Thanks, arnjer, for the publicity!
There are a number of animals that 'flash' like this...
As brittlestars commonly stick arms out of holes in rock, it is a possibility. I think that Dr. Ron on RC also had a similar question awhile back, and listed other possibilities.
As far as the brittle is concerned, this sort of 'flash' is a defensive mechanism. You can see in the photo below, that the central disk of the animal is dark- this is the most important part of the animal. Arms are commonly grabbed by crabs, or in your case, forceps. Often, the brittlestar will drop the arm, and it will keep flashing (the arm, of course, will eventually grow back). The crab is left with this odd meal while the rest of the brittle makes an escape. Ingenious, I would say...
But I am a bit biased.
I believe there are worms with the ability as well.
Here is a brittlestar, Amphiura filiformis (from site:
http://lifesci.ucsb.edu/~biolum/forum/ydewael1.html)
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Brittlestars!!
http://home.att.net/~ophiuroid
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[ August 08, 2001: Message edited by: ophiuroid ]