Variation in Eusperm Length May Reflect Reproductive Barriers and Differences in Sperm Competition Intensity Among...

Reproductive barriers limit gene flow and drive population divergence. Sperm morphology plays an important role in reproductive barriers, as successful fertilization depends on how well sperm perform in the female environment. Specifically, sperm length must be adapted to fit the female reproductive tract and storage organs. We investigated sperm length in species and ecotypes of Littorina, a genus of promiscuous marine snails with internal fertilization. Various Littorina species have ecotypes adapted to different shoreline environments where reproductive traits like sperm length divergence might play a role in reinforcing these barriers. Due to their promiscuity, sperm competition likely plays a role in Littorina, and sperm length variation can give insights regarding sperm competition intensities. This study examined eusperm length in four species (L. fabalis, L. littorea, L. obtusata, L. saxatilis) and two different ecotypes in both L. fabalis and L. saxatilis. The ecotypes of both L. fabalis and L. saxatilis differed in eusperm lengths, indicating the potential for this trait to be involved in prezygotic reproductive barriers between ecotypes of these species. Among-species differences in eusperm length variation were observed and may be a result of different sperm competition intensities.