Microplastics (MP, defined as plastic particles smaller than 5 mm in diameter) have been increasing in the world ocean and observed to give negative impacts on invertebrate animals such as by interfering with their feeding. In natural marine environments, however, other types of natural particles such as seston already exist, and it is unknown whether MP impacts on animals are different from them. This study compares the effects of MP and seston (diatom shell) on a blue mussel (Mytilus trossulus). We measured respiration rate, filtration rate, byssus number and the body condition index (relative weight of soft body to shell weight) of the mussel exposed to different concentrations of MP and seston. No significant differences were found in either variable between MP and seston. Our results suggest that both MP and seston do not affect the mussel vital rates under the current level of pollution in the natural environments. Because the blue mussels can live in turbid waters, they may already have adapted to the presence of non-organic natural particles, and thus MP may not be regarded as a new stressor.