Phylogeny of the Genus Volachlamys (Bivalvia: Pectinidae) from Japan

Abstract

Following analytical studies on the two forms (Japanese names: Awaji-chihiro and Yami-no-nishiki) of Volachlamys hirasei, fossil specimens in genus Volachlamys from Japan were examined. The fossil specimens were from the Kazusa Formation (1.7 Ma), the Kitaarima Formation (0.9 Ma), the "Maiko Shell-Beds" (Early-Middle Pleistocene), the Takatsukayama Formation (0.41 Ma), the Atsumi Group (0.44 Ma), the Kioroshi Formation (0.125 Ma) and a seaside alluvium in Takamatsu (0.006 Ma). In addition to these, Recent specimens collected from the Seto Inland Sea were examined. Shells of the specimens were measured, and growth was analyzed by observing the annuli on the shell surfaces. In the fossil specimens, the Yami-no-nishiki form was detected in the Atsumi, Kioroshi and Takamatsu samples; it appeared in 1 of 44 individuals in the Atsumi sample, 1 of 2 individuals in the Kioroshi sample, and 17 of 39 individuals in the Takamatsu sample. The remaining specimens were comprised entirely of the Awaji-chihiro form, suggesting that the Yami-no-nishiki form first appeared around the middle Pleistocene. Morphologically, although shell proportions and costae number were unique by locality, there were no samples that were particularly specialized. On the other hand, the shell weight index (SWI) of the Maiko sample was prominently lower, indicating that these shells were much thinner and lighter than the other specimens. The growth analysis revealed distinctly greater growth in the Maiko specimens, although the other specimens (including the Recent ones) showed similar growth rates to one another. These results suggest that only the Maiko specimens were genealogically distinct from the others. The fossil specimens of Volachlamys from Japan were generally supposed to be Volachlamys yagurai, being treated as a species distinct from V. hirasei. The specimens from the "Maiko Shell-Beds", which is the type locality of V. yagurai, have biological characteristics so different from those of the Recent specimens that they are treated as a distinct species; however, in the other fossil specimens, significant characteristics that differ from those of the Recent specimens of V. hirasei could not be detected. These results indicate that only the fossil individuals from the "Maiko Shell-Beds" are referable to Volachlamys yagurai, and those from all other localities should be assigned to Volachlamys hirasei, which is common to the Recent specimens.