Population-Specific Differences in Respiratory Metabolism and Key Metabolic Enzyme Activities in Manila Clam (Ruditapes philippinarum)

Respiratory metabolism reflects energy demand and physiological variation in bivalves, but interpretation of among-population differences requires careful consideration of developmental stage, diet, breeding background, and previous rearing history. In this study, oxygen consumption rate (OR), ammonia excretion rate (NR), O:N ratio, and key metabolic enzyme activities were compared among three Manila clam (Ruditapes philippinarum) populations: Laizhou wild-origin stock (LZ), Fujian-origin stock cultured in Liaoning (FJ), and Zebra shell-color selected line (ZS). These populations were selected to represent three common germplasm categories used in Manila clam production: a wild-origin stock, a translocated aquaculture stock, and a Zebra shell-color selected line. Clams with nominal shell lengths of 10, 15, and 20 mm were used for respiratory measurements, and enzyme activities were determined in 15 mm individuals. Two-way ANOVA revealed significant effects of population on OR, NR, and O:N ratio (p < 0.05), whereas shell length significantly affected NR and O:N ratio (p < 0.05). OR decreased with increasing shell length in LZ and FJ, while ZS showed a different pattern. NR generally declined with shell length across all populations. The O:N ratio increased markedly in LZ but remained relatively low in FJ and ZS. Among enzyme activities, SDH was significantly higher in LZ (p < 0.01), whereas AKP activity was highest in ZS (p < 0.05). LDH activity did not differ significantly among populations. These results demonstrate that Manila clam populations with different source backgrounds exhibit distinct physiological profiles under standardized laboratory conditions. The observed variation represents population-associated physiological characteristics rather than direct evidence of genetic differentiation, providing baseline information for future germplasm characterization integrating physiological, genetic, and environmental approaches.