Sea slugs are beautiful, jewel-like animals with surprisingly violent mating habits. This tiny unnamed species in the genus Siphopteron is no different.

It all looks relatively peaceful at first. There’s a bit of mutual biting when a pair meets up to mate, but it settles down as each slug slips a penis into their opposite number’s gonopore. And then a sharp, needle-like structure snakes out from each penis to jab the other slug in the forehead.

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As the slugs exchange sperm, those needles inject hormone-rich prostate fluids into the tissue over the central nervous system. Evolutionary biologist Rolanda Lange (who studies these animals) thinks the hormones may influence how much of its mate’s sperm each slug will use in the brood that it later lays.

[Lange et al. 2013]

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Video Credit: Rolanda Lange, Johanna Werminghausen, Nils Anthes


Contact the author at diane@io9.com.