TAXONOMY OF MACRIDISCUS SPECIES (BIVALVIA: VENERIDAE) FROM THE WESTERN PACIFIC: INSIGHT BASED ON MOLECULAR EVIDENCE, WITH DESCRIPTION OF A NEW SPECIES

The genus Macridiscus Dall, 1902 contains a few species present in warm temperate to tropical faunas of the western Pacific. Macridiscus was widely accepted as a subgenus of Gomphina until a recent suggestion that it should be separated from Gomphina and elevated to an independent genus, based on morphology and molecular data. The taxonomy of the genus Macridiscus has in the past been based solely on shell characters and there has been no agreement about the number of valid species. In this study, we explore the taxonomy and phylogeny of Macridiscus species in order to resolve the systematics of the genus, based not only on shell characters but also on mitochondrial and nuclear DNA sequences. The morphological characters, the sequences of the cytochrome c oxidase subunit I of mitochondrial DNA and the first internal transcribed spacer region (ITS1) between the 18S and 5.8S ribosomal DNA were highly concordant and clearly suggested that three species should be recognized in the genus Macridiscus: M. multifarius new species, M. semicancellata (Koch, in Philippi, 1843) and M. melanaegis (Romer, 1860). The morphological characters and geographical distribution of the three species are redescribed based on the molecular data.