Elemental C and N balances evidence stoichiometric adjustments to dietary protein content in growth phenotypes of the Manila clam (Ruditapes philippinarum)

Juveniles from two separately bred families of the Manila clam Ruditapes philippinarum were used to constitute fast (F)- and slow (S)-growing groups by size segregation. The growth phenotypes that resulted from these 4 family*growth category combinations were then used to measure elemental C and N balances in the acute and chronic responses to two different microalgal diets that were isocaloric but differed (x3) in their protein:energy ratios. Both diets were based on the same phytoplankton species (Rhodomonas lens) that were harvested in the exponential (low C:N ratio) or stationary (high C:N ratio) phases of an indoor culture. Aims were to assess the differential acquisition and processing of elemental nutrients according to differences in their dietary availability and the variable requirements set by the growth trends exhibited by different phenotypes. Clams fed the high N diet achieved the highest C and N balances through a combination of higher feeding rates and absorption efficiencies of overall organics. However, the main differential effects were associated with increased absorption efficiencies for N relative to C, which were particularly observed in clams subjected to chronic N deficit in the diet. This occurred in all growth phenotypes and resulted in the partial homeostatic regulation of nutrient imbalance operating at the preabsorptive level. Further adjustments occurred at the postabsorptive level with increased N disposal in the form of ammonia excretion and the resulting decrease of metabolic C:N indices recorded in clams fed the high N diets. The main phenotypic differences were observed in the intrafamily (F vs. S growth groups) rather than in the interfamily comparisons, with F clams exceeding the absorption rate of nutrients (both C and N) of S clams by 50%. Physiological responses to the acute dietary change were found to be faster in F clams, indicative of a higher plasticity of this phenotype. Whereas, stoichiometric adjustments by S clams resulted in higher N release through excretion, suggesting less efficient protein turnover.