Hazards of rare earth elements in bivalves: From sub-individual biomarkers to individual-level responses

Rare earth elements (REEs) play an essential role in modern technologies, but their increased use and continued discharge to marine and freshwater ecosystems classify them as contaminants of environmental emerging concern (CEC). Bivalves are ecological engineers, important commercial aquaculture species, and one of the key bioindicators in monitoring programs, such as the “Mussel Watch”. Therefore, they are an ideal group for assessing REE contamination due to their filter-feeding behavior and ability to bioconcentrate pollutants. This review screens and synthesizes the current knowledge and gaps regarding the effects of REEs on bivalves, following a standardized methodology, which retrieved 39 studies. The findings reveal a significant bias with 95% of the studies focusing solely on sub-individual biomarkers, while only 5% investigated organismal level effects (e.g. changes in filtration/clearance, respiration, or growth rates). We highlight evidence for links between apical endpoints at individual levels (e.g, reduced rates linked to energy depletion), identifying plausible connections rather than establishing causal links, contributing to hypothesis generation for future multi-level studies. The importance of analyzing full sets of parameters and the need to expand the range of studied model species and their geographical origin is likewise emphasized. The analysis underscores a critical gap in hazard and risk assessment of REEs related to a lack of studies linking molecular and biochemical responses to organismal and population-level consequences, which may overlook important outcomes. Future research must integrate multi-level assessments to accurately predict long-term impacts of REEs on the health condition of bivalves and how these expand on ecosystem functioning.