Synonymy of Calyptogena solidissima with Calyptogena kawamurai (Bivalvia: Vesicomyidae) and Its Population Structure Revealed by Mitochondrial DNA Sequences

Abstract

Nucleotide sequences of part (1,101 bp) of the mitochondrial cytochrome oxidase c subunit I (COI) gene were determined for two specimens of Calyptogena kawamurai collected in Kashima Nada and Suruga Bay, respectively. These sequences were identical to each other and to those from many individuals of Calyptogena solidissima, i.e., 11 of 12 specimens from a seep area in Nankai Trough, two of 20 from hydrothermal-vent fields in Okinawa Trough, and one of 14 from a seep area on Kuroshima Knoll. The nucleotide sequences of the 5' part (about 700 bp) of the first internal transcribed spacer (ITS-1) also showed a close relationship between C. kawamurai and C. solidissima. The radiating threads on the shell surface that were emphasized in describing C. solidissima are not consistent throughout these local populations. Variation in cardinal dentition was confirmed to be intraspecific by observations of a series of specimens. The shell length-height and shell length-width relationships of both species all fit a single regression line. These results suggest that C. solidissima is a junior synonym of C. kawamurai. The populations of Nankai Trough, Okinawa Trough, and Kuroshima Knoll were shown to be diverging genetically from each other. Populations of Okinawa Trough and Kuroshima Knoll are suggested to have derived independently from the most common haplotype of Nankai Trough.

The smaller vesicomyid bivalves in the genus Isorropodon (Bivalvia, Vesicomyidae, Pliocardiinae) also harbour chemoautotrophic symbionts

“…Goffredi et al 2003 study on the "pacifica/lepta" species documented divergence levels between 0.2 and 2.8 % (intraspecific) and between 3.9 and 10.3 % (interspecific), levels that were consistent with the ones reported for other described species of vesicomyids (Peek et al 1997(Peek et al , 2000. Baco et al (1999) and Kojima et al (2006) documented much lower intraspecific levels of COI variations (from 0.1 to 1.8 %) in Calyptogena kilmeri, Calyptogena elongata and Calyptogena solidissima. The level of divergence reported here is congruent with inter-specific, rather than intra-specific, variations.…”

“…Genome sequencing of symbionts has emphasized several traits typical of maternallytransmitted bacteria, including genome size reduction coupled with the loss of certain genes (Kim et al 1995;Harada et al 2009;Kuwahara et al 2008;Stewart et al 2008). The systematics and evolution of Pliocardiinae hosts have also been studied but their taxonomy is still under debate as genera are typically found non-monophyletic in phylogenetic reconstructions (Kojima et al 1995;Baco et al 1999;Osborn 2001;Goffredi et al 2003;Kojima et al 2004Kojima et al , 2006Decker et al 2012). Yet, apart from large-sized Pliocardiinae vesicomyids, there are in this subfamily smaller-sized species which have been overlooked, and for which very little data is available, both for hosts and associated symbionts.…”

Vent Fauna in the Okinawa Trough

“…are distributed only in sedimentary fields (Kim and Ohta 1991) in the hydrothermal vent fields of Minami-Ensei Knoll and Iheya Ridge and Iheya North Knoll in the Okinawa Trough. Hitherto, three species have been reported, and the population genetic structure has been examined for C. kawamurai and C. okutanii (Watanabe et al 2010, based on the data in Kojima et al 2001Kojima et al , 2006. In both cases, populations in the Okinawa Trough were significantly different from those outside of the Okinawa Trough.…”

Phylogeny and Diversification Patterns among Vesicomyid Bivalves

“…Phylogenetic relationships of Pacific and Atlantic vesicomyids reconstructed using the mitochondrial gene cytochrome oxidase subunit I (COI) supported the monophyly of this group [12], [13], [14], [15], [16], [17], [18], [19]. However, within the family Vesicomyidae, the morphologically defined genera Calyptogena and Vesicomya are not supported by molecular phylogenies which have revealed polyphyletic branching [12].…”