The Reason One Type Of Sea Slug Has Detachable Anatomy

After sex, a human male might find himself disposing of a condom. But what about throwing away the entire penis?

That's the situation that one sea slug finds itself in. Chromodoris reticulata is a species of sea slug that lives in the Pacific Ocean, according to the BBC. The slug makes its home in coral reefs, including around the coast of Japan and near Indonesia (via the University of Wisconsin La Crosse). This type of sea slug can only survive in saltwater conditions, so they're not found in freshwater bodies of water. With a red body and white underbelly, the Chromodoris reticulata's coloring scheme serves as camouflage from predators. The slugs can also produce mucus as a defense mechanism and are even able to obtain venom from their diet which they can use to deter predators.

The Chromodoris reticulata is part of the nudibranch order, meaning "naked gills." This order is composed entirely of invertebrates who share an interesting common sexual characteristic: they are all hermaphrodites.

Anatomy of the chromodoris reticulata slug

A 2013 study by Japanese researchers provided new, intimate detail into the anatomy and mating process of the Chromodoris reticulata. As a hermaphrodite, all members of the Chromodoris reticulata species contain both male and female sexual organs, according to the BBC. Their penises and vaginas are always found on the right side of their bodies, as the National Geographic notes.

When two slugs want to have sex with each other, they line up with their right sides pressed together. Then both slugs extend their penises so that they can simultaneously receive sperm into their vaginas and deposit it into their partner's vagina. The entire process can be over in seconds or can take a few minutes (via the Los Angeles Times). This mating ritual is unique enough without considering the extra step that the Chromodoris reticulata species takes: After slugs are done mating with each other, they detach their penises, and abandon them on the ocean floor.

Why these slugs dispose of their penises

It might seem illogical for slugs to dispose of their penises. But there's actually a few reasons this process might make evolutionary sense. For one, an investigation of the detached penises under a microscope revealed that these particular Chromodoris reticulata appendages are covered in spines, according to National Geographic. These spines might serve the purpose of scooping out rival mates' sperm from the partner's vagina, but it also makes it hard to retract the penis back into the body after sex.

Moreover, the detachment of the penis doesn't mean the animal can never have sex again. Instead, the slug's penis is composed of three parts, each about a centimeter long, which are kept coiled inside the slug's body. The slug only ever extends one part at a time, according to Mental Floss. That means, within 24 hours, the slug is raring and ready to go again. It can have sex up to three times before it has detached its entire penis. At that point, the penis is thought to grow back, but it's unclear in what sort of timeframe.