Divergent Selection for Shell Length in the Manila Clam, Ruditapes philippinarum

To determine whether genetic improvement could be attained through a selective breeding programme, divergent selection for shell length was conducted in a base population of the Manila clam, Ruditapes philippinarum. The 10% largest and 10% smallest clams were selected as parents for the upward-selected and downward-selected lines, respectively. Equal numbers of clams were randomly chosen to serve as parents for the control line before the selection. The selected and control lines were reared under the same environmental conditions at larvae, juvenile, and grow-out stages. The values of standardized responses to selection (SR) and realized heritability ( at the age of 360 d were 0.268 and 0.165 for the upward-selected line and 0.341 and 0.260 for the downward-selected line, respectively. Asymmetric differences were evident between the upward-selected and downward-selected lines in both SR and . Performance in shell length is improved by 3.55% in the upward-selected line after one generation, indicating that mass selection for shell length in a base population of the Manila clam, R. philippinarum, is effective and encouraging.