sea slug
Stephen Childs

The Chromodoris reticulate, a Pacific-dwelling sea slug, has a complicated sex life. It is both male and female. Its male parts fall off after sex. And unlike any other creature — as far as we know — it can regrow its penis.

Let’s begin with a pretty basic fact about this slug’s sexual anatomy: Its penis is spiny. Whatever, lots of creatures have spiny penises.

But fewer creatures are “simultaneous hermaphrodites,” i.e. have male and female sexual organs. Sea slugs can lay claim to that distinction as well. The BBC had Bernard Picton, a museum curator specializing in marine invertebrates, explain this:

The genital apparatus is on the right hand side of the body. So two nudibranchs come together and one faces one way and one faces the other way, with the right hand side of their bodies touching.

The penis from one fits into the female opening of the other one, and the penis from that one fits into the female opening of the first one, if you see what I mean.

Basically, whenever two of these slugs have sex, they’re having sex twice, at the same time, in a sort of hermaphroditic undersea 69.

When sex is done, the slugs discard their penises. Japanese scientists studying the creatures watched couples get it on 31 times, according to the BBC, and each time, at the end, the animals’ penises would float gently to the floor of the tank.

Again, that’s not so unusual. Believe it or not, plenty of creatures get rid of their sexual organs after they’re done using them.

Here’s the really crazy part: Within 24 hours, the penises grew back.

It turns out that slugs store part of their spiny, disposable penises inside their bodies. When they’re done with the outermost part, they get rid of it, and use the inside part to grow a new exterior penis. It’s sort of like human fingernails — you can cut a bit off and throw it away, but it’ll grow back, and then you can use it to have sex with a slug.

Scientists still don’t know exactly how many times sea slugs can do this, though. It may be that, like fingernails, these slug penises will grow back an indefinite number of times. Or it may be that they’re more like push-pops: Once you’re done eating the top part, you can squeeze more tasty ice cream out the tube, but there’s only so much there. Now please excuse me while I never eat a push-pop again.