Natural Experiment Reveals Lower Metal Levels in Older Oysters: Disentangling Age, Size, and Growth Effects

Allometry describes the disproportional scaling of physiological processes with the body size. This challenges the assumption of a direct relationship between metal concentrations in organisms and environmental levels, a premise commonly used in biomonitoring programs. However, deciphering the complex drivers of metal allometry is difficult because size and age often confound each other. In this study, we took advantage of a natural experiment provided by populations of oysters of known ages that exhibited substantial size variation within the same age cohorts. Using a stable metal isotope tracer technique, we examined the bioaccumulation of six metals (⁶⁰Ni, ⁶⁵Cu, ⁶⁸Zn, ¹⁰⁷Ag, ¹¹⁴Cd, ²⁰⁶Pb) under both dissolved and dietary exposures. Our results indicate that bioaccumulation efficiency primarily drives the allometric patterns of metal concentrations with age exerting a more pronounced influence than size. Older oysters showed a diminished bioaccumulation capacity, leading to negative correlations between the metal concentration and size. Meanwhile, indirect factors, such as growth dilution, played a minor role, and exposure duration had a negligible effect. This study enhances our understanding of the complex mechanisms governing metal bioaccumulation in marine organisms and provides critical insights into improving the interpretation of biomonitoring data.