You might want to check out the work of Edward Ruby. Any number of reasons
have been proposed. One is that the light is used to attract other
potential mates. Second the emission of light could make it hard for a
predator to detect the fish as the predator, looking down on his prey
cannot make out a defined shadow to attack. Of course the obvious
suggestion is that it acts as a flashlight. I don't know if anyone really
knows the answer to your question. I really suggest you look up E. Rubys
studies or any of the review articles on vibrio fischeri and
bioluminescence. Good luck.
Lou
Lou Passador
Dept. of Microbiology and Immunology
Univ. of Rochester
Rochester NY.
