Dear Kinsley,

Have you ever seen a diving beetle? They’re one of my favorite animals. They live underwater and breathe air from a bubble attached to their butts.

I talked about all the ways animals breathe underwater with my friend Wes Dowd. He’s a marine biologist and animal physiologist. He studies how living things interact with the world around them.

Animals need oxygen. For air-breathers like us, oxygen is mixed into the air. For water-breathers, oxygen is mixed into the water. To get oxygen into our bodies, we all need organs and tissues made of very thin material with lots of surface area. That means lots of places that touch the air or water where oxygen can pass through. Like inside lungs or gills. Or the skin of frogs, toads, newts and salamanders that can live on land or underwater.

This fish has a birth defect that makes it easy to see its gills. Photo: Wiki/Guitardude012
An immature mayfly’s gills look like feathers along its back end, behind its back legs. Photo: Wiki/Ian Alexander

“That’s why our lungs have little air sacs inside them—to increase surface area,” Dowd said. “If you look at a fish’s gills, it’s basically that structure flipped inside out. It’s a bunch of really small structures with high surface area that are exposed to the environment.”

Most underwater animals use gills to breathe. That includes fish, crustaceans, immature amphibians and immature insects that live underwater. As the water flows over gills, oxygen moves from the water into the animal’s body. If you peeked inside a fish’s gill slits, you’d see gill filaments with lots of folds. The gills of immature water insects look like feathery tufts around their legs and back ends.

Some underwater animals get their oxygen from the air. Whales, dolphins and porpoises are underwater mammals now, but their ancestors lived on land. So, they still have lungs like we do. They poke their nostril-like blowholes above the water to take a breath. That one breath will last up to two hours.

A diving beetle carrying a bubble of air from the surface.

Some adult insects live underwater—like diving beetles, whirligig beetles, water boatmen, backswimmers, giant water bugs and water scorpions. They don’t have gills or lungs. They have a network of air tubes inside their bodies and air holes along the sides of their bodies. They stick their back ends out of the water to get air. They can save air for later by collecting a bubble of air under their wings or on their back end or belly. Because insects are so small, sometimes the air bubble works like a gill. It pulls oxygen out of the water—like an air tank that refills itself.

There’s even one group of fish that have lungs and gills. They’re called lungfish. They can stick their noses and mouths out of the water and take a gulp of air to fill their lungs. Some lungfish breathe air that way all the time. Others breathe air sometimes and sometimes use their gills to breathe water.

Lungfish are some of our closest living fish relatives. Scientists think they could help us understand how life started underwater then moved to land—and back again for some animals. Maybe they can even help us understand all the amazing adaptations animals use to get oxygen.

Now that’s a whale of a thing to think about.


Dr. Universe