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BluegillOur Indiana lakes and streams are an amazing ecosystem filled with what may seem to be alien life forms compared with what we see on land.

I work at Versailles State Park. I see many fish in our lake, some as small as minnows and some as large as channel catfish and bass. No matter which fish I see, they all have some of the same characteristics. It's these traits that make fish unique and help them live underwater. Let's come up on the land by our nature center and explore some ways fish differ from land animals.

People can swim underwater, but there is a point at which even the best swimmer can no longer stay there; they have to surface for air. So how can fish live underwater all the time?

Humans have lungs to get us the oxygen we need, but fish have gills that take oxygen out of the water and provide air so they can breathe.

That's not the only special adaptation fish have. Since fish are constantly swimming, they need something to keep them from either floating to the top or sinking to the bottom. This organ is called the swim bladder, which can fill up or release air, allowing the fish to go up or down in the water. As a fish swims deeper, the pressure on its body increases. The swim bladder helps its body compensate for the pressure changes.

You might have known about gills and swim bladders, but did you know that fish can hear? Fish also have something called the lateral line system that is separate from their ears but works together with their ears to help them hear.

The lateral line system picks up vibrations made in the water, whether those movements come from a predator, an angler's lure hitting the water, or something else. Fish have to watch for predators and search for food. The lateral line system allows them to do both.

Fish can detect the tiniest movement in the water. This ability helps them avoid becoming a bigger fish's meal or assists them in finding dinner. Such a sense is important for fish because the sound doesn’t travel as well through the water. Fish rely less on their hearing and more on the lateral line system.

Hearing and lateral lines are two independent but related systems used by fish to help them "hear" underwater. The next time you visit a lake or stream, take time to watch the fish and see these amazing adaptations in action.