Why Sea Slugs Dispose of Their Own Penises

It’s cute and brightly colored, but this species of sea slug has a macabre sex life. Photograph by Jon Aguirre, My Shot


When it comes to kinky sex, nature has quite the imagination. Some animals devour their suitors after doing the deed, while others attach themselves, in a parasitic fashion, to their mates in order to reproduce.

But a new study finds that the ostentatious sea slug, or nudibranch, may take the cake—one species of this marine invertebrate cuts its own penis off after mating and regrows a new reproductive organ within 24 hours, whereupon it’s ready to mate again. (See sea slug pictures.)

“I have been working on the anatomy of nudibranchs for 20 years and I have never seen anything like that,” said Angel Valdes, a sea slug expert at California State Polytechnic University in Pomona, who was not involved in the study.

Valdes added in an email that sea slugs are known to sever other parts of their body in a process called autotomy. Some species will shed protrusions called cerata while others will drop the frilly “skirt” that runs around their body in order to distract a predator while the slug makes its escape. (Read about nudibranch defenses in National Geographic magazine.)

Ayami Sekizawa, of Osaka City University in Japan and lead author on the study, wrote in an email that while other animals, such as some species of octopuses, also break off their reproductive organs after mating, they can’t grow them back as far as researchers know.

Removable Penises

Most nudibranchs are simultaneous hermaphrodites, meaning individuals have both male and female reproductive organs—and they can deliver and accept sperm at the same time.

They can also store sperm from several mates and choose which ones to use in fertilizing their eggs by digesting the sperm they don’t want. (Also see pictures: “Fiery Sea Slug Discovered, Lays Lacy Egg Case.”)

When Sekizawa and her colleagues studied the mating habits of Goniobranchus reticulata (known as Chromodoris reticulata until last year), collected from shallow coral reefs near Sesoko Island (map) in the East China Sea, they noted that individuals would only sever their penis after disengaging from their partner.

And when researchers examined the discarded penises, they found sperm entangled in the backward-facing spines that cover the organ.

Sekizawa speculated that by removing their penis from their mate, the sea slugs were increasing the chances that their partner would use their sperm to fertilize its eggs, rather than a competitor’s sperm.

“If the sea slug left the penis in the mating partner’s female organ, it could not remove sperm of preceding mates,” wrote Sekizawa, whose study appeared February 13 in the journal Biology Letters. The researchers would need to conduct DNA tests in order to confirm this though.

Three Times a Charm

Sekizawa and her colleagues also found that intact individuals who hadn’t severed their male organ had a spiral structure in the middle of the inner duct of the penis. Discarded penises were missing the spiral structure.

They speculated that the spiral provided enough length for the sea slugs to grow and sever their penis at least three times, based on the fact that one G. reticulata in the study discarded and regrew its penis three times. (See “Barnacles Can Change Penis Size and Shape.“)

Valdes noted that other sea slug species have similar reproductive structures and couldn’t understand why this behavior hadn’t been observed before. But he’s excited to see whether other nudibranchs share this macabre habit.

Jane J. Lee is a news writer and editor at National Geographic.
  • Quattrone

    Detachable penis….

  • Sisifo

    Us primates have a similar system. The point is to remove the other guy’s semen from the vagina before leaving one’s own. The shape of the primate glans penis does exactly that. Here’s how: (1) Shove penis in vagina. This compresses previous semen and forces it behind glans rim (corona glandis). (2) Retract penis from vagina to move previous semen out of vagina. (3) Repeat as many times as required to get all previous semen out. (4) Put new semen in vagina. (5) No need to cut off own penis.

  • Brian skinner

    It’s disappointing that you conclude the article with a negative judgement that this behavior is “macabre”. Do you know that many male dragonflies before mating, will grasp their chosen one and flip them with enough of a G-force to eject the sperm of those who have come before? What would you make of that? The ways of nature should evoke wonder and and curiosity for it’s stratagems.

  • Severed in Woodland

    I’m guessing most men wish they do this on occasion.

  • Ke$ha

    That’s just weird- like polygamy…sexy sexy

  • Niki

    No fear for STDs I gues !

  • kathleen hilman

    If this practice is a normal process of this species, why should it be referred to as “macabre” ? Compared to what?

  • Emen jakarta

    So very amazing……

  • Jane

    ARE YOU KIDDING ME?!?!? While this story may be interesting, why in the world would you place it on the front page of your site today? I am a classroom teacher, and I use your site as my home page. My classes and I frequently peruse your top stories and discuss them. Imagine the unnecessary brouhaha in a classroom full of freshmen that this story’s headline would provoke. Really?!?! You couldn’t find another selection for the front page?!?!? I am huge supporter and promoter of National Geographic, as they have been very good to me, but seriously—consider your audiences when posting. The intern and/or the web producer needs his/her “chain rattled” for selecting and placing this story where it is. Thank you for listening.

  • arizona sucks

    natgeo has the right to put whatever they want on their homepage. valentine’s day is a holiday americans celebrate and it is about SEX. get over it

  • S Johnson

    Shut up, Jane.

