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

## TL;DR

Mussels form seasonal aggregations linked to reproduction, and plastic pollution disrupts this behavior, threatening ecosystems.

## Contribution

Demonstrated a novel link between mussel aggregation behavior, reproductive cycles, and plastic contamination effects.

## Key findings

- Mussel aggregation rate follows a seasonal cycle tied to reproductive maturity stages.
- Exposure to plastic leachates disrupts the cyclical aggregation pattern, potentially affecting reproduction.
- Aggregation behavior supports mussel reproduction and ecosystem stability.

## Abstract

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.

Mussels play a key role in coastal ecosystems by forming dense aggregations that provide habitats and enhance biodiversity. Here, we showed that the mussel aggregation rate varies seasonally according to their reproductive cycle, increasing during gamete maturation and declining postspawning, supporting a reproductive function for this behaviour. In this context, we subsequently found that exposure to plastic contaminants disrupts this seasonal cycle, potentially threatening their reproductive success, mussel bed stability and associated ecosystem services.