Why do cooked lobsters turn red? Why are there blue lobsters?

Lobsters have a variety of pigments in their shells, to help them blend in to their environment and avoid predators.

The main source of red is a pigment called astaxanthin, a relative of beta carotene, which makes carrots orange and lycopene, which contributes to the red of watermelons.

The astaxanthin looks blue when it binds to protein in a normal lobster - but when that lobster is cooked, the heat releases the astaxanthin from the protein, and turns it red.

Some lobsters have a defect that overproduces the blue combination of astaxanthin and protein. The result is a rare blue lobster - estimated to number one in a million.

If you cook them, they still go red, of course.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.