all crustaceans molt, except for barnicals. they lose their shell and then puff them selves up with water and secret a new shell so thwy have room to grow in it. young crabs that are growing rapidly can molt as often as every other week. most adult crabs do it about every 6 weeks. it depends on the crab and how much it is getting to eat. i love when my anemone crab molts because he grows a little algae on him and then he molts and gets all clean.