Why do crabs and lobsters turn red when they are cooked?

Crabs and lobsters turn red when cooked due to the chemical transformation of a pigment called astaxanthin.1

In their raw state, these crustaceans contain astaxanthin, which is a carotenoid pigment. While this pigment is naturally red or pink, it is typically bound to proteins within the animal's shell, which masks its true color and results in the dark, brownish, or bluish hues often seen in live specimens.1

When the animal is exposed to high heat during cooking, these proteins undergo denaturation—a process where the protein structure unfolds and breaks down. As the proteins denature, they release the astaxanthin pigment. Once freed from the protein complex, the astaxanthin is no longer masked, allowing its natural, vibrant red color to become visible.21