Glacial-driven vicariance in the amphipod Gammarus duebeni L. Krebes a, * , M. Blank a , K. Jürss a , M.L. Zettler b , R. Bastrop a a University of Rostock, Institute of Biology, Albert-Einstein-Strasse 3, D-18051 Rostock, Germany b Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Biological Oceanography, Seestrasse 15, D-18119 Rostock, Germany article info Article history: Received 15 April 2009 Revised 28 July 2009 Accepted 29 July 2009 Available online 3 August 2009 Keywords: Phylogeography Allopatry Glacial refugia Multiple invasions Molecular marker(s) Cryptic species abstract We have examined the genetic diversity using mitochondrial COI and ND2 sequence data from 306 spec- imens of the amphi-Atlantic-distributed amphipod Gammarus duebeni. Marine populations from the Atlantic Ocean, the Baltic and North Sea, as well as freshwater populations from Ireland, Cornwall and Brittany were analysed. G. duebeni is a complex of five allopatric lineages. Freshwater populations result from multiple inva- sions of marine ancestors, represented by distinct lineages. We interpret the recent distribution of lin- eages as the outcome of a series of spatio-temporal vicariant events caused by Pleistocene glaciations and sea level changes. The freshwater lineages are therefore regarded as ‘glacial relicts’. Furthermore, inter-specific competition with, for example, Gammarus pulex (which is absent in Ireland and western Brittany) may be another important determinant in the distribution of freshwater G. duebeni. In Ireland and Brittany, three freshwater refugia are suggested. The significantly limited gene flow detected among marine populations is more likely due to inter-specific competition than to salinity. The G. duebeni-com- plex represents a model system for the study of allopatric speciation accompanied by major habitat shifts. The pattern of spatio-temporal origins of the freshwater entities we describe here provides an excellent system for investigating evolutionary adaptations to the freshwater environment. Our data did not confirm the presently used subspecies classification but are only preliminary in the absence of nuclear genetic analyses. Crown Copyright Ó 2009 Published by Elsevier Inc. All rights reserved. 1. Introduction It is accepted that several factors, including historical events or ecological skills, determine the geographic distribution of a species as well the spatial arrangements of genetic diversity within this species. The ice ages are one such potent historical factor. Habitat and population fragmentation, several consequences of the ice ages, are major factors in determining the present day distribution patterns of genetic variation within species (Hewitt, 1996, 2000, 2004; Schmitt, 2007). The study of the spatial adjustment of intra- specific diversity, also called phylogeography (Avise et al., 1987; Avise, 2009), is one key discipline for the identification of glacial refugia and for the detection of species subdivisions promoted by ice ages. Further such studies yielded insights into postglacial recolonisation patterns of animals and plants in Europe (Taberlet et al., 1998; Stewart and Lister, 2001; Provan and Bennett, 2008). The amphi-Atlantic-distributed amphipod Gammarus duebeni Lil- ljeborg, 1852 has a large natural distribution and its recent area was undoubtedly strongly influenced by the ice ages. Because of the resistance of G. duebeni to various stresses, such as aerial expo- sure, hypoxia, anoxia, hyperoxia, heavy metal contents and X-radi- ation (Gaston and Spicer, 2001), it is of interest for physiologists and ecologists. However, most striking is the exceptional tolerance to different salinity regimes, ranging from freshwater to full mar- ine conditions. Like the ice ages as a historical factor, salinity as a limiting ecological determinant also defines the distribution of aquatic species. The interface between marine and freshwater hab- itats is a formidable barrier that few species are able to penetrate (reviewed by Lee and Bell, 1999). Biological transitions from mar- ine to freshwater conditions are of particular interest because they are often accompanied by shifts of physiological, morphological and life-history traits. Such transitions have initiated the radiation and speciation of many taxa (Lee and Bell, 1999). Marine or brackish water populations of G. duebeni are known from the Atlantic coasts of northern and mid-Europe as well as North America. Here it lives in the upper littoral zone of tidal areas, estuar- ies, shallow ponds, rock pools and other marginal habitats (Buln- heim, 1979). Aside from the marine or brackish water populations, freshwater populations of G. duebeni exist in Cape Breton Island, the Gulf of St. Lawrence, Canada (Shoemaker, 1930), Iceland (Poul- sen, 1939), the Faroes (Poulsen, 1929), the Shetlands (Stephensen, 1928), the Outer Hebrides (Forrest et al., 1936), the Inner Hebrides (Beadle and Cragg, 1940), the Isle of Man (Jones, 1948), Ireland (Reid, 1939), Western Cornwall (Crawford, 1937) and Brittany, France (Pacaud, 1945; Hynes, 1954, 1959). Reid (1939) first described 1055-7903/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.ympev.2009.07.034 * Corresponding author. Fax: +49 381 498 6112. E-mail address: lukas.krebes@uni-rostock.de (L. Krebes). Molecular Phylogenetics and Evolution 54 (2010) 372–385 Contents lists available at ScienceDirect Molecular Phylogenetics and Evolution journal homepage: www.elsevier.com/locate/ympev