A new lineage of fresh- and brackish-water mussels (Bivalvia, Mytilidae) from Southeast Asia

Three species of mussels collected from fresh- and low salinity (10 psu or lower) brackish-water environments in southern Thailand, Singapore, northern Borneo and south Sulawesi formed a new, well-supported and distinct mytilid clade based on molecular phylogeny. All are small (< 30 mm) species with either radially ribbed or smooth shells. Internally, the upper edges of the ascending lamellae of the outer and inner demibranchs are, respectively, fused to the mantle lobes and visceral mass. A new genus Parabrachidontes is proposed to accommodate the three species. Of the species in the new genus, one is described as new (Parabrachidontes amnicus sp. n.), and the other two poorly known species (P. leucostictus and P. cochinensis) are re-described. The Parabrachidontes clade is closely related to Mytella, Perna and Arcuatula. Together, they formed a well-supported clade, most members of which have ctenidial edges attached to the mantle and/or visceral mass. These genera are clearly distinct phylogenetically from two other mytilid clades containing brackish- and fresh-water species with ctenidia free of the mantle: one comprising the genera Limnoperna and Sinomytilus (together constituting the Limnoperninae), and the other Xenostrobus and Vignadula (forming the Xenostrobinae). We therefore suggest the existence of at least three lineages of mytilids living in rivers and estuaries of East Asia and Australasia.