Xenostrobus or Vignadula (Bivalvia: Mytilidae)? A taxonomic re-evaluation of small black mussels inhabiting the upper intertidal zone of the estuaries of Southeast Asia

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.

Automated summary

The species identities, taxonomy, geographical distributions, and genetic relationships of small black mussels in the upper intertidal zone of estuarine seashores in Southeast Asia have been clarified. Three closely related species in East and Southeast Asia are found to form a sister clade to Australian and New Zealand Xenostrobus species. A new subfamily Xenostrobinae is proposed to accommodate the two genera Xenostrobus and Vignadula.