Asymmetric dispersal of southern bull-kelp (Durvillaea antarctica) adults in coastal New Zealand: testing an oceanographic hypothesis CATHERINE J. COLLINS, CERIDWEN I. FRASER, ANNA ASHCROFT and JONATHAN M. WATERS Allan Wilson Centre for Molecular Ecology and Evolution, Department of Zoology, University of Otago, 340 Great King St, Dunedin 9016, New Zealand Abstract Coastal populations are often connected by unidirectional current systems, but the biological effects of such asymmetric oceanographic connectivity remain relatively unstudied. We used mtDNA analysis to determine the phylogeographic origins of beach- cast bull-kelp (Durvillaea antarctica) adults in the Canterbury Bight, a 180 km coastal region devoid of rocky-reef habitat in southern New Zealand. A multi-year, quantitative analysis supports the oceanographically derived hypothesis of asymmetric dispersal mediated by the north-flowing Southland Current. Specifically, 92% of beach-cast specimens examined had originated south of the Bight, many drifting north for hundreds of kilometres, and some traversing at least 500 km of ocean from subantarctic sources. In contrast, only 8% of specimens had dispersed south against the prevailing current, and these counter-current dispersers likely travelled relatively small distances (tens of kilometres). These data show that oceanographic connectivity models can provide robust estimates of passive biological dispersal, even for highly buoyant taxa. The results also indicate that there are no oceanographic barriers to kelp dispersal across the Canterbury Bight, indicating that other ecological factors explain the phylogeographic disjunction across this kelp-free zone. The large number of long-distance dispersal events detected suggests drifting macroalgae have potential to facilitate ongoing connectivity between otherwise isolated benthic populations. Keywords: biogeography, cytochrome c oxidase I, marine ecology, oceanography, phylogeogra- phy, rafting Received 5 July 2010; revision received 24 August 2010; accepted 27 August 2010 Introduction Relationships between oceanographic connectivity and biological similarity represent an important but under- studied topic in marine ecology (Briggs 1995; Gaylord & Gaines 2000; Kinlan & Gaines 2003). In particular, the potential role of oceanographic forces in driving direc- tional asymmetry of biological dispersal (see Cook & Crisp 2005) could have major ecological implications. Indeed, such asymmetric dispersal (also known as directional dispersal; Sanmartin et al. 2007) can substan- tially impact patterns of biological recruitment, levels of biodiversity and also—by extension—associated conser- vation management policy (e.g. James et al. 2002; Salo- mon et al. 2010). Although a number of recent oceanographic modelling studies have estimated poten- tial levels of marine biological connectivity across a range of regions and geographical scales (e.g. Chiswell et al. 2003; Siegel et al. 2003, 2008; Condie et al. 2005; Chiswell 2009), such dispersion models remain largely untested in terms of empirical biological data (but see Correspondence: Jonathan M. Waters, Fax: +64 0 3 4797584; E-mail: jonathan.waters@otago.ac.nz Ó 2010 Blackwell Publishing Ltd Molecular Ecology (2010) 19, 4572–4580 doi: 10.1111/j.1365-294X.2010.04842.x