Effect of temperature on the growth and sex steroid concentration of the half-smooth tougue sole(Cynoglossus semilaevis)

The influence of temperature on growth and the expression levels of sex steroids were studied in juvenile half-smooth tongue sole(Cynoglossus semilaevis Günther,1873).Tongue soles at 32 days post-fertilization(dpf)were exposed to temperatures ranging from 19 to 27 ℃.Measurements of length and body weight of all individuals in each temperature group were conducted 32-142 dpf.Student-Newman-Keuls software was used to analyze the growth rhythm of juveniles and relativity of temperature and length and body weight of half-smooth tongue sole. Results showed that the juveniles of 98 dpf grew fastest in 25 ℃ group but grew fastest in 23 ℃ group when cultured between 129-142 dpf.The specific growth rates of body weight of half-smooth tongue sole were 4.42 and 4.6 respectively at 23 and 25 ℃ and obviously higher than other temperature groups.The Pearson correlation coefficients between temperature and total length and body weight were 0.271 and 0.260,respectively.Moreover,the relationships of temperature(T)and length(L)and body weight(W)were given by the equations L=1.637 6T^0.3782(R²=0.082)and W=0.0285T^1.0781(R²=0.075),respectively.The expression of sex steroid hormones in fertilized egg and juveniles of 3-142 dpf at 23 ℃ were detected.The results showed that the concentrations of testosterone(T)and 17β-estradiol(E₂)were very high in fertilized eggs,but decreased gradually and reached their lowest levels at 32 dpf(4.6 ng/mg for T and 5.43 pg/g for E₂)and then increased to their highest levels(average 40.80-94.70 ng/mg for T and 42.04-105.80 pg/g for E₂)at 46 dpf,but decreased again later.In all temperature groups,the concentrations of T and E₂ increased obviously at 32-46 dpf and reached highest levels at 46 dpf.The concentrations of T and E₂ in 19 and 21 ℃ groups were much higher than that in other groups and there were no significant differences in these groups.The concentrations of sex steroid hormones decreased significantly at 98 dpf in all temperature groups and the expression of E₂ had no significant differences,but the expression of T were higher in 19 and 27 ℃ groups than that in other groups.Therefore,it was speculated that the gonad of juvenile half-smooth tongue sole began to differentiate when the body length was(4.99±0.75)cm at 4 dpf and 19 and 27 ℃ may be the temperature which could promote the juvenile to masculinization.