Fish Out of Water

By Daniel Maas

show/hide words to know

What’s in the Story?

We all know that fish have fins and tails, but how close are those to your own arms and legs? Some fish like mudskippers can flap their fins to travel across solid ground and mud puddles, but most fish just use theirs for swimming. Although fish can’t walk on land, scientists do believe that fish are the ancestors of land animals. In the Public Library of Science article, “Development and Evolution of the Muscles of the Pelvic Fin,” scientists discuss how ocean creatures developed the ability to walk out of the water and onto land.

Swimming to Walking?

Before we can understand how fish evolved, we need to understand the different ways land animals and ocean animals move. If you have ever seen a fish swim, you probably noticed that the front two fins and the tail fin move a lot. You can think of the two front fins like fish arms and the tail fin like fish legs.

The front two fins are called pectoral fins, and are used to move the two front fins, or “arms.” A fish’s tail fin is called the caudal fin, and is like a fish’s hips and legs. Think of your own body. Because you walk standing up, almost half of your body is hips and legs. But if you look at a fish, its “legs” are just a tiny bit of tail. If you were a fish that tried to walk on land, you would have to pull yourself forward with the big muscles in your arms and chest. The muscles in your fishy hips and legs would be much too small and weak to move your forward.

If you look at animals that walk on earth, like dogs and cats, you will see that they do not move by pulling themselves forward with their arms. Land animals mostly push themselves forward with their back legs. For your dog to walk, it needs to have lots of leg muscle, but less muscle in their arms and chest. That is the opposite of the fish muscles we just talked about.

From Water to Land

Now that you know how fish move, we can talk about how fish evolved from ocean dwellers to land-walking critters. You may have heard of evolution, or natural selection. It is the idea that some creatures with traits that help them survive, live to pass those traits on to their babies. Over many, many generations, those traits make that creature distinct from what it used to be.

Fish have very different muscles than land animals, so the evolution of fish would have required the change of many different kinds of body parts. One important idea in evolution is that creatures in the oceans evolved to live and walk on land. One way scientists examine this is by looking at fossils, like the dinosaurs you see in museums. Fossils are an ancient record of creatures that lived on earth hundreds of thousands of years ago. Most fossils are only the hardened skeletons of creatures though, and how muscles and other softer parts of animals do not become fossils.

Glow in the Dark Fish!

To see how fish muscles may have evolved, scientists conducted an experiment with several species of fish eggs. Scientists used fish species like paddlefish, zebrafish, and lungfish (a type of fish that has lungs similar to humans). They also wanted to see if there was a difference with sharks, because shark skeletons are made of flexible cartilage instead of hard bones.

First, scientists collected several fish embryos. Embryos are the early cells that will eventually grow to into babies, in this case, creating baby fish eggs. The researchers put bright green dye or red dye into the fish embryos, then let the embryos grow into fish babies that actually glowed red or green depending on the color injected. Because they were interested in seeing how fish “legs” evolved, scientists examined muscle cells from those areas of the fish.

The scientists took glowing red muscle cells from one baby fish and put them into the bright green baby fish. As the colors were both different, the scientists could actually see how different muscles grew as the fish aged. Look at the picture on the right and you can see both of these color cells near a fish's fin. Scientists did this because they wanted to see when, where, and how much fin muscle was growing.

When scientists looked at how the hip muscles grew, they found that bony fish like zebrafish and lungfish grew differently than sharks. The cells that made up the muscles in the bony fish grew in a way that was similar to some birds and land animals. That similarity is very important when trying to figure out how animals evolved. Since sharks have a cartilage skeleton instead of a bone skeleton, they have their own method of growing hip muscle that is not at all like land animals. By examining the differences in how fish muscles grow, scientists have a better understanding of how species evolved to live on land over many hundreds of thousands of years.

Goldfish image- Gullfisker, Wikimedia Commons.

Mudskipper image- OpenCage, Wikimedia Commons.

Additional images public domain on Wikimedia Commons.