WHY DO CRABS, LOBSTERS, CRAYFISH, SHRIMP TURN RED WHEN COOKED?

Most of the time, the exoskeleton of most crustaceans has a blue-green to graying colour and sometimes they appear a brown or olive green, with just a hint of red; with a few exceptions like the blue and yellow lobsters and crabs. The exoskeletons of such creatures are made up of several pigments, one of which is a carotenoid called astaxanthin, which provides its reddish colouring (astaxanthin is the same carotene that gives salmon its color).

At normal temperatures and when alive (when they are not yet dumped in boiling water or grilled), the astaxanthin pigments are hidden because they are covered with other protein chains that give their shells the bluish-gray or brownish-green colour we see.

Exposure to heat destroys this protein coating, while the carotenoid pigment, astaxanthin still remains stable. So when you cook a crab or lobster or its other tasty crustacean friends and family for your scrumptious meal, the heat breaks down all the other pigments except for astaxanthin; thus, causing the bright red colour we see in cooked lobsters, crabs, and crayfish or the reddish-orange colour of cooked shrimp.

All lobsters eventually turns red when cooked except of the albino crab and lobster. For the obvious reason that they don't have pigments so therefore, they remain the same colour even when cook.

Source: Today found out.