Both individual and collective anti-predator behaviours are essential for the survival of many species. This is particularly true for ecosystem engineers like intertidal mussels, which through their collective behaviour create new habitats for a range of organisms and biodiversity hotspots. However, contaminants may disrupt these behaviours and consequently indirectly affect exposure to predation risk at the population level. Among these, plastic litter is a major and ubiquitous contaminant of the marine environment. Here, we assessed the impact of microplastic (MP) leachates of the most produced plastic polymer, polypropylene (PlasticsEurope, 2022), at a high but locally relevant concentration (i.e. ca. 12 g L-1) on the collective behaviours and anti-predator responses of small and large Mytilus edulis mussels. Indeed, in contrast to large mussels, small ones reacted to MP leachates, showing a taxis towards conspecifics and stronger aggregations. All mussels reacted to the chemical cues of the predatory crab, Hemigrapsus sanguineus, but with two different collective anti-predator behaviours. Small mussels only showed a taxis towards conspecifics when exposed to predator cues. This response was also found in large ones with a tendency to form more strongly bound aggregations and an important activity reduction, i.e. they significantly delayed their time to start to form aggregations and decreased their gross distance. These anti-2 predator behaviours were respectively inhibited and impaired in small and large mussels by MP leachates. The observed collective behavioural changes are likely to reduce individual fitness and may enhance predation risk, especially for small mussels, that are H. sanguineus favourite preys. Given their key role as ecosystem engineers, our observations suggest that plastic pollution is likely to have implication on M. edulis at the species level, but also enhancing a cascading effect towards a higher level of organisation such as population, community and ultimately structure and function of intertidal ecosystem.