Sea slugs with disposable penises…or is it “penes”?

For a species of sea slugs (or nudibranchs) that calls the Pacific Ocean home, penises are apparently disposable. In fact, disposable penises are a natural part of sex for them. That’s according to a paper published recently in the journal Biology Letters.

Scientists first discovered this bright pinkish orange slug — called Chormodoris reticulata for the latin speakers out there — in the 1800s. Many sea slugs are simultaneous hermaphrodites, so have both both male and female parts, so to speak. Also, in terms of the female bits, they typically have two pouches to store sperm — one of which can destroy sperm. So, they can mate with two males and pick with sperm stash to dump. In the human world, we might call this the purest form of rejection, but in animal behavior terms, it’s called “female choice”.

Lots of sea slug species mate in weird ways, so it makes them an interesting group for animal behavior scientists to study. In this case, a team of Japanese researchers went scuba diving to collect Chormodoris specimen during their mating season — in 2005, 2006, 2009, and 2010. Back in the lab, they stuck the slugs in tanks to watch what happened. They either paired two slugs that had been isolated for 24 plus hours, or stuck an isolated slug in a tank with one that had recently mated. In addition, to the observation, they took photos and tissue samples of the slugs’ reproductive layout.

So, based on their study here’s how this particularly brand of kinky sea slug sex works: the slug has a really long penis that’s curled up inside it’s body, with a little bit hanging out. When two slugs are ready to get it on, the little bit hanging out elongates and develops prickly spines on the end. After either seconds or minutes of sex, the end of the penis falls off. It takes at least 24 hours for a new “penis” to uncurl from the spiral section inside the body, but then they’re ready to go at it again. A slug can do this up to three times, and this particular species is the only organism in the world that can chuck it’s penis and mate again 24 hours later. (Other slug species drop their penises after mating season is over, but they have a much longer time to replenish).

Weird sex happens all the time in the animal world, so what makes this particular instance a big deal. Sexual selection rarely happens with hermaphrodites — they have the best of both worlds, so they’re all kind of on an equal playing field. But, because it takes 24 hours for the slug to bounce back, replenished slugs have the advantage.

Smithsonian does a nice job of summing it all up, here.