How is it that crabs can breath under water and on land ? ?

Question

in response to number three .. some crabs (hermit crabs are coming to mind ) .. don't have wet gills .. they just live in the sand in aqariums

Answer 1

I don t think crabs breath at all

Answer 2

very few crabs live in water and land

most live either in water or on land

one set has gills the other have lungs of a sort

Answer 3

to answer this quickly, crabs have gills like fish that pull oxygen out of the water. Thats how they breathe under water.  For land, as long as the crabs gills remain moist, the gills will but the oxygen out of the air and convert it to oxygen that can be used by the CRAB.

Answer 4

Reference 2:  A hermit crab is not a "true" crab. "There are almost 5,000 different species of crabs; about 4,500 are true crabs, plus about 500 are hermit crabs."   Reference 1:  Whereas a true crab has a short abdomen protected by a wide flat plate, a hermit crab has an elongated soft abdomen which it protects by hiding inside a shell it adopts as its moving house.

Reference 3:  "Hermit crabs prefer shells that fit snugly, because a snug-fitting shell protects their soft abdomen and also helps prevent evaporation of moisture on their bodies. Hermit crabs' lungs aren't as efficiently developed as those of other land-based air-breathing creatures, and they require a certain amount of moisture retention for their lungs to operate properly. If hermit crabs become too dry, they can suffocate."

Reference 2: "Marine crabs breathe underwater using gills, which are located in a two cavities under the carapace. True land crabs have enlarged, modified cavities that act like lungs so that the land crabs can breathe air."  

Land crabs also have various methods of keeping their inner selves moist, like closing up the clefts to the outside, being nocturnal, and staying in moist places.

I also found out while researching this that hermit crabs are social and you shouldn't keep them alone -- who would have ever thought?  (reference 3)