Why does the lobster turn red when it get cooked?

Lobsters and many crustaceans contain a protein called astaxanthin that is also found in some fish, shrimp, and krill. Astaxanthin is bound to a protein in the lobster's shell and is colorless when raw due to its molecular structure. When the lobster is cooked, the protein denatures and the coiled astaxanthin molecules straighten out, reflecting light of the red-orange portion of the spectrum and making the lobster turn bright red.