The mechanism of sex differentiation is unknown in the mud crab, Scylla paramamosain. In the present study, the results showed that the gender could earliestly be distinguished by the characteristics of pleopods at the third stage juvenile crab. We first confirmed that crustacean female sex hormone (CFSH) played crucial role in the development of external sexual characteristics in S. paramamosain through RNA interference knockdown. In addition, it showed that the expression levels of Sp-cfsh was highest at the pre-molt stage in both sexes along the molting cycle at the third stage juveniles. With the development of external sexual characteristics (from the third stage to the forth stage, the main sexually differentiated phase of the pleopods), the expressions of Sp-cfsh were similar between the two sexes. At the fifth stage, when the gonopores occurred, the expressions of Sp-cfsh significantly increased in females but decreased in males compared with the forth stage, respectively. The combined results revealed that Sp-cfsh was involved in regulating the development of external sexual characteristics in S. paramamosain, especially for the gonopores in females.
Keywords：Scylla paramamosain, crustacean female sex hormone (CFSH), external sex characteristics