From paper plates to daily contact lenses to disposable nappies, our lives are full of things that are meant to be used once and then thrown away. Thankfully, penises don’t fall into the list.

The same can’t be said for Chromodoris reticulata, a type of sea slug (or nudibranch) found in Indo-Pacific waters. It looks like a red-and-white baby’s slipper, with two tentacles on its head and a flowery set of gills on its backside. It also has the world’s only known example of a disposable penis, which snaps off every time it finishes mating. But don’t worry! It’s got spares—up to two back-up penises coiled away within its body.

Like almost all nudibranchs, C.reticulata is a hermaphrodite. Every individual is both male and female. When they have sex, both partners sidle up to each other (always on the right side) and penetrate each other at the same time. From afar it looks like a single white thread unites them. Zoom in closer, and you can make out two distinct parallel penises. They fill each other’s vaginas with sperm for around ten minutes.

View Images Two mating Chromodoris reticulata. From Sekizawa et al., Biology Letters

Ayami Sekizawa from Osaka City University watched the mating slugs, she noticed something odd. After they finished mating, they would crawl away with their penises still extruded from their bodies. Twenty minutes later, these trailing genitals broke off completely.

That ought to be a fairly ruinous end to the slug’s sex life, but after a day, C.reticulata is ready to mate again. Its secret is a penis that’s much longer than it lets on. When it mates, it extrudes around a centimetre of penis, but the full organ is three centimetres long. The rest remains coiled up inside the animal. Each segment may be disposable, but C.reticulata has three of them.

The cells in the coiled portion are different from those in the uncoiled part, which suggests that these replacements must undergo some changes before they can be deployed. It takes a day for this to happen. If Sekizawa allowed a recently mated animal to have sex with one that had been kept in isolation, the first slug had no penis to offer and the sex was purely one-way.

Nudibranchs have two pouches that branch off from their vagina where they can store sperm. When a male has sex with a female, there’s no guarantee that his sperm will actually fertilise her eggs. She could digest them. Alternatively, they could get flushed or scooped out by the penis of another male. Indeed, when Sekizawa looked at the detached penises under a microscope, she found that they are covered by many backward-pointing spines, which had entangled many sperm.

View Images Left: Spines on a nudibranch’s penis. Right: Sperm entangled in the spines. From Sekizawa et al., Biology Letters

These features—the transformation of the coiled penis, and the backwards-pointing spines—may provide the answer to the most obvious question: Why amputate your penis at all? Sekizawa thinks that it’s too difficult for the animal to retract a used penis, and is forced to discard its one-shot organ. Once all three segments are spent, the penis will eventually grow back although Sekizawa doesn’t know how long that will take.

Penis amputation occurs in other branches of the animal kingdom. For example, I’ve written about a spider whose males leaves their severed penises inside their partners, fertilising them from afar and becoming better fighters in the process.

And in one of those unfortunate freaks of scheduling, there’s yet another bizarre animal penis coming your way tomorrow…