Abstract

Biogenic reefs created by Modiolus modiolus (Linnaeus, 1758) (horse mussel reefs) are marine habitats which support high levels of species biodiversity and provide valuable ecosystem services. Currently, M. modiolus reefs are listed as a threatened and/or declining species and habitat in all OSPAR regions and thus are highlighted as a conservation priority under the EU Marine Strategy Framework Directive (MSFD). Determining patterns of larval dispersal and genetic connectivity of remaining horse mussel populations can inform management efforts and is a critical component of effective marine spatial planning (MSP). Larval dispersal patterns and genetic structure were determined for several M. modiolus bed populations in the Irish Sea including those in Wales (North Pen Llŷn), Isle of Man (Point of Ayre) and Northern Ireland (Ards Peninsula and Strangford Lough). Simulations of larval dispersal suggested extant connectivity between populations within the Irish Sea. Results from the genetic analysis carried out using newly developed microsatellite DNA markers were consistent with those of the biophysical model. Results indicated moderately significant differentiation between the Northern Ireland populations and those in the Isle of Man and Wales. Simulations of larval dispersal over a 30 day pelagic larval duration (PLD) suggest that connectivity over a spatial scale of 150km is possible between some source and sink populations. However, it appears unlikely that larvae from Northern Ireland will connect directly with sites on the Llŷn or Isle of Man. It also appears unlikely that larvae from the Llŷn connect directly to any of the other sites. Taken together the data establishes a baseline for underpinning management and conservation of these important and threatened marine habitats in the southern part of the known range.

Highlights

  • The marine bivalve Modiolus modiolus (Linnaeus, 1758) is an Arctic-Boreal species with a distribution that extends from the seas around Scandinavia and Iceland southward to the Bay of Biscay [1]

  • Larval dispersal patterns and genetic structure were determined for several M. modiolus bed populations in the Irish Sea including those in Wales (North Pen Llŷn), Isle of Man (Point of Ayre) and Northern Ireland (Ards Peninsula and Strangford Lough)

  • Existing data places the southern limit of these reefs in the Irish Sea [1] where well-documented locations include the Ards Peninsula and Strangford Lough (Northern Ireland), the Point of Ayre (Isle of Man), and the North Llŷn (Wales) [3] (Fig 1)

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Summary

Introduction

The marine bivalve Modiolus modiolus (Linnaeus, 1758) (horse mussel) is an Arctic-Boreal species with a distribution that extends from the seas around Scandinavia and Iceland southward to the Bay of Biscay [1]. M. modiolus reefs are considered a type of Annex I biogenic reef under the Habitats Directive (Council Directive 92/43/EEC on the conservation of natural habitats and of wild fauna and flora) [2]. These reefs are scarce and limited in their distribution in contrast to records of individuals and comprise dense continuous beds, or scattered aggregations of this large mussel [2]. Existing data places the southern limit of these reefs in the Irish Sea [1] where well-documented locations include the Ards Peninsula and Strangford Lough (Northern Ireland), the Point of Ayre (Isle of Man), and the North Llŷn (Wales) [3] (Fig 1).

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