Genetic analysis of horse mussel bed populations in Scotland

Horse mussels (Modiolus modiolus) form biogenic reefs or beds, which are a conservation priority under national and international legislation (UK BAP, 2008; Rees, 2009; Tyler- Walters et al., 2016). Horse mussel beds are biodiversity hotspots and provide a number of ecosystem services including carbon sequestration and storage, and habitat provision for commercially important species (Rees, 2009; Burrows, 2014; Kent et al., 2017a). However, a decline in the extent of beds has been noted at a number of places across the UK (e.g. Strain et al., 2012) and further loss of this habitat is predicted over the next 100 years due to increased seawater temperatures (Gormley et al., 2013).
The UK is committed to creating an ecologically coherent network of MPAs in accordance with the OSPAR Convention, which highlights connectivity as a key element to assist the interpretation of ecological coherence. Connectivity has also been considered a fundamental element in the development of the Scottish MPA network. Effective spatial management therefore relies on an understanding of the linkages between protected features. However, very little is known about connectivity of benthic features between MPAs and beyond MPA boundaries. Unlike mobile species, mussels remain fixed to the seabed and rely on ocean currents for larval dispersal. Consequently, certain populations may act as larvae sources for other populations and thus, are of substantial value to maintaining a viable network. Horse mussel aggregations acting as sink populations are important settlement sites for larvae. Clarification of such relationships will help to determine the extent to which MPAs act independently or are dependent on other MPAs or features falling outside protected areas.
Analyses of genetic connectivity and diversity can be used to determine how populations relate (i.e. are connected) to one another and shed light on gene flow between areas. This report provides the methodology, results and discussion for an in-depth analysis of genetic connectivity of horse mussel beds in three distinct marine areas (West Coast, North-east and Orkney, Shetland Isles) in Scottish waters. Horse mussels also occur as individuals and clumps across Scotland, however, aggregations defined as ‘beds’ are considered of conservation importance and key settlement and recruitment sites. Therefore a selection of horse mussel bed populations were sampled for the purposes of this study.
Genetic connectivity results are discussed in relation to hydrodynamics, demographics and coastline geography as well as the implications for the MPA network. Recommendations for future work on horse mussel connectivity are provided, and adaptive capacity of the horse mussel is also considered in the context of future climate change.