Why Do Crabs Foam at the Mouth?

If we see a crab foaming at the mouth, it means the crab is having trouble breathing. Like all animals, crabs need oxygen to live. They take oxygen from the water, through their gills, as fish do. Even when crabs are on land they can use the water stored inside their gills. But they can do this for only a little while. When their gills start to dry out, it becomes hard for crabs to breathe. They begin to move their mouth and gills, trying to take in water. But only air comes in, and the water still in the gills turns to foam. Crabs like fish, get oxygen through their gills. A crab's gills are spongy, and it can easily take in water. The air is mixed with whatever water is still in the gills, and foam is produced. The staler the crab's water, the more foam there is.