The blanket octopus
The male blanket octopus spends his existence drifting along waiting to meet with a female. If the male meets a female, he fills one of his tentacles with sperm and tears it from his body. He gives this sperm-filled tentacle to the female which she then uses to fertilize her eggs. Afterwards, the female leaves the male who floats away and dies.
An unusual defense mechanism in the species has evolved: blanket octopuses are immune to the poisonous Portuguese man o' war, whose tentacles the female rips off and uses later for defensive purposes.