How Do Crabs Breathe Underwater?

Even though they breathe with gills as fish do, crabs can survive out of water for a much longer period of time. So, how do crabs breathe, and how long can they stay out of water?

Crabs Have Gills

Crabs breathe through gills. For gills to work, they must be able to take in oxygen and transport it into the animal's bloodstream. The gills of crabs are located under the carapace near the first pair of walking legs. The oxygen that crabs need is taken into the gills either through water or moisture in the air. 

Breathing Underwater

Crabs breathe underwater by drawing water (which contains oxygen) over their gills using an appendage called a scaphognathite, which is located on the crab's underside, near the base of its claws. The water passes over the gills, which extract the oxygen. Blood passes over the gills as well and transports carbon dioxide into the water, which releases near the crab's mouth.

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Breathing Out of the Water

Out of the water, crabs have plates called articulating plates that can keep their gills moist by sealing them in, storing moisture. Have you ever seen a crab blow bubbles? It is thought that crabs above water blow bubbles to keep oxygen flowing to the gills—the crab draws in air, which passes over the gills and supplies them with oxygen, but since the air is going over the moist gills, it forms bubbles which are released near the crab's mouth.

How Long Can a Crab Stay out of Water?

Land Crabs

The length of time a crab can stay out of water depends on the type of crab. Some crabs, like coconut crabs and land hermit crabs, are terrestrial and breathe well without water, although they still need to keep their gills moist. As long as their gills stay moist, these crabs can spend their lives out of the water. But if they were submerged in water, they would die. 

Aquatic Crabs

Other crabs, like blue crabs, are primarily aquatic and are adapted to receiving their oxygen from the surrounding water. Yet, they can still survive for 1-2 days out of the water.

The European green crab is a species infamous for surviving out of water for a long time—at least a week. These species seem indestructible, which is a problem since they have invaded many areas of the U.S. and are out-competing native species for food and space.

Habitat Challenges

Many crabs also live in intertidal zones. There, they may find themselves out of water for several hours at a time. At that point, the key to survival is keeping their gills moist. How do they do this? Out of the water, a crab's favorite place is a cool, moist, dark place where their gills won't get dried out and where they have shelter. The crab has special plates, called articulating plates, that keep their gills moist by shutting the opening in the exoskeleton so that dry air can't get in. In addition, the crab may drink water from puddles or even obtain it from dew. 

References and Further Information

• Fisheries and Oceans Canada. Underwater World: Green Crab. Accessed December 31, 2015.
• Florida Fish and Wildlife Conservation Commission. Blue Crab FAQ. Accessed January 31, 2015.
• Mahoney, P.M. and R.J. Full. 1984. Respiration of Crabs in Air and Water. Comp. Biochem. Physiol. 79A:2, pp. 275-282.
• Marine Education Society of Australasia. The World of Crabs. Accessed December 31, 2015.