend Ashley. She tells you that there are many animals that reside in water that are not fish at all.

There are many recognizable characteristics shared by all fish
Picture of a fish

You look surprised and ask Ashley how she knows the difference. She tells you that there are several ways to distinguish fish from other animals in and out of the water. You thought this was going to be a leisurely boating trip, but it has just turned into an interesting educational learning trip as well.

Characteristics of Fish

Ashley begins to go through the characteristics that all fish share. The first one is one that you already knew: all fish live in water their entire lives. From the time when a fish is developing inside of an egg through the end of its life, it lives in the water. Ashley only emphasizes that fish are in water their whole lives because there are other animals that live in water, but they only do so for part of their lives or just to reproduce. None of those animals are classified as fish because fish do everything in the water for their entire lives.

The next characteristic that Ashley tells you is that all fish are vertebrates, meaning they have a backbone.

All fish have a backbone
Picture of fish vertebrate

She explains that there are some differences in the composition of the backbone among fish. You jump in at this point because you do remember hearing that there are some cartilaginous fish. These are fish that have vertebrae made of cartilage rather than one made of calcium hardened bone. Regardless of what the vertebrae is made of, it is present in every fish.

Since fish are always in the water, Ashley explains that they all have gills which are slits on both sides of fish that allow for respiration. Fish need to have respiration, or take in oxygen and release carbon dioxide, just like all other animals. But, since they are in the water, they have to extract the oxygen from the water and this requires gills. Some other animals have gills during early development when they live in water, but fish have gills their entire lives.

Ashley says that fish are ectothermic. You think that is a strange-sounding word. She breaks it down to tell you that 'ecto' means outside and 'thermic' means pertaining to heat. This simply means that fish are cold-blooded animals and they cannot regulate their own body temperature. They absorb heat from their outside environment, hence the name 'ectothermic'. This also means that if the water in their environment is cold, then they are also going to be cold.

You are readily able to see the next characteristic Ashley discusses. Fish have fins, which are paddle-like extensions on the body of a fish that help the fish to turn and maneuver through the water. Different types of fish have fins in different areas of the body, but most will have at least one fin toward the rear of the body and one on each side toward the front of the body.

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