Crabs are somewhat similar to fish in that they are equipped with gills for breathing. The gills themselves are feathery type structures located at the top of the crab’s walking legs. They are derived from part of the jointed walking leg. In order to breathe on land, crabs must keep these gills moist so that oxygen in the air can still be absorbed. Gills work because oxygen is a very small molecule. During respiration these oxygen molecules first dissolve into a layer of moisture surrounding a thin membrane. Then these molecules, (because they are so small) cross through the membrane and enter the circulatory system (blood) of the crab. The source of oxygen can then either be used as gas in the air or already dissolved into another liquid such as the sea.
It does not necessarily matter where the oxygen originally comes from as much as that the surface of the oxygen molecules cross is wet. Crustaceans that reside in the water do not typically have trouble when it comes to keeping their gills moist. Terrestrial crustaceans can usually keep their gills wet by using fluids from inside of the body and by having their protective chambers well sealed so that very little moisture is lost. Crabs such as the hermit crab use their claws and antennae to keep themselves moist by placing water onto themselves.
Because these creatures need to keep their gills moist to breathe properly, it is important that a water source of some kind always be nearby. One of the most common reasons crabs such as the hermit crab dies in captivity is because crab owners neglect to provide them with this. The difference between marine crabs and land hermit crabs is that land hermit crabs have much smaller gills but also need air to breathe. They must be kept moist to survive but would drown if submerged completely in water. This is why it is also important when keeping them in captivity to provide a broken up piece of sponge in their water dish so that they have something to grasp onto to keep from losing their footing and falling into the water and/or drowning. If the crab’s gills become too dry, the crab will die. Another interesting fact about hermit crabs is that they carry an extra supply of water around in their shells to help keep them moist.