Before you learn how to eat soft shell crab, you should know what it is, right? Well, it's a hard shell crab in undies. That's right.
Crabs grow out of their shells, and when there's no more room left, they need to shed 'em and grow new ones. It's called "molting". Until the new coat is hardened, the poor things have nothing to wear but a flimsy piece of lingerie. Seabirds love it, and so do we. Now we can eat the whole thing just as it is. Yum.