The Gills

Crabs have gills on both sides of their bodies, which they use to breathe. Gills are featherlike structures which allow gas to be passed from the outside world into the crab.

The gills have a very thin layer which allows dissolved oxygen to enter the crab and carbon dioxide to leave.

In crabs that live in wet environments, the water is washed over the gills by small flappy structures in the chamber called scaphognathites. Crabs can reverse this motion to clean any debris or gunk that has entered the gills.

When the oxygen enters the crab it attaches itself to a pigment called hemocyanin - you can think of these like our human blood cells, hemoglobin.

Hemocyanin is copper based so a crab’s blood is blue. Human blood is iron based, which is why ours is red.

The circulatory system of a crab is known as an ‘open’ one. This is because blood flows all over the crab’s organs without travelling through veins.

Terrestrial crabs, ones that live on land, have a different way of breathing, although they still use gills.

These land crabs take water from damp soil and store it in their gill chambers to keep them wet. Their gill walls are much thicker, because there is no shortage of air on land - in fact there is 30 times as much!

The crab pictured is actually a molt of a Dungeness Crab - when crabs molt, they leave behind their old shell, along with the outer layers of many internal parts.

This helps remove toxins that may be in their shells and gills. However crabs often eat their old shell after molting which reintroduces the toxins to their system.