How do fish breathe...and hermit crabs too! (a gill experiment)

How do fish breathe? How do hermit crabs breathe? They certainly don't take a breath of air like you or I! Use this easy science activity to demonstrate how fish and hermit crabs (and other animals like octopuses, sharks, and some snails) breathe. Then, invite your kiddos to do their own science experiments based on the process!

Fish, hermit crabs, octopuses, sharks, and some snails all breathe through gills. Gills can vary a lot, but they kind of look like feathery coffee filters. If you have access to a real fish (or want to buy one for demonstration purposes!), it is really fun to look under a gill slit and see the actual gills...but, today's experiment does not use a real fish!

How do fish breathe? Simply: water moves through the gills. As it does so, the gills pull oxygen out of the water and send it through the fish's body. Hermit crabs also use gills! In fact, we did this experiment as part of the Virtual Book Club for Kid's Ocean Week, featuring the book A House for Hermit Crab! (Amazon affiliate link below:)

The hermit crab in this story (and the fish and snail) would have definitely used gills to breathe, so try out this easy science demonstration and experiment to get an idea of how it works:

How do fish breathe? (and hermit crabs too!) Science Experiment:

First, use an elastic to secure a coffee filter to a small beaker or cup:

Next take another beaker or cup and fill it with water. Then stir in a spoonful of dirt! The water represents water in the ocean and the dirt represents dissolved oxygen...and anything else in the water! You'll see little pieces of dirt suspended in the water and larger pieces either floating or sinking.

Here's the fun part! Carefully pour the dirty water into the coffee filter.

The coffee filter will "catch" the dirt while the water passes through it.

Similarly, gills "catch" oxygen and pull it out of the water as the water passes through the gills. If the dirt were really oxygen and the coffee filter really a gill, the gill would collect the oxygen and pass it to the circulatory system to be delivered to the rest of the fish's (or hermit crab's or octopus's) body!

The water simply passes through the gills and comes out of the fish with a little less dissolved oxygen!

It's really fun to remove the coffee filter and see the clean water underneath!

Like any model, this one is not perfect, but it does show kids that water moves through the gills while oxygen moves into the fish. It also helps to visualize the fact that water has dissolved oxygen in it. (For teachers, parents, and older kids: In reality, the gills are very, very thin membranes with deoxygenated blood on the other side. Oxygen, like any molecule moving across a semi-permeable membrane, diffuses from the water {where it is at a higher concentration} to binding sites in the blood {where it is at a lower concentration.})

Now let your kiddos experiment with variations! Does sand separate like dirt? Does the coffee filter "catch" it the same way? What if you included little pebbles? Sticks? What else do your kiddos want to try?

Other Extensions/Explorations/Discussions:

1- How does the coffee filter "catch" the dirt? How is this similar and different to gills?

2- Why can't fish and hermit crabs use lungs like you or I?

3- Sometimes hermit crabs climb out of the ocean...why don't they die? They can only do this in very humid environments, where the moist air moves across their gills.

4- How can you see oxygen in water? Try blowing bubbles into water with a straw...oxygen and other gasses from your breath will make the water bubble!

Are you working on an ocean theme? Be sure to check out all my preschool ocean activities...there are art, science, math, literacy, and more ocean themed projects and resources for preschoolers!