Mass Selection for Fast Growth in the Third Generation of the Orange Line of the Manila Clam Ruditapes philippinarum

The Manila clam Ruditapes philippinarum is natively distributed along the Pacific coast of Asia, where it is one of the most important bivalve species for local fisheries. This species has been intentionally introduced due to its high adaptability to various coastal environments and is currently widely cultivated in Europe and North America. We have initiated a selective breeding program to improve the growth rate of the Manila clam since 2007, and a full-sib family with orange shell color was selected. This family features high survival but slow growth. Thereafter, two generations of mass selection were conducted in this family by 10% upward selection for faster growth. In 2011, three types of lines were produced by selecting the breeder from the second-generation of selected lines. These lines are SS (subject to three generations of selection for growth in shell length), SC (selected for the second but not the third generation), and C (randomly sampled individuals from the national population). The genetic parameters of the shell length including hR2 (realized heritability), SR (selection response), and GG (genetic gain) were analyzed. Results showed that the shell length of the SS line was significantly larger than those of the SC and C lines at all developmental stages (P < 0.05). For the larval stage, the values of hR2, SR, and GG were 0.42, 0.73, and 6.66, respectively. For the juvenile stage, the values of hR2, SR, and GG were 0.40, 0.69, 21.76, respectively. For grow-out stage, the values of hR2, SR, and GG were 0.48, 0.83, and 18.22, respectively. The results of hR2 indicated the good potential of the SS line in selective breeding. The level of GG improvement was encouraging and consistent with the previous expectations. No inbreeding depressions in the shell length were observed in the SS and SC lines at all sampling days. All of the results indicate that maintaining selection pressure in successive generations may be effective in the selective breeding program of this family.