  • Melany

    Jane, I can think of no better way to get kids interested in biology than a lively and ribald discussion of removable penises – take a pill. Quattrone, this has been a favorite of mine since the first time I heard it in 1992, thanks!

  • Michael

    Seriously Jane. Come on. This isn’t advertised as a kids site. Regardless, if you actually prepped for your class you could just bookmark the exact page you wanted to use and there would be no problem. Also, penis is and never has been a bad word. Interesting article.

  • Michael

    *is not*

  • carl harr

    the bobbit bug

  • Sjon Tiljær

    In response to the comment posted previous to this by Jane in Tempe, Arizona, perhaps such a classroom moment would give you the valuable opportunity to steer the students under your care from likely giggles and onto a more mature reception of the information in this interesting article. We educators lead by example, and our youth at that age group often relish an opportunity to grow up a bit. Just a thought. 🙂

  • Sloth Lover

    I do a similar thing

  • Chris

    @Sisifo- if that were true, then there’d be no need for birth control, simply continue intercourse after ejaculation.

    But yeah, not actually true.

  • Astrogal

    As I began to read Jane’s post, I thought she was going to say she was an elementary school teacher. I really had to laugh when she said she had a classroom full of freshmen. First and foremost, as a teacher, it would be advisable to peruse the site before use in the classroom so you can see if there’s anything you may deem inappropriate. But sheesh! Get a life, this was not a wildly inappropriate article and, I concur with other posters, it was a sex-related article on Valentine’s Day. It made perfect sense to post on the front page. I think Nat Geo did consider their audience and have no issue with this whatsoever.

  • Polona

    Sorry Jane, but if you were a good teacher you would see the benefits of humor and provocative subjects in getting students interested. It seems to me you are juts a control freak afraid of a little loose atmosphere in the class. Chill.

  • sandip paswan


  • Dee

    Really, Jane? Isn’t sex and the word “penis” part of all of nature? Just because YOU have a hang up with it is NOT a good reason to bash NatGeo for posting the artcle. If it’s that important to you to avoid any discussions about sex in your classroom, then do some prep work to pre-select what pages you want to share with your class. Or how about figuring out a way to discuss topics like this with your class in a manner that doesn’t create the “brouhaha” (as you put it).

  • Michelle

    Yes, Jane, this should be an opportunity for you to teach and also encourage maturity. This lesson would actually be PERFECT for freshmen, and they may giggle, but would hang on your every word.

  • gmax275

    Has anyone asked whether this could be a defensive strategy to prevent STD’s? Are these animals prone to skin infections that might be transmitted to the penis during sex?

    The article specifically says the penis was only severed after the coupling was complete and the animals had separated, but then quotes one researcher as saying that severing the penis would make the partner more likely to use the sperm from that mating. This did not make any sense to me, however, dropping something that could have been infected would make sense, though again it seems that as simultaneous hermaphrodites both would be penetrated…

  • Julia

    So it’s always a fresh one! That must inspire confidence in the “ladies”. Seriously, Jane. This story might be confusing for kindergarteners, but by high school I hope everyone knows what a penis is. A raucous classroom moment just fuels the kids’ curiosity, which is always a good thing.

  • christabel

    this is quiet amazing

  • Ms Gertz

    I find it interesting that the nudibranch can shed a very important body extremity and it grows back. I guess he wouldn’t have to worry about disease transmission. Banana slugs participate in an equally odd behavior called apophallation – the act of chewing off the penis after copulation. This keeps the sperm inside and allows for a quick retreat from enemies. All of this penis severing sounds painful indeed!

  • Maryann Harvey

    Brings new meaning to the words all for you.

  • Swifty

    Interesting article and to Jane…really? Freshman are certainly mature enough to have a discussion regarding this subject. There may be an initial amount of giggling, but I’m pretty sure group of adults would have a certain amount of amusing discussion prior to the serious part.

  • Maja

    Sea Slugs are one of the most favorite things to me! And this article is awesome and very much interesting.

    On the other hand… It is the most amazing how some people can be so ashamed/funny when they hear the word “penis”, Like it is some dirty word… In biology this is normal term, but I guess we can call it a “Love stick” for the sake of those who can handle the word “penis”. 🙂

  • Gayle

    I’m just glad men don’t have this ability. I can’t get my husband to pick up his socks, nevermind his discarded penises.

  • CHUT


  • kate

    I am so glad someone else got “detachable penis” stuck in their head on this article. Jane, I don’t know if you’re a prude or your hands are tied by the Arizona school system, but either way banning the word “penis” when talking about animals is going to go poorly for your kids.

  • Dave

    Jane, I am an eighth grader currently in Junior High. I find it very unnecessary to tell you your job. However, you say you are teaching freshman. This term is only used in college and in high school. If they are in high school, they should be old enough to know that “penis” isn’t a dirty word, but the PROPER scientific name for the male reproductive organ. Did I laugh at this? No. I find this nature of the Sea Slug very interesting and confusing. NOTHING is comical about this article. Simply tell them the first one to laugh gets a detention. Second and so on goes to the principal’s office, as they should all be mature enough to handle such facts.

  • Jane

    I am a fifth grade student and don’t like that you are arguing Jane and Dave

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