RESEARCH ARTICLE A. D. Rogers Æ S. Morley Æ E. Fitzcharles K. Jarvis Æ M. Belchier Genetic structure of Patagonian toothfish (Dissostichus eleginoides) populations on the Patagonian Shelf and Atlantic and western Indian Ocean Sectors of the Southern Ocean Received: 9 April 2005 / Accepted: 10 January 2006 / Published online: 4 February 2006 Ó Springer-Verlag 2006 Abstract The genetic structure of Patagonian toothfish populations in the Atlantic and western Indian Ocean Sectors of the Southern Ocean (SO) were analysed using partial sequences of the mitochondrial 12S rRNA gene and seven microsatellite loci. Both haplotype frequency data (F ST >0.906, P<0.01) and microsatellite genotype frequency data (F ST =0.0141–0.0338, P<0.05) indicated that populations of toothfish from around the Falkland Islands were genetically distinct from those at South Georgia (eastern Atlantic Sector SO), around Bouvet Island (western Atlantic Sector SO) and the Ob Se- amount (western Indian Ocean Sector of the SO). Ge- netic differentiation between these populations is thought to result from hydrographic isolation, as the sites are separated by two, full-depth, ocean-fronts and topographic isolation, as samples are separated by deep water. The South Georgia, Bouvet and Ob Seamount samples were characterised by an identical haplotype. However, microsatellite genotype frequencies showed genetic differentiation between South Georgia samples and those obtained from around Bouvet Island and nearby seamounts (F ST =0.0037, P<0.05). These areas are separated by large geographic distance and water in excess of 3,000 m deep, below the distributional range of toothfish (<2,200 m). No significant genetic differenti- ation was detected between samples around Bouvet Island and the Ob Seamount although comparisons may have been influenced by low sample size. These localities are linked by topographic features, including both ridges and seamounts, that may act as oceanic ‘‘stepping stones’’ for migration between these populations. As for other species of deep-sea fish, Patagonian toothfish populations are genetically structured at the regional and sub-regional scales. Introduction Benthopelagic fish species, from bathyal depths, often have very wide geographic distributions. Populations occur on continental slopes, the slopes of oceanic islands and on axial and non-axial seamounts that maybe sep- arated by thousands of kilometres of deep ocean. These species are often strong swimmers as adults, are highly fecund and have long-duration pelagic eggs, larvae and/ or juvenile stages. As a null hypothesis it is assumed that such life-histories confer a high capacity for dispersal across large distances. Given the lack of obvious phys- ical barriers to dispersal in the oceans, this should lead to high levels of migration between distant populations and genetic homogeneity on regional or even oceanic scales. In some cases, there is evidence for gene-flow between populations of deep-water benthopelagic fish over large geographic distances. Examples include: slender ar- mourhead (Pseudopentaceros wheeleri Hardy 1983), wreckfish (Polyprion americanus Bloch and Schneider 1801) and alfonsino (Beryx splendens Lowe 1834) (Martin et al. 1992; Sedberry et al. 1996; Hoarau and Borsa 2000). However, in many cases, genetic differen- tiation has been detected between populations on in- teroceanic, intraoceanic and even intraregional scales (reviewed in Creasey and Rogers 1999; Rogers 2003; see also Stockley et al. 2005; Aboim et al. 2005). Communicated by J.P. Thorpe, Port Erin A. D. Rogers (&) Æ S. Morley Æ E. Fitzcharles Æ K. Jarvis M. Belchier British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, CB3 0ET Cambridge, UK A. D. Rogers Institute of Zoology, Zoological Society of London, Regent’s Park, NW1 4RY London, UK E-mail: alex.rogers@ioz.ac.uk Present address: K. Jarvis Institute of Genetics, Queen’s Medical Centre, University of Nottingham, NG7 2UH Nottingham, UK Marine Biology (2006) 149: 915–924 DOI 10.1007/s00227-006-0256-x