Dextral snail shells coil rightwards, and sinistral shells coil leftwards. Sinistral Satsuma snails cannot mate with right-coiling Satsuma species, leading scientists to wonder how sinistrality could have spread through dextral populations.
Masaki Hoso of Tohoku University in Sendai, Japan, and his colleagues show that sinistrality has arisen independently multiple times in Satsuma, and more often where snakes in the Pareatidae family occur.
The team found that the Pareas iwasakii snake, which preys on the molluscs, must stick to right-coiling species as its jaws are specialized for grasping them. (Snake jaw, with extra teeth on the lower mandible, pictured.) That gave sinistral individuals an adaptive advantage, allowing left-coiling species to emerge.