"Sexual selection continues after mating up to the point of fertilization (post-mating sexual selection, PMSS) through intersexual selection (cryptic female choice, CFC) and intrasexual selection between ejaculates (sperm competition, SC). Although PMSS is ubiquitous in nature, the mechanisms and selective processes underlying both CFC and SC have remained largely enigmatic. CFC is often thought to be mediated by interactions between reproductive proteins of eggs and sperm that mediate the fusion of gametes at fertilization. However, only a few empirical studies have directly tested this prediction. Similarly, our understanding of intrasexual PMSS is equally poor, because we lack a general understanding of how selection operates on the multiple ejaculate traits that make up the ejaculate as single integrated unit. I will clarify these weakly known parts of the sexual selection in broadcast spawning marine mussel Mytilus galloprovincialis. Specifically, I will utilize the latest technological and methodological advances of proteomics and recently devised method for fluorescent staining of living spermatozoa. Together, these approaches will enable me to characterize and identify the proteome of male and female gametes and investigate whether egg-derived sperm chemoattractants have the capacity to bias competitive fertilization success (i.e. the outcome of CFC and SC). In addition, I will characterize patterns of multivariate selection on ejaculate traits when sperm compete to fertilize the eggs, thus addressing one of the key challenges remaining in sexual selection research. Proposed methodological approaches are rarely applied to evolutionary studies and have never been combined into PMSS research. I anticipate that this research will reveal previously unknown mechanisms of PMSS and thus may lead to groundbreaking findings that broaden our current understanding of sexual selection."