Euprymna scolopes, also know as Hawaiian bobtail squid, is a nocturnal species of squid. In the moon lit waters E. scolopes would appears as a dark silhouette, making it easy for any other nocturnal predators to spot. To get around this E. scolopes produces light and directs it downwards which helps them to blend in with the moonlight. The light it is produce by the bacteria, Vibrio fischeri.
The juvenile Hawaiian squids are born with out V. fischeri and thus must acquire them from the seawater. V. fischeri are filtered from the seawater into the squid's light organ. Hours after V. fischeri enter the light organ V. fischeri loose their flagella, decrease in size, and begin to emit light. After a few weeks the light organ will be fully developed.
I chose V. fischeri because I found its symbiotic relationship with E. scolopes to be very unique. Most symbiotic relationships between bacteria and animals, that I have learned about, revolve around the bacteria breaking down a substance that the animal could not otherwise breakdown in exchange for protection. Producing light to help an animal camouflage its self in exchange for protection is a very unusual relationship, at least to me.