GREEN, A.J.; MANAHAN, D.T.; University of Southern California, Los Angeles; University of Southern California, Los Angeles: Metabolic efficiency in fast-growing larvae

Hybrid vigor for growth is an important biological phenomenon in many animals and plants. Larvae of the Pacific oyster Crassostrea gigas, obtained from different genetically-controlled crosses, had faster growth rates in hybrid larvae soon after feeding began. We measured growth, protein content, respiration, and enzyme activity in larvae reared under identical environmental conditions, but with different genotypes. Prefeeding larval stages of all genotypes tested had no difference in respiration rates per individual. Once feeding began at 2 days, larval respiration rate increased as a function of increasing size, but not as a function of differing genotypes. Genotype did, however, have a major effect on growth (�hybrid vigor�). Citrate synthase, an enzyme often used as an indicator of aerobic metabolic scope and mitochondrial density, increased in growing larvae in direct proportion to increases in oxygen consumption. There were no genotype-dependent differences in activities of citrate synthase on a per-larva basis for both prefeeding and later feeding stages of development. Also protein-specific citrate synthase activity did not change throughout larval development and growth for all genotypes tested. We conclude that faster growth can occur in these hybrid larvae without incurring increased metabolic costs. This growth/respiration efficiency does not appear to be related to mitochondrial density, but instead may be regulated by internal energy partitioning to allow faster growth for larvae of different genotypes under similar environmental conditions.