Crabs are red alien water tarantulas who regenerate like mutants. Just look at this crab literally climb out of its old shell and toss away that used exoskeleton like it's a dirty pair of pants. So gross but so cool. I kind of wish I could shed skin like this. It'd probably be the most refreshing feeling ever.
Pointed out by a Reddit user, the molting action starts around the 45 second mark in the video. The whole process of shedding its old body takes hours (it's been shrunk down to a minute in the video). When you see the arms come out, you'll probably shriek.