Relationship between Multiple Locus Heterozygosity and Physiological Energetics of Growth in the Estuarine Gastropod Thais haemastoma

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Abstract
The relationship between individual energy budgets and genetic variability was examined in Thais haemastoma acclimated to 7.5, 20, and 35‰S (parts per thousand salinity) at 21 C. Energy budgets were determined by measuring rates of ingestion, absorption efficiencies, oxygen consumption, and ammonia and primary amine excretion. Genotypes at six polymorphic loci coding for soluble enzymes were determined using starch gel electrophoresis. There was a significant positive relationship between total heterozygosity and snail scope for growth at all three salinities. The 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. The increased scope for growth measured in heterozygotes was caused by significantly greater feeding rates. Heterozygous individuals at 20 and 35‰S had lower routine metabolic "maintenance" costs, expressed as energy 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 because variation in metabolic efficiency was small compared with variation in feeding rate. These results provide physiological evidence of the superiority of heterozygotes over homozygotes for growth, survival, and potential reproductive output. The number of heterozygous loci per snail explains 16% of the variance of whole animal energy budgets at 7.5‰S, 14% at 20‰S, and 15% at 35‰S.