The presence of small black mussels in the upper intertidal zone of estuarine seashores in Southeast Asia is often overlooked, despite their numerical dominance. Here, we clarify their species identities, taxonomy, geographical distributions and genetic relationships. Our molecular results suggest that three closely related species in East and Southeast Asia form a sister clade to Australian and New Zealand Xenostrobus. Given their distinctness, we resurrect the genus Vignadula to distinguish these two clades. Vignadula atrata, first described from Japan and type species of the genus, is confined to north-east Asia, whilst two other Vignadula species occur natively in Southeast Asia. Of these two species, one is found throughout Thailand, Malaysia, Singapore and Java. We show that this species corresponds to Xenostrobus balani and Xenostrobus mangle described from the Malacca Strait, which are genetically indistinguishable and here considered as one and the same species, Vignadula mangle comb. nov. The other species, Vignadula kuraburiensis sp. nov., described in this study, has so far been found only in south-west Thailand. Members of Vignadula are, in turn, all clearly different from their closest morphological equivalent in Australia, Xenostrobus pulex. A new subfamily Xenostrobinae is proposed to accommodate the two genera Xenostrobus and Vignadula.
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.
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