The red pigment is the most stable component of the coloring in a crab shell. The greens, blues, and browns which darken the shell in a live crab are destroyed by cooking. The red pigment common to all decapod (shrimp, prawn, crab, lobster) exoskeletons (shells) is astaxanthin, a carotenoid (like Beta-carotene, the pigment that makes fruits red-orange) found in many organisms from bacteria to birds. Astaxanthin was first identified in the exoskeletons of crayfish, Astacidea, hence its name. In crabs, as in many decapods, astaxanthin is not a free pigment, but is complexed with a protein called Alpha-crustacyanin, which alters the resonance of astaxanthin such that the complex acts as a blue-green pigment. As mentioned above, astaxanthin is heat stable, while the Alpha-crustacyanin protein is not, so boiling the crab shell denatures the blue-green Alpha-crustacyanin releasing the red-orange astaxanthin.