The population structure and biology of the ocean quahog, Arctica islandica, in Belfast Lough, Northern Ireland i.d. ridgway, c.a. richardson, j.d. scourse, p.g. butler and d.j. reynolds School of Ocean Sciences, College of Natural Sciences, Bangor University, Menai Bridge, Anglesey, LL59 5AB The spatial distribution, density, growth rate, longevity, mortality and recruitment patterns of the long-lived clam Arctica islandica in Belfast Lough, Northern Ireland, UK are described. The A. islandica population at Belfast Lough appears to be restricted to a small area at the mouth of the Lough. Additional searches for specimens further into the Lough and into deeper waters found no evidence of a larger more widespread population and we report population densities of 4.5 indi- viduals m 22 . The ages of the clams were determined from the number of internal annual growth lines in acetate peel replicas of shell sections. The population growth curve was fitted using the Von Bertalanffy growth equation: L t ¼ 93.7 mm (12e 20.03(t – 1.25) ). Based on catch curve analysis, the Belfast Lough population has an estimated longevity of 220 years and a natural mortality rate of 0.02. We compare growth characteristics and life history traits in this population with other analogous A. islandica populations. The overall growth performance and the phi-prime index were used to compare growth parameters with data from the literature and we observed no significant relationship between the growth performance indices and longevity or latitude. Analysis of the age-structure and reconstructed dates of settlement indicate that this population has experienced almost continual recruitment over the last century with a gap in successful recruitment into the population 90 – 100 years ago and another 140 – 150 years ago. The size-structure revealed a scarcity of small individuals which we believe may be an artefact of the dredge sampling process. Keywords: Arctica islandica, distribution, density, recruitment, mortality, longevity Submitted 24 September 2010; accepted 7 December 2010; first published online 15 February 2011 INTRODUCTION The ocean quahog, Arctica islandica (Linnaeus, 1767), is a large long-lived suspension feeding infaunal bivalve mollusc found burrowed in the top 5 cm or deeper of sand and muddy substrates in the shelf seas of the North Atlantic, com- monly at a water depth of 25 to 80 m (Mann & Wolf 1983). In Europe the species occupies a wide latitudinal range from the Bay of Biscay in the south (488N), to Icelandic waters (668N) and even as far north as of the southern coast of the Svalbard (768N) in the Arctic Ocean (Dahlgren et al., 2000), whilst on the eastern coast of North America it occupies a latitudinal range from Labrador to Cape Hatteras (Franz & Merrill, 1980). The ocean quahog is of both commercial and academic interest. Commercially it is harvested in the USA, Canada and Iceland; conversely it is not a very marketable species in European waters and no commercial fisheries exist. Academically, however it is becoming an increasingly impor- tant species being utilized both as a high resolution marine palaeoclimate archive (Wanamaker et al., 2008; Butler et al., 2009) and a model organism for research about ageing (Strahl et al., 2007; Abele et al., 2008). The species has recently piqued the interest of biogerontologists because it is not just the longest-lived bivalve, it is also the longest lived non- colonial animal known to science (Wanamaker et al., 2008). The known maximum life span potential (MLSP) for this clam continues to increase (Thompson et al., 1980a; Jones, 1983; Ropes, 1985; Scho ¨ne et al., 2005; Scourse et al., 2006) and currently stands at 410 years (Wanamaker et al., 2008). As a result of commercial and academic interest, research on the population characteristics of A. islandica has become increasingly important in the past two decades. Knowledge of the species population-structure, recruitment history, mor- tality rates, longevity and reproductive cycle has largely been obtained from those regions where the species is commercially harvested. For example the age of the species has been inves- tigated for the Mid-Atlantic Bight population (Thompson et al., 1980a, b; Kennish et al., 1994), near New York (Ropes et al., 1984), eastern Canadian waters (Kilada et al., 2007) and in Iceland (Steingrimsson & Thorarinsdo ´ttir, 1995; Thorarinsdo ´ttir & Steingrimsson, 2000; Kilada et al., 2007) and the population-structure and seasonal reproductive cycle were described for the southern New England shelf (Mann, 1982), New Jersey (Fritz, 1991; Kennish & Lutz, 1995), Iceland (Thorarinsdo ´ttir & Einarsson, 1996; Thorarinsdo ´ttir & Johannesson, 1996; Thorarinsdo ´ttir, 2000), the Baltic Sea (Zettler et al., 2001), Georges Bank (Lewis et al., 2001) and the northern North Sea (Witbaard & Bergman, 2003). Despite this wealth of knowledge there have been no investigations into the population structure of any Irish Sea populations. Corresponding author: I.D. Ridgway Email: iain.ridgway@bangor.ac.uk 539 Journal of the Marine Biological Association of the United Kingdom, 2012, 92(3), 539–546. # Marine Biological Association of the United Kingdom, 2011 doi:10.1017/S0025315411000154