Contrasting demographic histories of European and North American sea lamprey (Petromyzon marinus) populations inferred from mitochondrial DNA sequence variation Martin J. Genner A,F , Robert Hillman B , Matthew McHugh C , Stephen J. Hawkins D and Martyn C. Lucas E A School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK. B Environment Agency, Sir John Moore House, Victoria Square, Bodmin, Cornwall, PL31 1EB, UK. C Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, PL1 2PB, UK. D Ocean and Earth Science, National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH, UK. E School of Biological and Biomedical Sciences, Durham University, Durham, DH1 3 LE, UK. F Corresponding author. Email: m.genner@bristol.ac.uk Abstract. Populations of anadromous sea lamprey (Petromyzon marinus) have been found to be largely genetically homogeneous across western Europe, and across the eastern seaboard of North America. However, comparatively little is known of the relationship between the European and North American populations. We quantified the extent of population structuring present over a transatlantic scale using mitochondrial DNA sequences. We found clear segregation of the populations on either side of the Atlantic, and considerable genetic homogeneity within Europe over a spatial scale of over 2000 km. The North American populations contained larger genetic diversity than those from Europe, and coalescent analyses showed a corresponding greater overall effective population size. Employing calibration points based on a dated phylogeny of the Petromyzontiformes, our analyses indicated that the North American population has been increasing in effective size since establishment ,500 000 years ago, while the total European population has only undergone population expansion only within the last 125 000 years. This evidence is consistent with a colonisation of Europe from an older North American population, and with the European population persisting through the last glaciation within regional refugia. Additional keywords: Bayesian skyline plots, lamprey phylogeny, migratory fishes, regional panmixia, transatlantic divergence. Received 4 March 2012, accepted 23 August 2012, published online 8 October 2012 Introduction Anadromous fishes that migrate between freshwater breeding habitat and marine growth habitat tend to exhibit a strong degree of natal homing (Waters et al. 2000), and many stocks exhibit local adaptations (Landry and Bernatchez 2001). An exception to the finding of considerable stock structure among anadro- mous fishes is the sea lamprey Petromyzon marinus (Waldman et al. 2008). The species has a North Atlantic distribution encompassing coastal waters of both Europe and North America, and there is evidence for strong population genetic divergence between the continents (Rodrı ´guez-Mun ˜oz et al. 2004; Bryan et al. 2005), but little or no genetic structuring among anadromous populations in Europe (Almada et al. 2008) or North America (Bryan et al. 2005; Waldman et al. 2008). Despite the lack of natal homing, adults appear to be effective at identifying suitable breeding habitats (Bergstedt and Seelye 1995), and there is good evidence that larval ammocoetes release pheromones that aid location of suitable breeding grounds by adults (Bjerselius et al. 2000; Sorensen et al. 2005; Vrieze et al. 2010). Feasibly detection of such pheromones represents an alternative adaptation for the location of suitable breeding grounds, and such an adaptation may also enable individuals to range widely during the marine phase, and still have a good prospect of identifying suitable breeding grounds. During this marine phase sea lampreys tend to be ini- tially predatory, feeding on small fish, but later become opportunistically parasitic on larger prey, including widely- ranging sharks and marine mammals (Gallant et al. 2006; Nichols and Tscherter 2011; Samarra et al. 2012). In Europe, the sea lamprey is a species of conservation concern (Renaud 1997; Beaulaton et al. 2008; Mateus et al. 2012). Anthropogenic disturbances are believed to have led to reductions in population size following blockage of migration routes, pollution, alteration of spawning habitat and CSIRO PUBLISHING Marine and Freshwater Research, 2012, 63, 827–833 http://dx.doi.org/10.1071/MF12062 Journal compilation Ó CSIRO 2012 www.publish.csiro.au/journals/mfr