Animal Respiration: Gill Breathing

In this article you can find information and pictures on gills as a respiratory organ of animals.

What Animals Use Their Gills to Breathe?

Primarily fish, but also crabs and mollusks use their gills to breathe. Larvae such as tadpoles and the larvae of dragonflies, which live in the water at first, also use their gills to breathe.

What are Gills?

Just like the lungs, gills are organs which supply animals with oxygen. The gill arches are equipped with countless comb-, leaf-, tuft-, or tree-shaped filaments. Thanks to their shape they provide a large surface – the larger the surface, the more (oxygen-rich) water can be processed. The gill lamellae of fish can cover a surface 10 – 60 times as large as their entire skin surface. The gills are protected by a gill cover.

How Do Animals Breathe With Gills?

The water streams into the gills via the mouth. The oxygen is absorbed from the water by the lamellae. The mouth closes, the gill cover opens and the water is pressed out of the body – together with the carbon dioxide as a by-product (see picture).

Gill breathing Gill breathing - Illustration: Silke/

Where are the Gills?

At the sides of the head? Mostly. But gills – like those of crabs – can also be situated at the legs.

How Many Gills Does a Fish Have?

Most fish have four gills on each side. Bony fish can have between 5 and 7 gills on each side.

What are the Disadvantages of Gill Breathing?

The water prevents the gills from drying out and collapsing. This is why fish suffocate when taken out of the water. Crabs that are primarily living ashore, have a special trick: they “store” water in their body to keep their gills wet.

Goldfish Goldfish - Photo: Mirek Kijewski/Shutterstock

How Long Can Fish Survive Outside the Water?

Most fish can only survive for a few minutes outside the water, because their gills do not work on land. Yet, there are “specialists” such as the toothcarp „Kryptolebias marmoratus“, which can live ashore for up to 66 days, because it is able to adapt its gills.

The mudskipper is another example. It is a fish, but it lives like an amphibian. With its pectoral fins it can even walk on land. It has particularly large gill cavities that function like the oxygen flasks of scuba divers. This enables the mudskipper to survive ashore for quite a while.

And there is the lungfish that has simple lungs in addition to its gills. This enables it to survive a few days outside the water if it does not dry out completely.

Does the Body Size of Fish Affect Their Respiratory System?

Small fish breathe faster and more often than large fish. They have smaller gills and cannot absorb as much oxygen from the water at a time.

Does it Matter Whether the Water is Clean or Not?

Some fish can only survive in clean water, and their presence signifies the good quality of the water. Other fish can also cope with foul water. Yet, foul water can damage the gills and make the fish more prone to illnesses.

Animals That Drown in the Water Despite Having Gills

There are actually animals with gills that would drown in the water after a short while – such as woodlice and several species of hermit crabs (such as the coconut crab). These animals have “adapted” gills and absorb oxygen from the air. But they still need access to water to keep their gills moist.

What Does the Term „Gill“ Mean?

The term “gill” signifies some kind of lamella. Some mushrooms also have “gills” under their cap. Yet, they do not use them for breathing but for dispersing their spores. Characteristics like the shape and color of the gills can also help to identify the mushroom species.