Fish breathe through gills, which are specialized organs that extract oxygen from water. Gills are located on either side of the head and are made up of thin, delicate filaments that are covered in tiny blood vessels. When a fish takes in water through its mouth, it passes over the gills and the oxygen in the water is absorbed into the blood vessels. The deoxygenated water is then expelled through the gills and the process is repeated.

In order to breathe efficiently, fish need to constantly move water over their gills. This is accomplished by using their mouths and opercula, which are bony covers that protect the gills. When a fish opens its mouth, water is drawn in and passes over the gills. The opercula then close, forcing the water out through the slits on either side of the head.

The number of gills that a fish has varies depending on the species. Most fish have four gills, but some species have as many as seven. The size and shape of the gills also varies depending on the species. Fish that live in warm, tropical waters typically have larger gills than fish that live in cold, northern waters.

Gills are essential for fish survival and play a vital role in gas exchange. Without gills, fish would not be able to breathe and would quickly die.