2012 Lange et al.
Reciprocal copulation of Siphopteron quadrispinosum (sea slug). The bipartite penises, which are everted as largely translucent structures at the right front of the head (h), are reciprocally inserted into the partner. While the actual penis (p) is inserted into the gonopore (located behind the right parapod), the penile stylets (s) are hypodermically inserted into the foot of the partner. Markings are only shown for the lower animal.
Sea slugs, despite their diminutive size and slow-moving nature, engage in violent, physically damaging mating routines, according to a new study published in scientific journal PLoS ONE.
"Let's say its waged at a small scale, but it is violent," said marine biologist and sea slug expert Pat Krug on Brand & Martinez. "The first act is an injection of what I would call a 'slug roofie,' it's a mix of secretions that actually make the partner slow down and not try to get away. Because the follow-up genitals, when the slug is acting like a male, they look like medieval torture devices, we're talking spines hooks needles. It's kind of brutal and it hurts, it physically damages them to engage in the act of mating."
According to Scientific American, in order to reproduce, a sea slug "pierces the skin of its partner with 'a syringe-like penile stylet that injects prostate fluids.'
Krug says there's another twist to this: sea slugs are hermaphrodites, so there's an epic struggle during mating where each slug tries to avoid playing the female role.
"They will engage in a behavior called penis fencing, where they'll literally fence using their junk as dueling implements, each one trying to be the boy, because the loser ends up being the female," said Krug. "It's thought that these male armaments evolve as a way to take choice away from the other partner and kind of force them to be the female."
The study also shows that sea slugs mate many more times than they need to, and that those who mated too much produced fewer eggs. Having multiple male partners has evolutionary affects, ensuring that the slugs produce more and healthier offspring.
"What females evolved to do is mate with multiple males because it maximizes the chance that they'll find one male who covers their genetic bases," said Krug. "The data show that a female who mates with multiple males has healthier offspring that mature faster and are more likely to survive than a faithful female who only mates with one partner."
Pat Krug, marine biologist for Cal State University, Los Angeles