I think "skullfucking" might be another word that could be used here, if only these weren't invertebrates. Still, they are hardly the only creatures with odd mating habits.
Photo by Johanna Weminghausen
As Ed Yong notes with characteristic restraint on Not Exactly Rocket Science:
If you're an aficionado of sea slugs, you're probably used to weird and cringe-worthy sex acts. When two of these hermaphrodite animals penetrate each other at the same time like an R-rated yin-yang symbol, you won't bat an eyelid. When several of them link up in a long mating chain, you'll have seen it all before. When one amputates its disposable penis after sex only to uncoil a spare, your only reaction will be, "Meh."
Still, when two sea slugs penetrate each other in the head while having sex, even the most jaded expert might do a double-take.
Read the rest of Yong's article to get the full story on how this amazing new species of sea slug was discovered.