White's Hypselodoris (Hypselodoris whitei)

Madidir III, Lembeh.
Two of the nudis are mating. You can see their genital in blue.
I have just learned the craziest thing about how nudis mate: they are both hermaphrodites and dispute to be the male. They have developed a strategy by which they let their penis stuck on the other nudi (they loose it and grow a new one later) to avoid loosing the opportunity to inject their own sperm. You can find this cool info in these links:
http://legacy.earlham.edu/~brickte/
https://www.nationalgeographic.com/science/phenomena/2013/02/12/sea-slug-amputates-its-disposable-penis-but-has-two-spares/