Modiolus modiolus beds are biodiversity hotspots and may function as essential fish habitats in Danish marine waters

The large-bodied, filter-feeding northern horse mussel, Modiolus modiolus (Linnaeus, 1758), forms aggregations on soft and mixed substrates between 0-200 meters depth. The longevity of the individual mussel and the horse mussel beds may exceed 45 and 100 years, respectively. Horse mussel beds are currently protected under the EU Habitats Directive habitat type “1170 Reefs”. Across its pan-boreal distribution, international sea conventions have documented declines caused by bottom trawling. Both the species and its biogenic habitats are considered threatened and in need of further protection and restoration throughout Danish and European seas. While these habitats are well known to creating local biodiversity hotspots of benthic invertebrates, knowledge of their functioning as fish habitats are remarkably limited. We conducted an experimental field study of the fish and invertebrate epifauna associated with a M. modiolus habitat in Danish waters. Historically, the biodiversity associated with M. modiolus habitats was first reported upon from the Sound and Kattegat as part of the Modiola-community (e.g., Petersen, 1913). Several recent M. modiolus habitat studies in the Atlantic and adjacent seas have recorded high diversities of associated invertebrate fauna and macroalgae species and documented these habitats as being local biodiversity hotspots in different biozones. Other studies have linked variation in habitat structure and complexity to different functioning as substrate for sessile organisms and nursery grounds for mobile invertebrates, such as welks and decapods, and other common prey for fish. Our field study showed several fish and invertebrates species were associated with the M. modiolus habitat. The initial results from our experimental study suggest that M. modiolus habitats are essential habitats not only for epifaunal invertebrates but also for several fish species. Studies that integrate habitat-forming species, invertebrates, fish and their interactions may add great value to habitat protection and restoration actions and support ecosystem-based management solutions. This study was supported by the research projects, MarHab (Horizon Europe 2024-2027) and ECOSPACE (European Maritime, Fisheries and Aquaculture Fund 2023-2027).