Gamete traits can vary widely among species, populations and
individuals, influencing fertilisation dynamics and overall reproductive
fitness. Sexual selection can play an important role in determining the
evolution of gamete traits with local environmental conditions
determining the strength and direction of sexual selection. Here we test
for signatures of post-mating selection on gamete traits in relation to
population density, and possible interactive effects of population
density and sperm concentration on sperm motility and fertilisation
rates among natural populations of mussels. Our study shows that males
from high density populations produce smaller sperm compared with males
from low density populations, but we detected no effect of population
origin on egg size. Our results also reveal that females from low
density populations tended to exhibit lower fertilisation rates across a
range of sperm concentrations, although this became less important as
sperm concentration increased. Variances in fertilisation success were
higher for females than males and the effect of gamete compatibility
between males and females increases as sperm concentrations increase.
These results suggest that local population density can influence gamete
traits and fertilisation dynamics but also highlight the importance of
phenotypic plasticity in governing sperm-egg interactions in a highly
dynamic selective environment.