2. The crab we usually eat can't die naturally, because it is a highly immune creature. Once on the verge of death or already dead, toxins and bacteria will invade the body of river crabs, producing harmful substances, which will be poisoned when absorbed by our bodies, so dead river crabs cannot be eaten. But because sea crabs are special, they can still be eaten as long as they are preserved, and most of what we can eat at ordinary times is dead.