UK United Kingdom

Who keeps the penis? Slugs and the battle of the sexes

Mating slugs and snails - like Chromodoris reticulata - undergo complicated negotiations over what goes where. Bernard Dupont

Last week a group of Japanese scientists published a paper describing a sea slug with a disposable penis.

As strange as this sounds to us, the concept of the detachable penis is not a surprise to malacologists, which is what we call people who study slugs, snails, octopus and other soft creatures.

Actually, I was thinking about snail sex a couple of weeks ago because our department ran a taxonomic workshop where, among other expert presentations, a malacologist named Winston Ponder told us how to tell one gastropod from another. I learned for the first time that many snails have an external cephalic penis.

Think about what that means. A cephalic penis doesn’t have a head, it grows out of your head. The snail penis looks like a fleshy dreadlock growing right near their stalked and googly eyes. At least they can see what they are doing when the deed is being done.

And suddenly I realised that snails have to carry their penis on their head, because if it were tucked inside their shell, how would they use it? I wondered why I hadn’t thought of this before, while Winston enlightened us about other anatomical oddities of our molluscan brothers.

You might think that two days of this kind of thing would send a roomful of human beings into a paroxysm of giggles, but taxonomists are made of stern stuff. That fact is that people who name and describe species spend a lot of their time illustrating and discussing the shape of sexual organs. (Unless you work on fish or birds, but that’s another story.)

The sexual organs are often the most distinctive feature of a species. We often know plants by their flowers. While you may recognise a rose bush from its foliage alone, you cannot be sure of the exact type of rose you until the flowers come out. For some reason, you are not ashamed to pluck that sexual organ and stick your nose in it. So don’t judge scientists for mounting snail pricks on slides, and categorising them.

Gastropod penises come in different forms, including the bilobed, or two-pronged version. Some snails have internal penises, and others are aphallate. That is a word that means “without penis”. These would be the dick-less snails. One might be tempted to call them girls, except that they all really are, even the ones with penises. This is because snails and slugs are hermaphrodites, with both girl and boy reproductive parts, completely functional, and often in multiples.

I wanted to ask Winston Ponder about if the aphallate species were considered “lower” gastropods because as humans we believe that anything without a penis cannot be highly evolved. I never got around to it, but my head was unaccountably full of questions about sex in snails, which may be why the latest news about slug sex caught my attention.

Slugs are snails that have lost their shells, and this has allowed them to put their penis back where it belongs, which to say away from their head. When slugs have sex, they line up head to toe because their male and female bits are always on the same side of their body. The logistics of two penises and two vaginas creates a complex set of sexual politics, including something quite sinister in land slugs, called apophallation.

The Greek prefix a- means without, and phallate means to have a penis, so it makes sense that aphallate snails have no penises. The Greed prefix apo- means separation, so apophallate slugs had a penis at one point but lost it somehow. This seems particularly tragic in the case of the well-endowed banana slug, whose scientific name means “big penis”. The truth is that apophallate is a fancy word for a violent act.

Sometimes land slugs get stuck during the sexual act. When they cannot separate after sex, according to Wikipedia, “one slug gnaws off either its own, or its partner’s penis”.

I find this hard to believe.

Even a slug is going to hesitate about gnawing off its own penis when it could bite off another slug’s penis instead. The alternative is a level of altruism that I cannot believe: slugs in love protecting their partner by making the ultimate sacrifice. I prefer to believe this quote from an actual malacologist who has considered the pay off involved:

“The apophallated slug … cannot regrow his penis and is now obligated to be a female and forced to offer eggs. It may be that the castrator can raise his reproductive success by increasing locally the density of females.”

This brings up the idea that although slugs start out with the equipment to be male or female, they can be forced to take the role of the female. Growing and nurturing eggs is an energetic burden, and if the decision of who gets to be the male happens during the sexual act, then we have a true battle of the sexes.

Some flatworms “penis fence”, which means that they fight off their partner’s penis while trying to make contact with their own. The first to penetrate remains male, while the other has to carry and care for their developing eggs. The Australian species that does this is not only two-faced, it has two penises and two-tailed sperm.

Which brings us to the sea slug with the disposable penis. Chromodoris reticulata is a sea slug only a few centimetres long, and when the scientists brought specimens into the lab for observation, they were not surprised by the sight of two penises reciprocally inserted, but by the fact that their penises fell off about 20 minutes after the sexual act.

By offering these individuals mating partners at various intervals, they learned that it took about 24 hours for these slugs to regrow their penises. One individual was able to mate three times in three days, regrowing his (or her?) appendage each time.

