HARD CLAM, MERCENARIA-MERCENARIA, BROODSTOCKS - GROWTH OF SELECTED HATCHERY STOCKS AND THEIR RECIPROCAL CROSSES

Crosses between and within two hatchery stocks of hard clams, Mercenaria mercenaria, were created in three independent experiments and the offspring reared to 2 years of age. Environmental variation between experiments strongly influenced early growth, but mean sizes at 2 years were similar for the three experiments. Offspring from all crosses had above-average growth rates. Offspring were assessed electrophoretically at seven enzyme loci, and gene frequencies and overall heterozygosities were compared to parental stocks. There was no correlation between heterozygosity and size at 2 years, nor between heterozygosity and variance of mean size. Reciprocal crosses were not consistently faster growing nor more heterozygous than purebred lines, nor was any relationship apparent between cross and variance of mean size. However, offspring from the reciprocal crosses were genetically distinct from each other and from the purebred lines and possessed the desirable traits of the parental stocks.