Remove the crab claws and legs and crack them to extract the meat. Open the crabs by pressing with your thumbs on the edge of the section of the shell to which the legs were attached. Pull out the central section.
Discard the stomach sac and feathery gills. Scoop out the meat from inside the shell and put it into a bowl with the meat from the claws and legs. Remove the meat from the leg, sockets with a skewer and mix with the other crab meat. Mash well with a fork.
Add 50 g/2 oz of the breadcrumbs to the crab meat, together with the red peppers, garlic, chilli, mace, allspice and parsley. Add the lime juice and rum, if using, and season to taste with salt and pepper.
Thoroughly scrub the empty crab shells, dry them, and fill with the crab meat stuffing mixture. Sprinkle with flip remaining breadcrumbs and dot with butter. Bake the crabs in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes, until browned. Garnish with parsley sprigs and serve immediately.