Left snail

The shells of almost all common garden snails coil to the right. Almost all.

Angus Davison, CC BY-SA 4.0, via Wikimedia Commons

Ninety-nine percent of all animals show bilateral symmetry, that is, they have left and right halves that are roughly mirror images of each other. I say “roughly” because most animals have some kind of asymmetry: human hearts are more on the left side than the right, for example. And common garden snail shells coil to the right.

But not always. Some people have their heart on the right side. Actually, all of their internal organs are mirrored, flipped along the left-right axis. This genetic condition is called situs inversus and it’s pretty bizarre. There is an equivalent (we think) in some very rare snails: the snail’s shell coils to the left instead of the right.

(Side note: this depends on snail species. Most common garden snails coil to the right; other species may coil predominantly to the left instead.)

Enter Jeremy. In 2016 a retired scientist discovered a snail whose shell went the other way, with a whirl on the left. A research team at the University of Nottingham studied Jeremy for genetic abnormalities in the hope of getting an insight into conditions like situs inversus. They put out a public appeal for other lefty snails so they could breed them. (Jeremy’s genitalia was also inverted, making breeding with regular snails a challenge.) And so an Internet celebrity was born!

Some more left-coiled snails were found and sent to the university. Jeremy got some kids… but they were all right-coiled. If the condition is genetic, why were there no more left-coiling snails? The current hypothesis is that the coiling direction is determined not by the genes of the animal itself, but by the genes of its mother (the so-called “maternal effect”). I’m going to write about that another time, because it’s quite a fascinating phenomenon in itself.

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