The crayfish (US: spiny or rock lobster) is a salt water crustacean, not to be confused with freshwater crayfish. It is similar to lobster in size and shape, but without the claws, and has longer, more pronounced antennae. The pure white tail meat has a wonderful, sweet flavour. The shell is pimply and reddish brown to dark blue before it is cooked, becoming scarlet on cooking. The grey-ish, blue/black colour of decapods is produced by a complex of the reddish carotinoid pigment astaxanthin (which colours carrots and other reddish foods) and the blue-green protein a(lpha)-crustacyanin. Astaxanthin is heat-stable while a(lpha)-crustacyanin is not. When the animal is cooked, the crustacyanin is denatured which releases the astaxanthin and results in the bright red colour we see. With many thanks to Lisa Fagg for this information.