system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:I've seen crabs living under water and also living out of water in the sand on the beach .How can they breath in both environments?
gpt
"Crabs" is a huge category with many species.  Aquatic crabs live permanently in water and have gills like fish.  They can go out of water but only for short periods, but then again so can some fish.  There are also terrestrial crabs that have lungs and live on land.  These guys can swim around and actually hold a bubble of air in front of their face, but again they can only go under for a short time.  Intertidal crabs have gills, and then also have cuticles on their legs that help them breathe on land.  They have to stay moist though and can't live away from water, but they can breathe in air or water fairly well.