The stylet also inserts something into the other slug that increases it’s fertility.
Reference: Anthes, N. and Michiels, N.K., 2007. Precopulatory stabbing, hypodermic injections and unilateral copulations in a hermaphroditic sea slug. Biology Letters, 3(2), pp.121-124.
This is a series of little illustrated factoids about animal reproduction, posted daily. I’d love to hear your suggestions/ feedback. I also post these on my insta.