Mesodon normalis hatchlings (totaling 569), exposed to a stress involving excess moisture, mucus, and feces exhibited a reduced mean growth rate, an increased mortality rate, and a significant positive association between juvenile growth and genetic heterozygosity. No significant genotype-dependent growth and mortality occurred in an unstressed control cohort of 458 offspring. The significant r² value 0.053 in the stressed cohort is consistent with values from comparable studies in marine bivalves. Two of the 5 enzyme loci (ALA and PGI) contributed significantly to the association in the stressed cohort, but neither heterozygous deficiencies nor level of heterozygosity were associated with the growth-heterozygosity correlation. When the growth-heterozygosity association was examined in each of the 8 broods comprising the stressed cohort, only one clutch from one parent showed a significant growth-heterozygosity association. In 6 of the remaining 7 broods in the stressed cohort and in 5 of the 8 broods in the unstressed cohort, the trend was in the direction of enhanced growth of heterozygotes over homozygotes or failure of the homozygote class to survive.

By comparing the genotypic structure of the parent to its offspring, it was determined that selfing did not occur and that multiple paternity was common during reproduction in this sample of the monoecious Mesodon normalis.

These findings have significance for previous work on the population biology of this species in that genotype-dependent growth and survivorship appear to influence more the timing of adulthood and reproduction rather the usual body size-dependent reproductive output reported for many European helicids.