Phylogenetic relationships of two freshwater pearl mussels, Margaritifera laevis (Haas, 1910) and Margaritifera togakushiensis Kondo & Kobayashi, 2005 (Bivalvia: Margaritiferidae), in the Japanese archipelago

Abstract

The phylogenetic relationship between Margaritifera togakushiensis Kondo & Kobayashi, 2005, a newly recognised species distributed in the Japanese archipelago, and its closely related species, M. laevis (Haas, 1910) widely distributed in the same area, was analysed based on the mitochondrial COI and nuclear ribosomal (18S rRNA, 28S rRNA) gene sequences. The sequence data of several margaritiferids registered in GenBank were also used in the analysis. Margaritifera togakushiensis and the North American species M. marrianae were the most closely related species, forming a clade with which the Japanese M. laevis and the North American M. falcata were sisters. The two Japanese and two North American margaritiferid species, which occur on opposite sides of the Pacific Ocean, form the newest lineage in the Margaritiferidae, a family thought to be the most primitive of the freshwater pearl mussels, originating in the Mesozoic. Our results also support recent suggestions that generic names Dahurinaia and Cumberlandia be combined into the genus Margaritifera. Based on our sequence and shell morphology data, we confirmed that Russian samples previously assigned as M. laevis were actually M. togakushiensis. Most margaritiferid species worldwide, not only those in Japan, are in a critical situation at present. It is important that these freshwater mussels and their host fishes, which have great evolutionary and ecological interest, be conserved into the future.