One northward, one southward: Contrasting biogeographical history in two benthic freshwater fish genera across Southeast Asia (Teleostei: Cobitoidea: Nemacheilus, Pangio)

Southeast Asia is one of the world’s biodiversity hotspots, and the high level of diversity and endemism was reached by colonisation events as well as internal diversification. We investigate the phylogenetic relationships and biogeographic history of the loach genus Nemacheilus, which is widely distributed and common across freshwaters of Southeast Asia. In addition we present the ancestral range reconstruction of the related loach genus Pangio that commonly occurs in the same region as Nemacheilus. Our results reveal that the species currently classified as Nemacheilus in fact are a polyphyletic assemblage; most species are now retaining in a monophyletic Nemacheilus sensu stricto and five species belong to different lineages. We further indicate the existence of hidden diversity within Nemacheilus in the form of several undescribed species. Three major clades (Selangoricus, Masyae and Ornatus) are found within the genus Nemacheilus sensu stricto. These clades generally correspond to the species groups formerly defined on the basis of their pigmentation pattern. The biogeographic analyses show that Nemacheilus most likely originated in mainland Southeast Asia and subsequently expanded in a southward direction to Borneo, Sumatra and Java and the southern Malay Peninsula. In contrast, the genus Pangio originated in Sundaland, from where it extended several times northwards into Indochina and to northern India. Our results demonstrate that small freshwater fishes with restricted dispersal ability are very helpful for the reconstruction of biogeographic history. The contrasting biogeographic history of these two groups of small, benthic and related fish show how complex and case-specific the processes that lead to the biodiversity richness of Southeast Asia are.