Age distribution, growth, and lifetime copulation frequency of a freshwater snail, Clithon retropictus (Neritidae)

Abstract

Clithon retropictus is a prosobranch snail species that reproduces in freshwater but spends its veliger period in the sea. The age distribution of this snail was investigated at three sites along Takase River in Japan by counting annual growth lines on their shells. Also, copulation performance was examined using sheaths of spermatophores stored in the bursa copulatrix of females. A mark–recapture census demonstrated that the number of growth lines was a good indicator of the age of a snail. The maximum number of growth lines was 20, suggesting that this species is one of the most long-lived freshwater gastropods. All the populations were female biased; the growth rate was higher and the life span was longer in females. Because spermatophore sheaths received by a female remained in her bursa copulatrix without being digested or discharged, the number of spermatophores implies her lifetime number of copulations. The number ranged from 0 to 91 and increased with age. Comparisons of population demonstrated that the average age was older, and the lifetime copulation number was significantly lower, at the upper stream site. These findings suggest that upstream migration had occurred only when the snails were very young and that copulation had been suppressed at the upstream site. The observed shorter longevity in males and the female-biased sex ratio are thought to be a result of the high cost paid by the male in multiple copulation.