How come crabs can live at the bottom of deep oceans?

Question

and also live on land, because other amphibians have to come up for air regularly?

Answer 1

Crabs are crustaceans, not amphibians. They breathe with gills that are under the shell. Even land crabs breathe with gills that must be kept moist in order to work.

Crabs at the bottom of deep oceans can find the food they need. They even live around undersea volcanic vents.