Snails with shells that coil anti-clockwise are less likely to fall prey to snakes than their clockwise-coiling cousins, scientists have discovered.
The arrangement of the snakes' teeth makes it difficult for the reptiles to grasp these "left-handed" snails.
The effect of this advantage on the survival of Satsuma snails is so great, say the researchers, that they could separate into a distinct species. Biologists in Japan report the finding in the journal Nature Communications.
Angle of attack
Satsuma snails come in two forms: those which have shells that coil anti-clockwise, considered sinistral or "left-handed" and those that coil clockwise, considered "right-handed".
Land snails copulate face-to-face, and a snail with a reverse-coiled shell has its whole body reversed - including the position of its genitals. This means that oppositely coiled individuals are anatomically incompatible when it comes to mating, so the scientists were puzzled as to why "reverse-coiled" snails continued to survive and evolve.
See the Snake Attack Video on BBC
To investigate, the team, led by Masaki Hoso from Tohoku University in Sendai, set up "predation experiments". They observed snail-eater snakes' (Pareas iwasaki) as they attempted to eat the snails. To consume the soft-bodied molluscs, the predators had to extract them from their shells.
"When attacking, the snake always tilts the head leftward," Dr Hoso told BBC News. The "right-handedness" of this sequence of movements, Dr Hoso explained, means that the snake "cannot grasp [left-handed] or sinistral snails well". The scientists wrote: "This study illustrates how a single gene for reproductive incompatibility could generate a new species by natural selection."