Highly biased sex ratios in the twelve species of the freshwater snail genus Semisulcospira in and around Lake Biwa

The sex ratio is nearly equal in most organisms, while several ecological and evolutionary factors skew the sex ratio. We evaluated the sex ratios of 12 species in the freshwater snail genus Semisulcospira in and around Lake Biwa. We found highly female-biased sex ratios in all 12 species. The sex ratios were nearly equal or slightly female-biased in the smaller shell-size classes, while they were highly female-biased in the larger size classes. We also found that females were significantly larger in most of these species. Further, the species with a larger size difference between the sexes tended to exhibit a more female-biased sex ratio. Considering the general correlation between shell size and age, this pattern can be explained by longer lifespans in females, shaping the observed female-biased sex ratios. We also found that some Semisulcospira species showed female-biased sex ratios in all shell size classes. This pattern suggests that the distinct longevity hypothesis can not solely explain the observed biased sex ratios. We discussed the possible contributions of the sampling errors and selfish genetic elements to the observed biased sex ratios. This study provides essential information to disentangle the mechanisms of the biased sex ratio in the Semisulcospira snails.