Protein turnover, physiological energetics and heterozygosity in the blue mussel, Mytilus edulis : the basis of variable age-specific growth

Genotype-dependent changes in whole-body protein turnover are shown to be of significance to metabolic efficiency, energetic maintenance requirements and associated rates of dietary absorption in Mytilus edulis. Faster growth rates were, on average, evident in more heterozygous mussels. Such variation did not stem from different efficiencies of nitrogen conversion, which remained high and independent of either genotype or age. Rather, faster growth derived from decreased energy requirements for maintenance. This decrease was associated with greater efficiencies of protein synthesis [(net protein balance/gross protein synthesis) $\times $ 100] and lower intensities of nitrogen metabolism. Energy `saved' by virtue of higher metabolic efficiency was used to effect increases of both ingestion and absorption efficiency. Alternatively, reduced synthetic efficiencies among more homozygous mussels necessitated elevated protein synthesis per unit deposition, high efficiencies of nitrogen conversion being sustained by a greater proportional recycling of breakdown products. Energetic costs were thus enhanced, with a consequent reduction in growth. Efficiencies of protein synthesis declined with age, whereas maintenance requirements grew. These findings are discussed in relation to individual fitness, and proposed as a mechanism by which genotype may mediate intraspecific variations of growth among animals generally.