# Population density effects on gamete traits and fertilisation dynamics under varying sperm environments in mussels

## TL;DR

This study shows how population density affects gamete traits and fertilization in mussels, highlighting the role of phenotypic plasticity in reproductive success.

## Contribution

The study reveals how population density and sperm concentration interact to influence gamete traits and fertilization dynamics in mussels.

## Key findings

- Males from high-density populations produce smaller sperm than those from low-density populations.
- Females from low-density populations show lower fertilization rates at lower sperm concentrations.
- Phenotypic plasticity plays a key role in sperm-egg interactions under varying environmental conditions.

## Abstract

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 density 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.

Our study not only shows that local population density can influence gamete traits and fertilisation dynamics but also highlights the importance of phenotypic plasticity in governing sperm–egg interactions in a highly dynamic selective environment. Our study highlights that the processes modulating fertilisation success are more complex than previously appreciated. Indeed, our work has important implications for interpretation of past studies on compatibility and intrinsic parental effects which have not taken into consideration the fertilisation environment. Furthermore, our results will change the way researchers in this field think about and study how sexual selection targets gametes.