When you dive underwater, you must hold your breath. This makes it hard for humans to stay underwater for long periods of time. As oxygen is scarce under water, why can such a large number of species thrive in water ecosystems? How do these animals get their oxygen? 

Many water-dwelling animals have gills. Gills are respiratory organs that filter oxygen from water and remove carbon dioxide from the organism.

Watch the video. Can you find gills? How do they work? Are the animals in the video crustaceans or insects?

The gills of crustaceans and insects are located in the back of the animal's body. Many "worms" gather oxygen directly through their skin. Clams actively pump water in and out of their shell. 

Some insects, such as water beetles, have no gills. As a result, they must occasionally come up to the surface to breathe in oxygen from the atmosphere with their breathing tubes.


A water beetle coming up to the surface to breathe.