MULTIPLE-LOCUS HETEROZYGOSITY, MORTALITY, AND GROWTH IN A COHORT OF MYTILUS-EDULIS

A positive correlation between the degree of individual heterozygosity (H) at five polymorphic enzyme loci and shell length was reported for a cohort of Mytilus edulis L. sampled in September, 1983, two months after settlement (Koehn and Gaffney, 1984). In the present study, the same cohort was resampled four and eight months after settlement in November, 1983, and April, 1984, respectively. Among individuals four months of age, there was a small, but statistically significant, negative correlation between H and shell length. In this sample, heterozygote deficiency in the cohort was less than that of any size group in the September sample. The reversal of the H-size relationship between September and November suggests that differential mortality, particularly among small, homozygous individuals, oocurred in this period. For individuals eight months of age, there was no correlation between H and shell length, there was no heterozygote deficiency in the cohort and the average heterozygosity was the same as that of the largest individuals in September. At the youngest age, there was a positive relationship between H and growth rate; as aging occurred, differential mortality altered the relationship between H and growth rate; viability differences among heterozygosity classes obscured the original effect of H on growth rate. If there is a positive relationship between heterozygosity and size, it can most likely be detected from sampling a large outbreeding population before differential mortality occurs and before energy is allocated to reproduction.