Stoichiometry of growth under variable scenarios of nutrient limitation: Differential homeostasis of body composition among growth phenotypes of the Manila clam

Abstract

Fast- and slow-growing phenotypes from two separate breeding families of the Manila clam (Ruditapes philippinarum) were alternatively fed two monoalgal diets with high and low N content (C:N ratios of 4.9 and 13.5, respectively). After 35 days of food conditioning, clams were sacrificed, and the soft body was dissected out into five different tissue fractions to determine the corresponding ponderal ratios (tissue wt./body wt.) and a separate analysis of the elemental composition of these tissues. Previously reported C and N balances performed with the same conditioning diets were integrated and compared with tissue composition of the same phenotypes in order to assess the efficacy of mechanisms elicited to compensate for N deficit. Broad differences in dietary N content resulted in only minor changes in whole-body C:N composition which suggests a noticeable degree of homeostatic regulation of nutrient balances. This regulation was found to be stricter in fast-compared to slow-growing phenotypes and differed among the various body tissues. Using the threshold element ratio approach, physiological mechanisms were identified that partly compensate for large stoichiometric mismatches between low-N food and body tissues.