Introduction Vestimentiferan tubeworms (Annelida: Siboglinidae) endemic to hydrothermal vents of the eastern Pacific are grouped into four monotypic genera (see Fig. 1 for distributions), Ridgeia, Riftia, Tevnia, and Oasisia (Black et al., 1997; 1998). However, previous investigators suspected the existence of multiple species in two of these genera. Ridgeia was thought to comprise several species (Jones, 1985; Tunnicliffe, 1988), but genetic studies revealed only a single species, Ridgeia piscesae Jones, 1985, with developmentally plastic tube morphology (Southward et al., 1995; Black et al., 1998). Similarly, differences in plume morphology led Jones (1985) to suspect that Riftia from the Gulf of California might be distinct from Riftia pachyptila Jones, 1981 on the East Pacific Rise (EPR) and Galapagos Rift (GAR). Again, genetic analyses revealed only a single species along the GAR and EPR from 27°N to 32°S (Black et al., 1994; Hurtado, 2002). Comparable morphological differentiation has not been noted for Tevnia, which also comprises a single genetically cohesive species, Tevnia jerichonana Jones, 1985, along the EPR from 13°N to 32°S (Hurtado, 2002). The genus Oasisia also has only one described species, Oasisia alvinae Jones, 1985 (Jones, 1985), but the existence of a second morphologically discrete species at 13°N on the EPR is suspected (Tunnicliffe, 1991). The purpose of this study was to assess genetic variability of Oasisia throughout its distribution range. During the past 12 years, we have collected vestimentiferan samples from most of the known hydrothermal vent locations between Guaymas Basin (27°N) in the north to 32°S latitude along the EPR and on the GAR. Oasisia were obtained from four EPR localities in the sampled range: 21°N; 13°N; 9°N and 32°S. This study focuses on variation in DNA sequences of the mitochondrial Cytochrome Oxidase Subunit I gene (mtCOI) from these samples. Unlike the other three vestimentiferan vent genera of the eastern Pacific, Oasisia exhibits extraordinarily high levels of mtCOI diversity among sympatric and allopatric lineages. The present data are consistent with the existence of multiple species in this genus and warrant further studies of morphological and genetic differentiation to confirm whether these lineages indeed correspond to different species. Material and methods Oasisia specimens were collected using the submersible Alvin. Individuals were stored at -80°C prior to genetic analyses. For the present study, we examined 22 individuals from 21°N (dives 2233 and 3347), 11 from 13°N (dives 2227 and 2228), 15 from 9°N (dives 2351, 2356, 2359), and five from 32°S (dives 3338, 3339, 3341, and 3342) on the East Pacific Rise (Fig. 1). We followed the manufacturer’s protocol for the DNEASY kit (Qiagen, Inc., Valencia, CA) to extract total DNA from a vestimentum sample of each individual. An approximately 710-bp fragment of mtCOI was amplified using published primers and PCR conditions (Folmer et al., 1994). Both strands of each PCR product were sequenced on an ABI 377 automated sequencer (Perkin-Elmer/ABI, Foster City, CA). Sequences were proofread, aligned and assembled with Sequencher v. 4.1 (Gene Codes Corp., Ann Arbor, MI). Assembled sequences were truncated to 649-bp, a fragment that contained only clear and readable nucleotides. Cah. Biol. Mar. (2002) 43 : 377-380 Molecular evidence for multiple species of Oasisia (Annelida: Siboglinidae) at eastern Pacific hydrothermal vents Luis A. HURTADO 1 *, Mariana MATEOS 1 *, Richard A. LUTZ 2 and Robert C. VRIJENHOEK 1 (1) Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA (2) Institute for Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA *Present address: Department of Ecology and Evolutionary Biology, University of Arizona, 310 Biosciences West, Tucson, AZ 85721-0088, USA Fax: (1) 520 626 3522 - email: lhurtado@u.arizona.edu