Originally I had wanted to write about the relationship between male and female angler fish, a sexual suicide for the males during the mating process. However, upon looking into symbiotic relationships that the angler fish holds, I found out about the relationship between female angler fish and bioluminescent bacteria. I suppose the tiny male angler fish will have to wait until next time...
Prior to becoming informed I always thought that the light produced at the end of the female angler fish lure (the antennae on the head) was a natural phenomena that the female herself produced. In reality, the light that is vital to her hunting and mating habits is being produced by symbiotic bacteria. Photobacteria, as it's called, enter the end of the lure through pores to gain nutrients off the angler host. Once inside they live in the pouch and produce the valuable light for the female. In addition, the chemical reaction that provides the light can only happen when this symbiotic relationship occurs. Because both parties benefit from this endosymbiotic mutualism, it's a win-win for the female fish and the bacteria... somewhat better than the gruesome relationship between the male and female angler fish.
Deep Sea Angler Fish - Queens of the Glowing Depths (The Naked Scientists)
A Light in the Darkness (Understanding Evolution)