The most famous detached appendage is the hectocotylus, or octopus penis, first described by Aristotle. Because it remained inside the female, it was long thought to be a parasitic worm.

Argonaut octopus males are so small compared to the females that scientists did not realise they were the same species for hundreds of years. They use their modified arm to transfer the sperm to the female, but it breaks off and they never get it back. As a consequence they mate only once in their lifetime.

Athough losing your penis is not unusual among molluscs, growing it back again is a neat trick.

Join the conversation

14 Comments sorted by

  1. Kim Darcy


    Thanks Susan. What a fascinating article. Now, this is the sort of reading that makes the conversation worthwhile. One memory trick I have learnt over the years is to focus on two words or phrases in an article. I found that saying or writing those words/phrases down later, provokes memories of a great deal more info from that article. From your article, I need remember only one new word in its adjectival form - "apophallated slug". I can assure you, your name and this article shall be spread far and wide from my lips and keyboard. What a corker of a word and concept. The imagery!

  2. Peter Ormonde
    Peter Ormonde is a Friend of The Conversation.


    Cor - this evolutionary biology is a sordid smutty business isn't it?

    Raises some interesting moral questions for those "intelligent design" enthusiasts doesn't it? What on earth was He thinking on day five... hermaphrodites, peripatetic pricks and all that mucous... most glad He got his act together later on in the week. Well a bit of an improvement anyway.

    But as you will have realised from some commenters here over recent weeks there are still a few of us about who believe that anything without a penis cannot be highly evolved - yep that's you Dale. Seems some of us are still lugging about a few cephalic penis genes.

    God you lot must have strange dinner parties.

  3. Sue Ieraci

    Public hospital clinician

    (Is one permitted to make this observation here:

    This adds a whole new meaning to the term "dick head".)

    1. Susan Lawler

      Head of Department, Department of Environmental Management & Ecology at La Trobe University

      In reply to Sue Ieraci

      It even gives us a new insult to use on certain dickheads, as in "you deserve to become an apophallated slug."

  4. Anthony Nolan

    logged in via email

    This article is fascinating. Ain't nature wonderful. I used to work in Child Protection in NSW; many people in that department also had the ability to lose a dick, when it seemed expedient, and then grow it back when environmental circumstances deemed dicks to be an advantage. I can't tell you how happy this article has made me - it confirms my suspicions that I was working in the understory - among human slugs; by which I mean my colleagues, not the 'client' community. But hey, dickless slugs, they're everywhere! I also used to teach at Usyd and let me tell you ya need gumboots to wade through the human slugs in that place.

  5. Dale Bloom


    This must be the 10th article appearing in The Conversation on penises, and I think all of them written by female authors from universities.

    Most probable reasons for this number of articles on penises by female authors from universities.

    1/Attempts to denigrate, mock or malign the male gender (I give this 50 % probability).

    2/ Penis envy (I give this 25 % probability).

    3/ Castration desire (I give this 25 % probability).

  6. Comment removed by moderator.

  7. Greg Young

    Program Director

    Thanks Susan for a truly informative and entertaining article.

    Your argument about the apophallated slug seems to assume that being male enhances the chances of reproductive success. Is that actually correct? If the inverse were true, then there might be an evolutionary driver for a slug to self-castrate. (Sorry if this is a dumb question).

    1. Dale Bloom


      In reply to Greg Young

      If you were to self-castrate, I am certain you would be more than welcome in an Australian university.

    2. Susan Lawler

      Head of Department, Department of Environmental Management & Ecology at La Trobe University

      In reply to Greg Young

      Hi Greg,
      We usually measure reproductive success in quantity, and it is possible to produce many more offspring by donating sperm than it is to nurture the developing eggs. This is obvious in a human context, where men can have hundreds of offspring if they have access to a harem.

      The case is not nearly as clear cut for slugs, particularly since they have both ovaries and testes, but it is still easier to produce sperm than it is to make eggs.

      An apophallated slug is stuck with the egg option and loses the ability to fertilise other slugs, which is the cheaper way to become a parent.

      It has occurred to me that there may be a need to self castrate, if your own penis is stuck you may not have a choice.

    3. Greg Young

      Program Director

      In reply to Susan Lawler

      Thanks for the explanation Susan. It was their hermaphroditic nature that prompted my question, and your answer makes sense.

      As to your last para, what a wince-inducing image! :-)

  8. Peter Hindrup


    Thank you Susan, This is interesting and amusing.

    Would it also follow that these creatures do not get fixated on their penises, as many humans seem to do?

  9. Arthur James Egleton Robey

    Industrial Electrician

    "and other soft creatures".
    Oh, I say. Steady on. That's a bit rough isn't it?