Why do some crusties turn red when cooked?

This question was brought up by someone on the 'professional' mailing list I am a subscriber of. I don't feel good not naming names of those who provided answers to the discussion, but I don't want found out via google should one of these academics (or their students) google their names so suffice to say, I am merely paraphrasing their answers (with apologies).

Anyway.. someone asked the above question and it is something I've always wondered about too. Some crustaceans e.g. prawns also turn red on preservation and when asked by students, I've always replied with the standard 'That's due to protein denaturation which causes the animal to turn red'.

But thanks to the list, I have more explicit answers now! And it will undoubtedly be useful for next sem's practicals.

The culprit? Carotenoids. More specifically, astaxanthin. It exists as a protein-astaxanthin complex in the exoskeleton which gives the animal it's 'live colour'. When cooked, the protein is denatured and dissociates from astaxanthin, thus revealing the red pigment colour of astaxanthin alone.

Some crustaceans do not turn red on cooking. Stomatopods tend not to - they turn purplish. Someone on the list (he's my collaborator, as a matter of fact) suggests that this is due to the fact that these pigments are derived from diet and stomatopods tend to be more carnivorous than other crustaceans which tend to ingest more plant material or detritus.

Tadaaah! Mystery solved.