Here you go!

But by the time he has found her, there is a good chance he's been beaten to the punch. The small suitor will often share his partner with upwards of six other males.
Undeterred, he uses his small, sharp teeth to latch onto his intended's side, holding on for the rest of his (and her) life.
In some species the male becomes a permanent parasite on the female, his small body left simply trailing along in the water beside hers. Slowly he fuses to her, becoming an irremovable appendage.
In this relationship he is completely catered for. He no longer needs his eyes, so he loses the ability to use them and relies on his host for all of his nutrition. Even his bloodstream eventually connects with hers.
But he is an integral addition to her life, because the vital thing that that the male provides is a constant supply of sperm. This ensures that the female can produce fertile eggs for the rest of her life, making him vital to species survival.