Zoological Studies

Phylogenetic Position of Vesicomyid Clams from a Methane Seep off Central Chile (~36°S) with a Molecular Timescale for the Diversification of the Vesicomyidae

Francisco Valdés, Javier Sellanes, and Guillermo D’Elía (2012) Bivalves of the family Vesicomyidae are conspicuous inhabitants of sulfide-rich settings associated with both hydrothermal vents and methane seeps.  The taxonomy of the family has been the subject of recent reviews, and currently, 2 subfamilies, the Vesicomyinae and Pliocardiinae, are recognized.  Although the family has a worldwide distribution, material from the Southeast Pacific available so far for study has been scarce.  In the present study, we consider 2 species from the Concepción Methane Seep Area (CMSA; 36°27.87'S, 73°43.25'W), Calyptogena gallardoi Sellanes and Krylova, 2005 and a larger species, which based in its morphology was tentatively assigned to the genus Archivesica (Archivesica sp. 1).  We include new mitochondrial DNA sequences of the cytochrome c oxidase subunit I (COI) gene of these species, together with previously available sequences, in a phylogenetic analysis using maximum parsimony and Bayesian inference.  In addition, a relaxed molecular clock was calibrated to estimate the divergence times of the lineages from which these species originated.  The topologies recovered generally agree with a recent classificatory scheme of the family.  Calyptogena gallardoi falls within the clade of Calyptogena sensu stricto, corroborating the morphologically based generic assignation.  Similarly, Archivesica sp. 1, as expected from its morphology, falls within the clade corresponding to the genus Archivesica.  According to results of the molecular clock, lineages leading to both CMSA species date back to the Miocene: C. gallardoi to ca. 11.2 Mya and Archivesica sp. 1 to ca. 6.18 Mya.  Two independent vesicomyid invasions of the southeastern Pacific Ocean are proposed to account for the diversity seen at the CMSA.  In general, the chronogram shows that lineages of pliocardiine vesicomyds have accumulated at a regular pace since the Eocene with no signs of diversification pulses.