Prawns are large ten-legged crustaceans whose translucent grey-blue shells flush coral-orange the moment they meet heat. Meatier, sweeter and firmer-biting than the small shrimp they are often confused with, king and tiger prawns are the prized centrepiece of British, Australian and Asian tables — thrown on the barbecue, tossed through curries and laksa, or shelled into cocktails and stir-fries.