A question about crabs
 

Other day I took a couple of photos of a Semaphore Crab - Heloecius cordiformis

http://farm8.staticflickr.com/7070/6...964c635b4d.jpg

According to my book and looking at images on the web, this is a juvenile because of the bright colouring of the claws. As the crab reaches maturity, it becomes a more uniform shade of drab purple.

My question is why the young crabs would have the bright colours? Doesn't seem smart to draw attention to yourself when you're only small. Can't be a mating/courtship thing either.

Suggestions?

Red is one of the first colours to lose its - err - colour with increased depth.

This is neither an answer or a suggestions but something that perhaps can complicate it further http://www.discussworldissues.com/fo...lies/smile.png

Guess: The animals it's usually hiding from might not have colour vision, so red wouldn't be such a problem, maybe? It might match the background for other viewers.

Also, as nut says, how a colour is perceived depends on the type of light that falls on it, e.g. red looks black under a sodium lamp. IIRC. (I haven't seen one for ages.)

It may also be worth investigating UV light. If it's in shallow water UV can have a profound effect.
I've read about trout and their perception of UV. Corals can be pretty speccy under UV.

There may be a camouflage benefit our puny eyes can't see.