Limitations in the genetic variation of hatchery produced batches of the giant clam, Tridacna gigas

Eight production batches of juvenile giant clams (Tridacna gigas) were sampled from three different hatcheries where some batches were derived from single-pair matings and some derived from mixtures of gametes from multiple adults. Allozyme electrophoresis was used to assess parentage and genetic diversity of the batches. Genetic diversity measures were lower, and gene frequencies markedly skewed, in all the batches relative to those in the wild populations from which they were derived. Genotype frequencies in the batches recording single-pair matings did not fit those expected from a pairwise mating of only two individuals. Selfing of at least one parent must have occurred. Contamination by gametes from other clams at fertilization, or by later accidental introduction of individuals from other batches, was also suggested by the presence of additional alleles. Excesses of heterozygotes were frequent, suggesting selection for heterozygotes, ether at fertilization, or in the production process. However, there was no consistent correlation of individual heterozygosity and growth rate within batches.