Crab Molts Shellfish biologists on the north and south coast report early molting of male Dungeness crab. Crabs tend to molt all at once, so great numbers of their old shells can show up on the beach at one time. It can look like a major disaster occurred to the crab population, but it’s as natural as kids outgrowing their old shoes. You can tell the difference between a shell from a molt and a dead crab by looking at the molt line. If you are lucky, you may find a whole exuvia or crab molt. During the molt, the crab leaves even its old gills, antennae, and mouthparts behind. Every detail is there except the crab.