Small-scale variation within a Modiolus modiolus (Mollusca: Bivalvia) reef in the Irish Sea: I. Seabed mapping and reef morphology

Abstract

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.

An enigmatic kilometer-scale concentration of small mytilids (Late Miocene, Guadalquivir Basin, S Spain)

“…Large concentrations of the horse mussel Modiolus modiolus appear at the Bay of Fundy (NE Canada) and Strangford Lough (N Ireland) (Magorrian and Service, 1998;Wildish et al, , 2009Lindenbaum et al, 2008;Gormley et al, 2013;Elsäßer et al, 2013). These bioconstructions form kilometer-scale elongated structures, 1-3 m in height, oriented perpendicular to sand waves from shallow subtidal (5 m) down to about 100 m water depth, being 20-40 m the preferable bathymetric distribution (Elsäßer et al, 2013).…”

Review of the functional morphology, biology and perturbation impacts on the boreal, habitat-forming horse musselModiolus modiolus(Bivalvia: Mytilidae: Modiolinae)

“…Subsequently, biogenic habitats formed by M. modiolus have been recorded widely from northern Atlantic and Pacific waters. The ecological importance of M. modiolus as a habitat for a diverse associated biota has been reported upon from the North Atlantic and adjacent seas (Petersen 1918;Spärck 1929Spärck , 1935Spärck , 1937Einarsson 1941;Thorson 1950Thorson , 1957Jones 1951;Roberts 1975;Noble et al 1976;Warwick & Davies 1977;Comely 1978;Logan et al 1984;Sebens 1985;Witman 1985Witman , 1987Davoult et al 1988;Ojeda & Dearborn 1989;Brown 1990;Magorrian 1996;Dinesen 1999;Mair et al 2000;Dinesen & Bruntse 2001;Moore et al 2006;Lindenbaum et al 2008;Rees et al 2008;Sanderson et al 2008;Rees 2009;Roberts et al 2011;Solyanko et al 2011;Ragnarsson & Burgos 2012) and Northeast Pacific coastal shelf areas (Shelford 1935;Soot-Ryen 1955;Selin 2011). Modiolus modiolus aggregations include three major community components: (1) a single dense layer of living individuals residing on top of shell debris and supporting a community of sessile epifauna (and epiflora in the photic zone); (2) a mobile megafauna of suspension-feeders, grazers, predators and scavengers; and (3) an either mobile or sedentary macrofauna living in crevices and among the intertwined byssus threads.…”

“…Modiolus modiolus forms aggregations that range from a few individuals grouped in small clusters to extensive beds and reefs with densities reaching up to 400 individuals/m 2 (Lindenbaum et al 2008) and stretching patchily for between several square metres to kilometres of the subtidal shelf (Dinesen 1999;Bruntse & Tendal 2001;Dinesen & Bruntse 2001;Lindenbaum et al 2008). Petersen (1913) first noticed that M. modiolus aggregations harboured a rich epifauna.…”

Seafloor mapping trial in Princess Amalia Wind Farm : use of ME70 on board of RV Tridens

“…Several studies made use of the acoustic backscatter to derive sediment classification on a large scale (Lamarche, Lurton, Verdier, & Augustin, 2011;Lurton et al, 2015). Such an approach would allow one to detect hard substrate structures if present in the area (Lindenbaum et al 2008, Raineault et al 2012. If hard structures would be detected this way, the monitoring program could be adjusted to sample these areas specifically for identification of the organisms forming these structures.…”