Systematic revision of the balitorid loach genus Sewellia of Vietnam and Laos, with diagnoses of four new species

The highly specialized rheophilic loach genus Sewellia Hora, 1932 and its only previously known species, the Vietnamese S. lineolata (Valenciennes, 1849), are redescribed based on recently collected material. Four new species are described: S. pterolineata from Vietnam and S. speciosa from Laos in the nominal subgenus Sewellia, and S. diardi and S. elongata from Laos in the new subgenus Diardichthys. All species from Laos are from hill streams in the Se Kong watershed (Mekong basin). The species differ in size, scale counts, coloration, sexual dimorphism, and vertebral counts. Scanning electron micrographic observations are presented on tubercles and on unculiferous and non-unculiferous epidermal structures of the mouth and fins.