A Blog by Ed Yong

Single gene creates snake-resistant mirror-image snails, and maybe some new species


In the novel Dr. No, the titular villain explains to James Bond that he once survived an assassination attempt because his heart was in the wrong place. The good doctor had a condition called situs inversus – his organs were mirror images of their normal versions, found on the opposite side of his body. His heart, being on the right, was unharmed when his would-be murderer stabbed the left side of his chest. Having a mirror-image body can be useful when someone’s out to kill you and while that’s true for criminal masterminds, it also applies to snails.

In Japan, Satsuma snails have shells that mostly coil in the same direction. If you put your finger in the shell’s centre and follow the spiral outwards, you would probably move in a clockwise circle. And Iwasaki’s snail-eating snake knows it.

The snake, as its name suggests, eats snails and both its body and behaviour are beautifully adapted to prise these animals from their defensive shells. A clockwise shell (known as a ‘dextral’ one) sits on the snail’s right side, so the snake stalks from the left. It grabs the shell with the upper jaw and sticks its lower one into the gap (see above left). Both sides of the lower jaw can move independently, and by shifting them back and forth in turn, the snake slowly ratchets the snail out of its fortress.

But shells that spiral anti-clockwise (‘sinistrals’) protect the part of the snail that the snake attacks. They are much tougher to break into and the snake fails to switch its attack plan by striking from the opposite side. Not only that, but its equipment falls short. Its lower jaws are full of wicked, backwards-pointing fangs for hooking into snail flesh, but it has around a dozen more of these on the right side than the left (see above right). That’s great for attacking snails with clockwise dextral shells, but less so for those with anti-clockwise sinistral ones.

Masaki Hoso from Tohoku University thinks that this advantage explains why the sinistral snails are still around, and it may even lead to the two populations splitting into separate species, according to the coil of their shells. Why? Because shell shape doesn’t just determine a snail’s chances of fending off snakes, it affects their sex lives too.

The Satsuma snails mate face-to-face. Every animal has both male and female sex organs and when a pair lines up, they harpoon each other with their respective penises. All of this relies on them having their equipment on the same side and that, in turn, depends on the spiral of their shells. There is no position in the Satsuma snail Kama Sutra that will allow a dextral and sinistral snail to mate.

Amazingly, a single gene controls the way the shell spirals – one version creates a dextral snail and the other creates a sinistral. Over time, the less common version should lose out. Dextral snails are in the majority, so a dextral snail would have a far larger supply of mates than a sinistral one. They should eventually outcompete their minority rivals, driving them and their genes to extinction. That is, unless having a sinistral shell has some sort of benefit that outweighs the difficulty in finding a suitably shaped sexual partner – a benefit like snake resistance. In a series of feeding experiments, Hoso found that dextral snails always died at the jaws of Iwasaki’s snail-eater, while seven out of eight sinistral snails survived.

What of the snakes? They never changed their behaviour to deal with their mirror-image prey. It’s possible that ‘left-handed’ snakes with their own mirror-imaged bodies will eventually appear to take advantage of the sinistral snails. But no such animals have ever been found. With so many species of snail available, and most of them being dextral, it’s more likely that the snakes would just swap to a different menu.

At least six other species of snake – all members of the aptly named ‘slug-eaters’ – attack snails in the same way. By looking throughout Japan, Hoso found that where these snakes are found, the sinistral Satsuma snails have a stronger presence (see below left).

As the sinistrals gain a foothold, they should start to split away from their dextral cousins. Members of the two groups can’t mate with one another, so their genes are effectively isolated within their own kind. They are free to evolve in their own independent directions, even though they may live in the same place. Hoso found that this has already happened. He sequenced the DNA of all of Japan’s Satsuma species and built a family tree that shows their evolutionary relationships. The tree clearly showed that sinistral species have evolved from dextral ones on at least six different occasions (see below right).


The same thing has happened throughout the world. The majority of snails are dextral, but sinistral forms have evolved time and time again. In at least four of these events, this reversal was driven by a change in a single gene.

This change may be simple, but its consequences are vast. In a virtual instant, you get a group of snails that are adapted against their predators and separated from members of their own species, free to evolve down their own path. Hoso thinks that such events are examples of the “hopeful monster” idea coined by a geneticist called Richard Goldschmidt, where small genetic changes could bring about very large evolutionary ones.

Reference: Nature Communications http://dx.doi.org/10.1038/ncomms1133

All images and videos courtesy and copyright of Masaki Hoso.

