the spare

"When a male anglerfish is hatched, he has extremely well developed olfactory organs that detect scents in the water. He has no digestive system. His goal in life is to detect the pheromones that the female anglerfishes release. When he finds a female, he bites into her flank which releases an enzyme that digests the skin of his mouth and her body. The two then fuse together, including blood vessels. The male degenerates into nothing more than a pair of gonads that releases sperm when the female releases hormones into the bloodstream that signals she is ready to release her eggs. This is an extreme example of sexual dimorphism. If the male anglerfish doesn't find a female, he dies."

- taken from a description of Anglerfish on Wikipedia