REALIZED HERITABILITY OF GROWTH-RATE IN THE HARD CLAM MERCENARIA MERCENARIA

Native South Carolina wildstock clams (Mercenaria mercenaria) were mass-spawned to produce a large initial population of parents for a directed breeding program. At 2 years of age, the largest 10% of this population, and an equal number of mean size clams, were segregated to become selected and control-line parents. Three separate experiments were performed, usually involving 20-40 selected and control parents. Offspring were reared under standard hatchery, nursery and field grow-out conditions. Realized heritability was determined at 2 years of age. In one experiment, no response to selection was observed at 2 years, possibly due to reduced effective breeding number. Conservative estimates of realized heritability of growth rate for the other two experiments were consistent and high: 0.42 +/- 0.10 and 0.43 +/- 0.06. Mass selection appears to be a promising technique for improvement of hard clam broodstocks.