Abstract
Intrapopulation variability in growth trajectories among conspecifics provides a reservoir of phenotypic diversity that can buffer populations against environmental change. From a life-history and metabolic-scaling perspective, this variability reflects alternative resource-allocation strategies-ranging from rapid biomass accretion to energy conservation-that may represent bet-hedging under fluctuating environments. Biochemical metabolic processes may underpin these diverse strategies, though the mechanisms involved remain poorly understood.A cohort of Gammarus insensibilis, a broadly distributed aquatic macroinvertebrate commonly found in transitional water ecosystems, was reared under laboratory conditions for 75 days. Despite being of the same age, individuals displayed marked size differences and were subsequently classified by body size into Small (S), Medium (M), and Large (L) groups as proxies for slow, intermediate, and fast growers.To test the occurrence of a biochemical basis of this intrapopulation variability, a single-organism (1)H NMR metabolomics approach was applied to Gammarus insensibilis individuals.Metabolic profiling of each animal revealed discrete "fingerprints" rather than a uniform baseline state, with at least two major metabolic pathways significantly different between S and L size individuals. These pathways included alanine, aspartate, and glutamate as well as arginine metabolism. Medium-sized individuals displayed intermediate profiles with unique metabolite ratios pointing to phenotypic plasticity.These results highlight the utility of (1)H NMR spectroscopy in resolving individual metabolic states and identifying anabolic pathways linked to growth phenotypes, suggesting that phenotype-dependent performance under fluctuating conditions may maintain intrapopulation variability.