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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.