Aggregation Behaviour as an Adaptive Reproductive Strategy in a Marine Ecosystem Engineer

Aggregation behaviour is a key process shared by numerous organisms, providing benefits such as predator protection, resistance to abiotic stressors and enhancing reproductive success. In coastal waters, mussel aggregations shape ecosystems and increase biodiversity; however, many aspects of this ecologically key behaviour remain unexplored. Here, we investigated the potential link between aggregation rate and reproductive maturity stages in the blue mussel Mytilus edulis over a seasonal cycle. We showed that the seasonality of M. edulis aggregation rate was a significant sinusoidal function of the mean reproductive maturity stage. Specifically, aggregation increased during gamete maturation until the onset of spawning and subsequently declined, following a cyclical pattern, supporting a reproductive function of this aggregation behaviour in mussels. Given the ubiquity of plastics as marine contaminants and their known harmful effects on organisms, we subsequently assessed how the temporal patterns observed in aggregation behaviour may be impacted by an exposure to leachates from plastic pellets. Thus, an exposure to plastic leachates led to a loss of the cyclical pattern observed in control seawater, potentially compromising mussel reproduction and the stability of mussel bed habitat. Our study showed the relationship between aggregation behaviour, reproduction and contaminants in mussels. Understanding these complex interactions is crucial, given the pivotal role mussels and their aggregations play in shaping marine ecosystems, offering new insights into the resilience of this habitat facing contemporary challenges.