Most snails boast a shell that spirals out from the centre in a clockwise direction. But a few ‘lefty’ snails have shells that coil the opposite way, and now scientists know why.

Masanori Abe and Reiko Kuroda, now at Chubu University in Kasugai, Japan, used the CRISPR–Cas9 gene-editing technique in the pond snail Lymnaea stagnalis to knock out a gene called Lsdia1. This encodes a protein that helps to govern a cell’s internal framework.

The team found that snails missing this gene had offspring with the rare anticlockwise shells, suggesting that Lsdia1 is key to determining the creatures’ ‘handedness’. These snails passed on the mutated gene and shell type to subsequent generations; those with functioning copies of the gene had shells that coiled the normal way. The gene influenced the pattern of cell division from the stage at which snail embryos divided from one cell into two.

Research on genes such as Lsdia1 could shed light on a rare human condition called situs inversus, in which the positions of the organs in the body are reversed, the authors say.