ANGLER FISH - Wow, this is weird!
Anglerfishes are bony fishes. Some of them have a unique mating method: Since individuals are rare and encounters doubly so, finding a mate is a problem, especially at a time when both individuals are ready to spawn.
When a male anglerfish hatches, it is equipped with extremely well developed olfactory organs that detect scents in the water. They have no digestive system, and thus are unable to feed independently. They must find a female anglerfish, and quickly, or else they will die.
When he finds a female, he bites into her flank, and releases an enzyme which digests the skin of his mouth and her body, fusing the pair down to the blood vessel level. The male then atrophies into nothing more than a pair of gonads that release sperm in response to hormones in the female’s bloodstream indicating egg release. This is an extreme example of sexual dimorphism. However, it ensures that when the female is ready to spawn, she has a mate immediately available.
The entire story as told by theoatmeal— http://theoatmeal.com/comics/angler