| In order to explore the optimal conditions for large-scale breeding of artificial seedlings of Manila clam (Ruditapes philippinarum), the effects of different stocking density and diet species on the growth, survival and attachment metamorphosis of R. philippinarum larvae were investigated in the present study. In this experiment, 4 cultivation density gradients (5 ind./ml, 10 ind./ml, 15 ind./ml, 20 ind./ml) and 4 groups of diets (Group A Lsochrysis galbana, Group B Chaetocerossp, Group C Lsochrysis galbana + Chaetocerossp 1:2, Group D Lsochrysis galbana + Chaetocerossp 2:1) were set up. The larval shell length and survival rates were measured on the 3rd, 9th, 18th and 27th days, respectively. The larval settlement and metamorphosis rates were counted on the 27th day. The results showed that larval shell length (0.968 ± 0.002 mm) of diet group A was the largest on day 3, significantly higher than that of other diets groups (P < 0.05). The shell length (0.102 ± 0.013 mm) in the medium density group (10 ind./ml) was the largest, which was significantly higher than that of other density groups (P < 0.05). In terms of survival rate, the survival rate of bait group A (26.67%) on the 18th day was the lowest, which was significantly lower than that of the other groups. There was no significant difference in the survival rates between group C and group D, but the survival rates of group C and group B were significantly higher than those of group A and group B. There was no significant difference in the survival rate between density group of 5 ind./ml and density group of 20 ind./ml. However, the survival rates of density groups of 5 ind./ml and 20 ind./ml were significantly lower than those of density groups of 10 and 15 ind./ml. In terms of settlement and metamorphosis, the mixed diets groups of C and D were significantly higher than those of diet groups of A and B. The density group of 15 ind./ml had the highest settlement and metamorphosis rate (17.38%), followed by 16.65% in density group of 10 ind./ml. For larval settlement and metamorphosis rates, there was no significant difference between density groups of 15 and 10 ind./ml, but significantly higher than those in density groups of 5 and 20 ind./ml. The results showed that larvae fed at early opening with Lsochrysis galbana had the highest survival rate, and larvae fed with the mixed diets showed the highest growth rate and the highest rate of metamorphosis. Therefore, culture density of larvae should be maintained between 10 to 15 ind./ml. In summary, during the process of artificial seedling rearing of R. philippinarum, larval density should be maintained at 10-15/ml. To obtain the most suitable condition for larval growth and survival, the diet of Lsochrysis galbana should be selected for the early development stage and the mixed diets should be adopted for the late development stage. The present results not only provide the necessary scientific basis for the study on environmental factors of larval growth and development of R. philippinarum, but also have important guiding significance and application value for improving the seed yield per water unit and stable production of seedlings of clams. |
