When most people picture a crab or a lobster, they envision bright red pincers and an armor plated body. When these delicious sea creatures arrive on our plates, they usually are that vibrant shade of red. But when they’re roaming free on the ocean floor, crabs and lobsters are usually brown, muddy green, or dull blue. Hardly the gorgeous meal we’re used to seeing dressed up in cookbooks.
So why do these crustraceans turn red when they’re cooked? Lobsters and crabs have a pigment called astaxanthin in their shells, which has the ability to absorb blue light, making the shell appear red under certain conditions. When the lobster is alive, the pigment is safely stored inside a membrane hidden in the shell called the crustacyanin. It’s packed so tightly inside the lobster shell that the pigment is trapped inside the membrane, unable to flow freely. That means that crustacean usually appears in those muted dark blue or green tones.
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The pigment responds strongly to heat, though: Once you dump a crab or a lobster in a pot of boiling water, it’s body chemistry changes: The pigment—astaxanthin—gets separated from the membrane—the crustacyanin—transforming your dinner into that ruby-hued tone we know so well. Same goes for usually boring grey shrimp, which also have a pink pigment in their meat and shells that is released by heat.
There are also those very rare blue lobsters, which fishermen consider good luck charms. Scientists think that one in every two million lobsters are born with that brilliant blue color, which is triggered by a genetic abnormality that causes them to produce a certain protein in overabundance. Prized for their beauty, these lobsters are mercifully tossed back into the ocean by fisherman searching for their more common-colored brethren.
Even more rare are the lobsters that don’t have any pigment at all—they’re albino—and leave the pot the same way they entered it: a grayish-white color.
Maybe you’ll remember this science lesson when you’re sitting down to a plate of crab legs this summer, but hopefully you’re too distracted by pure bliss to recall any chemistry.