The direction in which a snail's shell twists affects much more than the animal's appearance. Angus Davison and his colleagues at the University of Nottingham, UK, have shown that the 'handedness' of great pond snail shells corresponds to the handedness of their courtship.
When hermaphrodites of the species Lymnaea stagnalis mate with each other, one takes a 'male role' by climbing onto the other and circling (pictured) before copulation. All 46 dextral males studied by Davison's team circled anticlockwise during courtship; all but one of 48 sinistral snails circled clockwise.
The researchers say that sinistral snails tend to have mirror-image brains, so the handedness of their behaviour is probably a direct consequence of the handedness of their brains. This probably also dictates the direction of twist in their shells.
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Zoology: Sex with a twist. Nature 456, 285 (2008). https://doi.org/10.1038/456285d