Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


The relationship between individual energy budgets and genetic variability was examined in Thais haemastoma acclimated to 7.5, 20 and 35('o)/oo S at 21(DEGREES)C. Energy budgets, after the equations of Crisp (1971), were determined by measuring rates of caloric ingestion, absorption efficiencies, oxygen consumption and ammonia and primary amine excretion. Genotypes at six polymorphic loci coding for soluble enzymes, of 25 total loci assayed, were determined using starch gel electrophoresis. The polymorphic loci examined were those encoding for (alpha)-L-amino peptidase (LAP), phosphoglucomutase (PGM), glucose-6-phosophate dehydrogenase (G6PDH), a non-specific esterase (Est) and two dipeptidases (L-leucyl-L-proline and L-phenylalanyl-L-proline dipeptidases). The genetic variability observed in T. haemastoma was similar to that reported for other marine invertebrates. There was a significant positive relationship between total heterozygosity and whole animal Scope for Growth at all three salinities. This relationship was due in part to the greater size of the more heterozygous individuals since Scope for Growth increases with size. Analysis of covariance also revealed a significant positive effect of total heterozygosity on weight corrected Scope for Growth. These results provide physiological evidence of superiority of heterozygotes over homozygotes, for growth, survival and potential reproductive output. The increased Scope for Growth measured in heterozygotes was caused by significantly greater feeding rates. Heterozygous individuals at 20 and 35('o)/oo S had lower metabolic "maintanence" costs, expressed as calories lost per unit weight than more highly homozygous ones. However, greater metabolic efficiency did not account for much of the differences in Scope for Growth between heterozygotes and homozygotes. Heterozygotic overdominance, or heterozygote superiority over homozygotes across all loci is the most likely explanation of the observed differences in Scope for Growth. The heterozygote contribution at each locus is small, but cumulative, resulting in a statistically significant relationship. Mean Scope for Growth was significantly greater for LAP heterozygotes at the stressful salinities, 7.5 and 35('o)/oo S, but not at the optimum salinity, 20('o)/oo S. This indicates the possibility of direct selective pressure from salinity extremes on genetic variability at the LAP locus.