Seasonal aggregation behaviour in the reef ecosystem engineer Mytilus edulisBiogenic Reefs

Marine Uguen¹, Sylvie Gaudron^1,2, Camille Henion¹, Laurent Seuront^1,3,4

¹Université de Lille, France - ²Sorbonne Université, France - ³Rhodes University, South Africa - ⁴Tokyo University of Marine Science and Technology, Japan

Aggregation behaviour is a fundamental ecological process that provides numerous benefits, including predator protection, resistance to abiotic stressors, and enhanced reproductive success. In the marine environment, aggregation benefits of species such as bivalves, polychaetes, corals, and sponges may extend well beyond the individual level as group formation generates complex three-dimensional biogenic reef structures that increase biodiversity and provide essential habitat for various marine species. In contrast to most reef-forming species, mussels retain their ability to move after settlement, forming dynamic aggregations called mussel beds that provide optimal habitats to a wide range of species. Despite the ecological significance of mussel beds, the dynamics of mussel aggregation remain poorly understood. Here, we investigated the seasonal patterns of aggregation in the blue mussel Mytilus edulis in the Eastern English Channel, where mussel reefs provide a habitat for ca. a hundred macrozoobenthic species, mainly annelids, crustaceans, molluscs and echinoderms. Through an eight-month study combining behavioural assays and histological analyses, we demonstrate that M. edulis aggregation follows a sinusoidal pattern closely linked to their reproductive maturity. Specifically, aggregation increased during gametes maturation until the onset of spawning, and subsequently declined, following a cyclical pattern, supporting a reproductive function of this aggregation behavior in mussel. These seasonal fluctuations in aggregation behaviour may have significant implications for the structure and stability of mussel reefs potentially leading to cascading effects on biodiversity. Our results underline the importance of understanding the dynamics of ecosystem engineer species, as they can act as a key factor for the conservation and management of these critical habitats, ensuring the continued functioning of coastal ecosystems.