Gamete interactions play crucial roles in the outcome of reproductive competition. Nowhere is this more apparent than in broadcast spawning marine invertebrates, where there is no opportunity for reproductive behaviours before gamete release. Among such taxa, the blue mussel (Mytilus galloprovincialis) offers a powerful model system for studying gamete interactions during sperm competition – where ejaculates from multiple males compete for fertilisations. This is because mussels inherit mitochondria from both the eggs of mothers and the sperm of fathers. This presents the opportunity directly to track the fertilisation success of different competing males by dyeing the mitochondria of focal males’ sperm. I present the results of a study testing the efficacy of this technique, by comparing swimming behaviour and fertilisation ability of dyed and undyed sperm samples from the same males. I found no effect of the dye on sperm traits or fertilisation and therefore propose it as a feasible technique for tracking the individual success of ejaculates during sperm competition. The technique was then used to explore the potential role of egg chemoattractants as a novel mediator of sperm competition. Sperm from different males preferentially swim toward chemoattractants from different females in M. galloprovincialis. The dye technique allows assessment of the success of competing sperm in the presence of chemoattractants from different females, and therefore determination of whether chemoattractants act in female interests, allowing them to choose among competing sperm. Finally, I examine the mechanisms underlying these gamete interactions. Sperm RNAs have been suggested to play roles in sperm competition and fertilisation, and I explore whether differences in RNA profiles underpin: (a) response to different environmental cues (chemoattractants); and (b) variation in male, sperm and offspring fitness. The approach described here examines in unprecedented detail the gamete level processes that play critical roles during reproductive competition in broadcast spawners.