Copulatory behaviour increases sperm viability in female spiders

One remarkable reproductive feature in animals with internal fertilization is a reduction in sperm viability over time in females. Whether this reduction is driven by male–male competition and/or cryptic female choice is unclear. From the perspective of cryptic female choice, we postulated that sperm viability is affected by a particular male copulatory behaviour. In this study, we investigated the following aspects: (1) sperm viability in mated females vs. males; (2) whether sperm viability varies temporally after mating; and (3) whether male copulatory behaviour covaries positively with sperm viability within females. We used the spider Holocnemus pluchei, whose males use several copulatory behaviours to court females. We found that females that stored sperm for 4 or 15 days showed no difference in sperm viability but had lower sperm viability compared with males, and males that performed a longer post-insemination behaviour had higher sperm viability inside the female. It is unclear how sperm viability is reduced and how male post-insemination behaviour affects this. It is possible that extending copulation allows males to induce females to keep sperm alive for longer. This result is predicted by theory whereby males induce females to facilitate sperm to reach and fertilize eggs based on male postcopulatory behaviour.

Evolution of sperm morphology in Daphnia

Sperm, the male most fundamental reproductive feature allowing egg fertilization, evolves under sexual selection. Two components of sperm are mainly under selection, their number and their morphology (including the associated quality). Here we study the evolution of sperm morphology in the genus Daphnia. Based on microscopic observations of sperm morphologies mapped on a Daphnia phylogeny, we found that increase in sperm length evolved at least twice in Daphnia, once in the Daphnia sensus lato clade and once in the Ctenodaphnia clade. Furthermore, Daphnia s.l. lost the ability of cell compaction by losing extracellular encapsulation and exposing large filaments. We discuss the potential reasons for such convergent evolution in sperm morphology.