J.M. Quattro á M.R. Chase á M.A. Rex á T.W. Greig R.J. Etter Extreme mitochondrial DNA divergence within populations of the deep-sea gastropod Frigidoalvania brychia Received: 16 January 2001 / Accepted: 3 July 2001 / Published online: 1 September 2001 Ó Springer-Verlag 2001 Abstract The deep sea supports a diverse and highly endemic invertebrate fauna, the origin of which remains obscure. Little is known about geographic variation in deep-sea organisms or the evolutionary processes that promote population-level dierentiation and eventual speciation. Sequence variation at the 16 S rDNA locus was examined in formalin-preserved specimens of the common upper bathyal rissoid Frigidoalvania brychia Verrill, 1884) to examine its population genetic struc- ture. The specimens came from trawl samples taken over 30 years ago at depths of 457±1,102 m at stations in the Northwest Atlantic south of Woods Hole, Massachu- setts, USA. Near the upper boundary of its bathymetric range 500 m), extremely divergent haplotypes com- prising three phylogenetically distinct clades average uncorrected sequence divergence among clades 23%, 3% within clades) were found at stations separated by a maximum distance of 80 km, suggesting the presence of high levels of intraspeci®c divergence or the possi- bility of morphologically cryptic species. Only one of these clades was found at two stations in the mid- to lower part of F. brychia's depth distribution 800± 1,100 m), suggesting lower clade diversity with increas- ing depth, although among-sample divergence, with a single exception, was minimal. One station was geneti- cally divergent from all others sampled, containing a unique suite of haplotypes including two found only at this site. Steep vertical selective gradients, major oceanographic changes during the late Cenozoic, and habitat fragmentation by submarine canyons might have contributed to an upper bathyal region that is highly conducive to evolutionary change. Introduction During the last several decades, much has been learned about the interactions of mutation, selection, migration and random genetic drift, and their impacts on popu- lation-level dierentiation e.g. Hartl and Clark 1997; Li 1997). Genetic structure is the most basic information for documenting the degree of population-level diver- gence and inferring its causes). The evolutionary pro- cesses of population dierentiation, speciation, adaptive radiation, and the geographic spread of taxa can be re- constructed by associating divergence patterns with spatial and/or temporal changes in the environment Avise 2000). The deep sea is the largest and most recently explored environment on Earth. Long thought to be unfavorable to life, sampling has instead revealed a rich, endemic invertebrate fauna Hessler and Sanders 1967; Sanders 1968; Smith et al. 1998). While patterns of biodiversity at the species level have been documented from local to global scales Rex et al. 1997, 2000), research on the genetic foundation of this biodiversity is just beginning. Genetic population structure is known for only a few invertebrate species from soft-sediment habitats e.g. France and Kocher 1996a; Chase et al. 1998a; Creasey and Rogers 1999), largely through the development of techniques to extract and sequence DNA from formalin- ®xed archived material France and Kocher 1996b; Chase et al. 1998b). With little comparative genetic data for deep-sea organisms, the mechanisms of population dierentiation and the potential nature and role of geographic isolating barriers in the deep-sea ecosystem remain speculative. In the present study we assessed the genetic popula- tion structure of the upper bathyal snail Frigidoalvania Marine Biology 2001) 139: 1107±1113 DOI 10.1007/s002270100662 Communicated by J.P. Grassle, New Brunswick J.M. Quattro &) á T.W. Greig Department of Biological Sciences, Program in Marine Sciences, Coker Life Sciences Building, Baruch Institute and School of the Environment, University of South Carolina, Columbia, SC 29208, USA E-mail: quattro@mail.biol.sc.edu Tel.: +1-803-7773240 Fax: +1-803-7774002 M.R. Chase á M.A. Rex á R.J. Etter Department of Biology, University of Massachusetts/Boston, 100 Morrissey Boulevard, Boston, MA 02125, USA