Sexual dimorphism in species of Schistura (Teleostei: Nemacheilidae) from the Mae Khlong basin and peninsular Thailand

Sexually dimorphic characteristics are described for four species of Schistura from the Mae Khlong basin and peninsular Thailand. Males of S. mahnerti have a suborbital flap and rows of unculi on the upper surfaces of the pectoral-fin rays. Females of S. mahnerti have a suborbital groove. None of the morphological measurements differ significantly between males and females of S. mahnerti. In the other three species, S. aurantiaca, S. cf. aurantiaca, and S. sexcauda, all individuals lack the flap or groove, but most males have a conspicuous black botch on the procurrent rays of the upper lobe of the caudal fin, a feature absent in most females. Some morphometric characteristics vary between sexes of these species. Sexually dimorphic traits presumably have a function related to reproduction; however, little is known about reproduction in Schistura, and variation in morphology in relation to habitat or other environmental factors has not been studied.