Crawfish actually glow bright white/blue when exposed to 365nm light. The light also seems to disorient/daze them, making it very easy to scoop them up with a net. This is the same phenomenon that you will see with scorpions, but it seems to be lesser known. Due to the wavelength, you cannot penetrate into deeper water(12 inches is probably the deepest a good light will penetrate) , but when shore hunting they light up like Christmas lights.

The same is true with ocean crustaceans like blue crabs and sand fleas, walking the beach at night with a 365nm light and it lights up all the hidden organisms like a scene out of avatar.

You can find fairly inexpensive lights on Amazon.