More on the origin of species:

And apropos of nothing, here is a video of Isabella Rossellini, of all people, enacting snail sex. I found this while doing some of the background research for this piece and I can’t scrub it out of my mind. NSFW, obviously.
If the citation link isn’t working, read why here

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6 thoughts on “Single gene creates snake-resistant mirror-image snails, and maybe some new species

  1. This brings up the muddy issue of what exactl constitutes a species again, but if the dextral and sinistral snails already can’t interbreed in the wild, could not the argument be made that they are already separate species, and not just in the process of potentially becoming so?

  2. As a teacher, I was very impressed by the article, particularly for its pedagogical virtues (even if not intended). For me, it’s a near perfect example of applying the scientific methodology to address questions everybody could easily understand or ask. Sure, it may be hard to find the answers, and that may require expert techniques, but once found, they can be put in layman’s terms.
    One could imagine the following scenario : a speaker or teacher outlines the situation, brings up the idea that the asymmetry of the snake jaw could explain why the sinistral snail thrives, then ask the audience how it would try to prove/disprove it. The answers are all in the article… A great moment of teaching science!

    But concerning the second thesis, that of a new species, I am not convinced yet. Maybe the host or another reader could first answer the following questions, for which I cannot find answers in the linked article or anywhere else (IANA biology teacher):
    – is that snail diploid (i suppose it is)?
    – if yes: is the “levogyre” gene recessive (probably, cannot find answer in article)?
    – if yes: how can one speak of a “new species”, if the old one can produce a member of the new one?

  3. Should snake predation not strongly favour sinistral variants? Or do the numbers simply not add up, allowing large numbers of dextral forms to survive regardless?

    Also, regarding “…the snake slowly ratchets the snail out of its fortress”. Mental image of the snail’s thoughts at that moment (grant me poetic licence on the point): NNNNNNNNNOOOOOOOOOOOOOOOOOOO!!!

  4. Sistral is an excellent word!

    Taar, as the snails reproduce sexually, they are diploid.
    The mutation that leads to a dextral snail producing a sistral offspring isn’t going to happen often enough to disrupt the production of a new species. That said, the evidence that this left/ right spiralling has led to new species is pretty strong but I don’t think they have the smoking gun yet!

  5. Taar – the sinistral form is actually probably dominant (see refs 6, 8, 9 of the original paper – they’re all different snail species, as far as I can tell, but they all seem to work the same way from my brief investigation).

    It’s also maternal effect, so a mother carrying the sinistral gene (e.g. from a de novo mutation, who was dextral herself) would produce all sinistral offspring – who may or may not carry the sinistral gene! Only if an offspring snail does carry the gene and mates with another snail that also carries it will S homozygous offspring be produced.

    I think the maternal effect complicates things more than the dominance/recessivity. Even with sinistrality being dominant, two sinistral heterozygotes could still produce dextral offspring. Speciation probably depends more on predation leading to the weeding out of dextral snails and a proportional increase in sinistral homozygous snails, which can only produce sinistral offspring.

    That’s my guess, anyway, but I’m no expert.

  6. Is there any species of snail that exists in practically equal numbers of dextral and sinistral forms because of evolutionary pressure to avoid snakes and such that the dextral form is a mirror image of the sinistral form? As long as the dextral form can produce an offspring of the sinistral form and vice versa, that doesn’t make one form a different species from the other. I think it’s possible for a species of snail to have evolutionary pressure to have pretty much equal numbers of both forms by following this evolutionary history: it was originally dextral, then sinistral forms starting becoming more common to avoid a type of snake with an asymmetrical head then once there are more of the sinistral forms, then inverted snakes started being selected for and in the long run, for which ever form of snail there’s slightly more of the form of snake designed to eat that form of snail, which in turn creates evolutionary pressure against the form of snail there’s more of. If the evolutionary pressure from the snakes is storng enough, it could exceed the evolutionary pressure of being the form there is more of for mating success because an individual of the form there’s fewer of can still find another individual of the form there’s fewer of and the main cause of there being more of one form than the other in the long run being random fluctuations in the population of each form. I’m also wondering if there is any species with a conch shell that mate in such a way that there’s a point of inversion between the 2 individuals and because of that, there’s evolutionary pressure to be which ever form there’s fewer of and because the orientation of one of the mating individuals has all 3 coordinates flipped compared to the other instead of only 2 of them, one is a mirror image of the other. That might not work very well because the 2 mating individuals would not be equal with respect to the direction of gravity and therefore would require one to do the hard climbing on top of the other but if it did occur, it’s still possible for one form to evolve as the mirror image of the other form because there could be randomness of which individual will climb on top of the other from genetic drift being stronger than natural selection and that would cause both forms to have equal evolutionary pressure of shape.

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