Plastic wastes are widespread pollutants in marine environments and several studies have focused on their impacts on different ecosystems. Microplastics (MPs, < 5 mm) have been the focus of a particularly extensive investigation because of their ubiquity, large surface area, interactions with organisms, and the challenges they present in terms of disposal and management. However, studies regarding their fates and life cycle in ecosystems are still limited. This study examined the effects of presence of food (the green microalga Dunaliella salina) on egestion rate of polyethylene MPs in the mussel Mytilus galloprovincialis. Ingestion and egestion rates were calculated after 6, 12, 18, and 24 h of depuration. The results suggest that MPs exposed to algal food persisted in the mussels. A single exposure of MPs without food induced relatively rapid excretion by the mussels compared to MPs exposure with food. This could be attributed to the ability of mussels to distinguish between nutritive foods and unusable suspended particles. Thus, environmental factors, such as food abundance, can affect the cycle or fate of MPs in marine environments.