Size-dependent response of the mussel collective behaviour to plastic leachates and predator cues

Both individual and collective anti-predator behaviours are essential for the survival of many species. This is particu-larly true for ecosystem engineers such as intertidal mussels, which through their collective behaviour create novel 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 both 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 con -specifics when exposed to predator cues. This response was also found in large ones with a tendency to form more strongly bound aggregations and a considerable reduced activity, i.e. they significantly delayed their time to start to form aggregations and decreased their gross distance. These anti-predator behaviours were respectively inhibited and impaired in small and large mussels by MP leachates. The observed collective behavioural changes may reduce individual fitness by enhancing predation risk, particularly in small mussels that are the crab H. sanguineus's favourite preys. Given the key role of mussels as ecosystem engineers, our observations suggest that plastic pollution may have implication on M. edulis at the species level, but also enhancing a cascading effect towards a higher level of organisa-tion such as population, community and ultimately structure and function of intertidal ecosystem.

Automated summary

Both individual and collective anti-predator behaviors are important for the survival of many species, including intertidal mussels. Plastic pollution, especially microplastics, can disrupt these behaviors and indirectly affect predation risk. This study found that microplastic leachates affected the collective behaviors and anti-predator responses of small and large mussels, with small mussels showing taxis towards conspecifics and stronger aggregations in the presence of microplastics. All mussels responded to predator cues, but with different collective anti-predator behaviors. These behavioral changes may increase predation risk and have implications for the species and structure of intertidal ecosystems.