Siphopteron sea slugs are hermaphroditic, but this is not all that's unusual about their sex lives. When they mate, one of the pair just straight-up stabs the other one in the forehead with a tiny, spiky thing. It's a very direct approach! No muss, no fuss, just hormones straight into the nervous system, convincing the recently stabbed slug that it has to be a mother to lots of little sea slugs, straightaway.
James Gorman of the New York Times calls it "a mating behavior so extreme it seems to be a scientific first." We call it efficient.
(Also, a side note: These tiny marine beings are so beautiful and diaphanous, like pretty ocean orchid creatures, that it seems like their mating rituals ought to be some sort elaborate swimming ballet. But you can't tell nature what to do.)