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.

Every individual is a hermaphrodite with both male and female genitals. When they have sex, they can simultaneously penetrate each other, with