A hybrid zone between Bathymodiolus mussel lineages from eastern Pacific hydrothermal vents

Shannon B. Johnson, Yong Jin Won, Julio Bj Harvey, Robert C. Vrijenhoek

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36 Scopus citations


Background: The inhabitants of deep-sea hydrothermal vents occupy ephemeral island-like habitats distributed sporadically along tectonic spreading-centers, back-arc basins, and volcanically active seamounts. The majority of vent taxa undergo a pelagic larval phase, and thus varying degrees of geographical subdivision, ranging from no impedance of dispersal to complete isolation, often exist among taxa that span common geomorphological boundaries. Two lineages of Bathymodiolus mussels segregate on either side of the Easter Microplate, a boundary that separates the East Pacific Rise from spreading centers connected to the Pacific-Antarctic Ridge. Results: A recent sample from the northwest flank of the Easter Microplate contained an admixture of northern and southern mitochondrial haplotypes and corresponding alleles at five nuclear gene loci. Genotypic frequencies in this sample did not fit random mating expectation. Significant heterozygote deficiencies at nuclear loci and gametic disequilibria between loci suggested that this transitional region might be a 'Tension Zone' maintained by immigration of parental types and possibly hybrid unfitness. An analysis of recombination history in the nuclear genes suggests a prolonged history of parapatric contact between the two mussel lineages. We hereby elevate the southern lineage to species status as Bathymodiolus antarcticus n. sp. and restrict the use of Bathymodiolus thermophilus to the northern lineage. Conclusions: Because B. thermophilus s.s. exhibits no evidence for subdivision or isolation-by-distance across its 4000 km range along the EPR axis and Galápagos Rift, partial isolation of B. antarcticus n. sp. requires explanation. The time needed to produce the observed degree of mitochondrial differentiation is consistent with the age of the Easter Microplate (2.5 to 5.3 million years). The complex geomorphology of the Easter Microplate region forces strong cross-axis currents that might disrupt self-recruitment of mussels by removing planktotrophic larvae from the ridge axis. Furthermore, frequent local extinction events in this tectonically dynamic region might produce a demographic sink rather than a source for dispersing mussel larvae. Historical changes in tectonic rates and current patterns appear to permit intermittent contact and introgression between the two species.

Original languageEnglish
Article number21
JournalBMC Evolutionary Biology
Issue number1
StatePublished - 2013

Bibliographical note

Funding Information:
The project was conducted with the expert help of captains, crews and pilots assigned to the R/V Atlantis, R/V Atlantis II, and research submersible Alvin (Woods Hole Oceanographic Institute). We thank Dr. Richard A. Lutz (Rutgers University) for contributing the N9 samples and participating in numerous oceanographic expeditions. W. Joe Jones, Phil Hoos, C. Robbie Young, and E. Anderson contributed to sampling, sequencing, and statistical analyses. Funding was provided by NSF grants OCE-9910799 and OCE-0241613 (to RCV), the David and Lucile Packard Foundation (to MBARI) and the National Research Foundation of Korea Grant funded by the Korean Government NRF-2011-013-C00063 (to YJW).


  • Bathymodiolus antarcticus n. sp
  • Bathymodiolus thermophilus
  • Deep-sea
  • Hybridization
  • Hydrothermal vent
  • Linkage disequilibrium
  • Recombination


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