Phylogeographic inferences concerning marine species are largely based on intertidal species. In high latitudes, intertidal species have been affected by ice coverage and ice scour, and therefore show northern range limitations during glaciations. In this study, we use the subtidal horse mussel (Modiolus modiolus) to investigate whether generalizations about genetic structure of high latititude intertidal species, specifically in the North Atlantic, are representative of other near shore taxa. We analysed genetic diversity, molecular variance, and geographical patterns of genetic relatedness using data from the mtDNA CO1 gene. Although we do find little to no haplotype structure in the North Atlantic, our results show that north-eastern Pacific individuals represent a different haplotype network with no haplotypes in common with Atlantic individuals. Thus, M. modiolus in the Pacific may represent an unrecognized species. Genetic diversity and population expansion times suggest a Pacific origin is most likely, with subsequent dispersal to the Atlantic. The lack of genetic structure in the Atlantic suggests that a rapid range expansion occurred less than 50 KYA, rather than a stepping stone mode of dispersal.

Certain members of the family Mytilidae have long been recognized either as major commercial species in aquaculture or as fouling pests. These sea mussels possess a remarkable capacity to get translocated to distant shores through driftwood and hull fouling in adult stages and through ballast water during larval stages. In a survey under the Ballast Water Management Programme, several individuals of the bivalve Modiolus auriculatus were collected from reinforced cement concrete harbour structures in Visakhapatnam. These collections constitute a new record of the species on the north-eastern coast of India.

The spatial heterogeneity of epifauna on a Modiolus modiolus reef off north-west Wales was investigated using divers. The community associated with these horse mussels was similar to that described previously from Loch Creran and the north basin of Strangford Lough. Some differences in epifauna may be attributable to the less sheltered nature of the site. Modiolus modiolus numbers and the associated epifaunal community were significantly different between ridge and trough sub-habitats. Troughs can be considered ‘reduced’ ridge communities whereas ridges have high densities of horse mussels and certain sessile taxa were correlated with their abundance. Modiolus modiolus aggregation as a competitive response to the feeding environment, enhanced food availability on ridges and sediment deposition amongst mussel clumps may start to explain the undulating bed-form. Patchiness in community composition and periodic cover by ophuroids has implications when considering the monitoring of the horse mussel community. Stratified, in situ recording of the highly populated ridges could improve the statistical sensitivity of monitoring horse mussel reefs whilst simultaneously focusing on the more sensitive indicators of fishing threats.

Beds of Modiolus modiolus, in areas of moderate to strong tidal currents, develop into reefs with a relief of wave like undulations 0.09–0.45 m in amplitude and length scales of 6–18 m. Cores taken by diver operated suction sampler were targeted at positions on the ridges and troughs of a reef, in the Irish Sea off north-west Wales, allowing the fauna to be compared between adjoining ridges and troughs. Sessile epifauna was mostly attached to the larger mussels clumped together on the ridges. The crevice fauna and infauna were also nearly three times more abundant on the ridges, but the lists of species were similar from the two sub-habitats. Species richness was higher on the ridges, however, diversity and evenness measures were similar for ridges and trough samples. The Modiolus sub-habitats were found to be distinct from other macrofaunal assemblages in the wider southern Irish Sea. Deposition of faecal pellets in the spatially complex habitat amongst the mussels provided conditions suitable for an infauna more typical of inshore muddy sands enriched by organic matter. The scale of the ridge and trough morphology may increase variability between replicates when grabs are used remotely to sample this type of biogenic feature.

Surveys by digital side-scan sonar, RoxAnnTM acoustic ground discrimination systems, multibeam echosounder and a sub-bottom profiling system showed that a Modiolus modiolus reef, in the Irish Sea off Pen Llŷn, north-west Wales, had a distinctive morphology and acoustic characteristics. The extent of the reef could therefore be determined and the benthic structure reliably mapped. The biogenic reef is in an area with moderately strong tidal currents and overlays lag gravel and cobbles with patchy sand veneers. The mussels form an undulating surface, orientated perpendicular to the current, with an average wavelength of 11.7 m and amplitude of 0.24 m that is significantly different from the surrounding seabed. Reef deposits reach a thickness of 1 m on top of the underlying lag gravels. The characteristic reef surface morphology helps distinguish the reef from the surrounding seabed on side-scan sonar and multibeam echosounder records and the undulations create the spatial complexity that influences the small-scale distribution of the associated epifauna, and infauna, reported in papers II and III of this series. The M. modiolus reef was recorded in the same location 40 y ago and has probably persisted there for over 150 y. Monitoring implications are discussed.