Abstract: Horseshoe crabs are precious marine ‘living fossil’. Little information is available on dynamics of Carcinoscorpius rotundicauda population. During the period of 2014 to 2020, wemonitored C. rotundicauda population at Shankou Mangrove Reserve in Behai, Guangxi. A total of 990 individuals of C. rotundicauda were recorded, with a declining trend of the population. In the sixth survey year, the population decreased by 94.60% compared with that in the first survey year, indicating thatthe conversation of this species is imperative. Male accounted for 80.28% of the total individuals, while females accounted for 17.50%. The population of C. rotundicaudafluctuated seasonally, and the population size was significantly positively correlated with temperature and the north and south wind direction states (0 and 4), suggesting biological characteristics of coastal vane for C. rotundicauda. This study help understand the population dynamics of C. rotundicauda and its influencing factors, providing reference for the protection of C. rotundicauda resources in the reserve.

Key words: Carcinoscorpius rotundicauda, wind direction, population size, seasonal variation.