Gasp!!

http://www.wildsingapore.net/chekjawa/text/g321-1.htm

Bursting out: When the new exoskeleton is ready, the crab swallows water to expand its tissues and the old shell splits along predetermined lines, usually near the abdomen. The crab then carefully pulls out of the split in its new exoskeleton which is still soft and wrinkly. The crab continues to swallow a lot of water to stretch the new exoskeleton. When the new shell hardens in a few hours, there is space in the new shell for the crab to grow. Moulting thus makes the crab 'watery' and less valuable as food. The 'soft-shell crabs' that we eat are crabs that have just moulted! Since crabs are vulnerable during moulting, they usually find a safe place to hide during this time.