One Gene Decides Which Way a Japanese Snail Coils, Left or Right

Biologists report that natural variation in one gene decides whether a Japanese snail coils its shell to the left or the right, and the left-coiling snails are healthy, viable animals ^[1][2].

The gene is called myosin I a/b. The finding comes from a study in Current Biology by Alec Martin Lewis and colleagues, published in 2026 and titled "An ancient polymorphism in myosin I a/b determines chiral variation in Japanese Euhadra snails" ^[1]. The team worked on wild Euhadra snails in Japan ^[2].

The word "polymorphism" in that title means a stretch of DNA that comes in more than one common version within a species, as opposed to a rare one-off mutation. "Ancient" says both versions have been around for a very long time, which is what the team reports about both coiling forms ^[1][2].

Myosin is a protein that helps cells move and change shape. The researchers propose that small natural differences in this one myosin alter how cells twist in the earliest stages of development, and that this sets the direction of the shell and the body asymmetry that follows ^[2].

My reading is that the interest lies less in the coil than in the cost. Nature has handed biologists a left-right switch that does not break the animal. That is rarer and more useful than it sounds.

A twist in the cell, a twist in the shell

Most of us think of left and right as a surface matter, which hand you write with, which side your shell opens on. In an embryo it is a structural matter. Somewhere in the first divisions of a fertilized egg, the cells have to agree on a direction, and everything built afterward inherits that agreement.

Scientists call this property chirality: a shape that cannot be laid over its mirror image, the way a left hand will not fit a right-hand glove. A shell that spirals clockwise and one that spirals the other way are the same object in mirror form.

The Euhadra study ties that agreement to a protein that works at the scale of single cells. Myosin helps cells move and reshape themselves, and the researchers think different versions of it change which way cells twist early on ^[1][2].

If that holds, a molecular difference too small to see becomes a shell that spirals one way instead of the other. That is a bridge from the smallest scale of biology to the shape of a whole animal, and such bridges are hard to find.

The pond snail that failed the test

Left-coiling snails are not new to science. The team set Euhadra against a pond snail in which a mutation in a different gene can also produce left-coiling shells ^[1][2]. In the pond snail, the same change also causes abnormal development ^[1][2].

That contrast is the heart of the paper's value. A flip that arrives with damage is hard to read, because you cannot tell whether the gene is steering asymmetry or merely breaking something nearby. A flip that arrives with no damage, in an animal that grows and lives normally, shows the machinery of asymmetry doing its ordinary job, only pointed the other way.

The researchers say both Euhadra forms develop normally as embryos and are healthy, viable animals ^[1][2]. They contrast this with reversed organ arrangements in other animals, which they say are often associated with problems in embryo development ^[1][2].

The usual assumption about a reversed body plan is that it is a defect that survives by luck. These snails say otherwise. In Euhadra, left is simply another way to be a snail.

A valley where lefts and rights mix

Two more findings make the case for taking that seriously. The team reports that both coiling forms are ancient, and that the left-coiling form shows evidence of natural selection ^[1][2]. A trait that carries a signature of selection is not an accident that happened once and persisted. Something has acted on it over a long time. The summaries of the study do not say what that pressure is.

The second is a place. The researchers found a mountain valley in Japan where left- and right-coiling Euhadra can mate and exchange genes ^[1][2]. That matters because snails with opposite-sided genitals often struggle to mate at all ^[1][2]. Such a mismatch would ordinarily be expected to keep the two forms apart. In this valley it does not.

The finding sets a clean natural experiment next to a laboratory mutant. Where the two forms interbreed, genes pass between lefts and rights, and a variant that shifts one coil to the other can be followed through a mixed population, without the animals having been engineered. I would watch this valley more closely than any other detail in the paper.

Two kinds of handedness

Here the headline writers will be tempted, and here the story needs the most care. The word "handedness" covers two different things.

One is chirality, the left-right pattern of a body as it forms, such as which way a shell coils, or where organs sit. The University of Nottingham's account links the snail question to wider animal asymmetry, including the position of the human heart ^[2]. The other is behavioral handedness, the hand a person prefers to write with.

The Euhadra study is about the first. It does not show that this gene determines human hand preference ^[1][2]. Nothing in what the researchers have reported connects a snail's coil to whether a person is left-handed, and the paper should not be read as if it did.

The human link, where there is one, runs through the body plan. An animal's whole left-right layout traces back to early cells making a directional choice. A snail that makes that choice differently, and lives, gives biologists a way to ask how the choice is made. Whether anything similar happens in people is a question this study opens and does not answer.

What the study does not yet tell us

The strongest objection is also the plainest. A claim that one gene "determines" a trait is only as good as the experiments behind it, and the public summaries of this study do not describe them.

The number of snails and populations sampled, and the tests that tied myosin I a/b to the coil, have not been set out outside the paper ^[1][2]. Specialists reading the full text will judge how strong the causal link is.

I think the objection shrinks but does not vanish. The result is published in a peer-reviewed journal, and the shape of the argument is sound. It pairs a gene found in wild animals with a normal, healthy outcome, set against a mutant pond snail that is not healthy. That pairing is hard to explain away as artifact.

But the cell-twisting mechanism is the researchers' proposal, and the summaries describe it as such ^[2]. It should be treated as a well-motivated hypothesis until the details are public.

There is also a plainer limit on what one species can say. A gene that sets the coil in Japanese Euhadra tells us how one lineage of snails does it. Whether other animals use the same myosin, or a different lever entirely, is not something this study claims to settle.

What to watch

The unanswered questions are specific. Which changes in the protein separate the left-coiling form from the right-coiling one, and how do they alter myosin's action? How do those changes make early cells twist one way rather than the other? How common are the two forms across Euhadra populations, and what is the selection acting on the left-coiling form?

The last question is the one I would bet on. A trait that is ancient, healthy, under selection and freely exchanged in a mountain valley is a trait with a story. Whoever finds what the pressure is will learn why, in at least one snail, the left side has been worth keeping.