Female reproductive fluids 'rescue' sperm from phenotypic ageing in an external fertilizer

Female reproductive fluids (FRF) serve key reproductive functions in sexually reproducing animals, including modifying the way sperm swim and detect eggs, and influencing sperm lifespan. Despite the central role of FRF during fertilisation, we know surprisingly little about sperm-FRF interactions under different environmental conditions. Theory suggests that, in external fertilisers, FRF may 'rescue' sperm from ageing effects as they search to fertilise eggs. Here, we test the interaction between these two fundamental properties of the fertilisation environment, ejaculate age (i.e., time since ejaculation) and FRF, on a range of functional sperm phenotypes in a broadcast spawning mussel, Mytilus galloprovincialis. We found that the effects of ejaculate age on multivariate sperm motility traits and total sperm motility were altered by FRF, and that longer-lived sperm exhibit stronger, likely more advantageous, responses to FRF after periods of ageing. We also detected significant among-male variation in the relationship between sperm motility traits and ejaculate age; notably, these patterns were only revealed when sperm encountered FRF. Collectively these findings underscore the importance of considering female reproductive physiology when interpreting ageing-related declines in sperm motility, as doing so may expose important sources of variation in sperm phenotypic plasticity among males and environments.