Abstract: Yunnanilus is a group of endemic fish inhabit in Yun-Gui Plateau and its adjacent areas. They show the characteristic of sex dimorphism, which could be an important reason for their adaptation to karst habitats. Here, we used Yunnanilus analis as the model to understand the sex dimorphism characteristics and its adaptation to the karst habitats. The sex dimorphism, female fecundity, and food specialization of Y. analis were investigated in Xingyun Lake, Jiangchuan, Yunnan. Our results showed that Y. analis exhibit sex size dimorphism (sex dimorphism index=0.23; female with larger body size). There were stains on the transverse sections at females body, but not in males. Males had dark longitudinal lines at their body sides. Morphological differences between males and females were confirmed by the results of one-way analyses of variance (ANOVA), multivariate statistical analyses, principal component analyses, discriminant analyses, and one-way analyses of similarity (ANOSIM) on total length, standard body length, fork length, head width vs. heal length, and the distance between the starting point of ventral fin to the starting point of pectoral fin vs. standard body length. Fecundity of female fish was 1364.5±489.3 (470-2430) eggs, which were positively correlated with their body size. Both female and male Y. analis mainly feed on Chironomid larvae and mayfly naiads. Their food composition was somehow similar, with significantly statistical difference. In conclusion, fecundity selection pressure and food specialization should be the main factors contributing to the evolution of Y. analis’ sex dimorphism. More importantly, sex dimorphism of Y. analis is a significant adaptation to the karstic habitats.

Key words: Yunnanilus analis, sex dimorphism, food specialization, fecundity, ecomorphology