Hydrobiologia 505: 41–48, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 41 Genetic population structure of Littorina brevicula around Korean waters Sook-Jung Kim, Mauricio Rodriguez-Lanetty & Jun-Im Song ∗ Department of Life Sciences, Ewha Womans University, Seoul 120-750, Korea Tel: 822-3277-2357. Fax: 822-3277-2385. E-mail: jisong@ewha.ac.kr ( ∗ Author for correspondence) Received 23 October 2002; in revised form 13 March 2003; accepted 28 April 2003 Key words: mtDNA markers, Genetic differentiation, Current Systems Litterina brevicula. Abstract Because Korean waters are influenced by different minor current systems, this may lead us to expect certain genetic differentiation among populations of marine invertebrates around Korean coastlines. To test this idea, we examined the genetic differentiation of 14 populations of a widely distributed marine gastropod, Littorina brevicula, using mtDNA markers, cyt b and ND6. Sequencing analyses of 500 bp fragment of the cyt b and 501 bp of the ND6 mtDNA revealed the existence of 14 and 24 haplotypes, respectively. Geographic association with population genetic variability was tested using a nested clade analysis (NCA). However, no significant association between geography and haplotype distribution was found, which suggests the existence of a high level of gene flow of L. brevicula around Korean waters. The lack of genetic structure and likely high gene flow among these populations, even under influences of different minor current systems, may be accounted for long periods of dispersal of the planktonic larvae from this gastropod species within major currents on the north western Pacific Ocean. Introduction The degree of genetic differentiation among local pop- ulations of marine invertebrate species provides im- portant indirect evidence, reflecting pattern and scale of effective local dispersal (Reid, 1996; Heipel et al., 1998; Bohonak, 1999). Most marine species with a high dispersal potential show limited population ge- netic differentiation, because gene flow is usually positively correlated with dispersal ability (Hansen, 1980). In marine molluscs, for example, species with planktonic larvae display higher levels of gene flow and less population genetic differentiation (Benzie & Williams, 1992; Brown & Murray, 1995), than nonplanktonic species with poor dispersal capacity (Johannesson, 1992; Johnson & Black, 1995). Nev- ertheless, even in species with high dispersal abilities there are several factors that may limit actual dispersal and/or gene flow, thus creating opportunities for ge- netic differentiation as well (Palumbi, 1994, 1996). These include abiotic factors, such as temperature, salinity gradient and wave action; isolation by dis- tance; and behavioral limit to dispersal and selection (Palumbi, 1994, 1996). The focus of this study is on the widespread mar- ine gastropod, Littorina brevicula, which is one of the most common snails found in the intertidal zone along the southeast Asian coast (Tatarenkov, 1992; Zaslavskaya & Takada, 1998; Park et al., 1999). Its abundance and distribution within Korean waters are quite remarkable (Song, 1991). It is one of the few marine invertebrate species that is present within the four coastal biogeographic regions described for Korean waters (Song, 1991; Seo, 1996) (see Fig. 1a). This wide geographical distribution range might be accounted for the high dispersal capacity that this species possesses, due to its planktonic larval devel- opment (Reid, 1996). Nevertheless, the present ocean current patterns around Korea suggest that gene flow among populations of L. brevicula might be to some extent restricted. The four main coastal regions around Korea, including the East Sea, Korean Strait, Jeju Island area and West Sea (Yellow Sea), are each in- fluenced by different ocean currents (Fig. 1). There is