Comparison of Toxicity of Heavy Metals and Rare Earth Elements in Zebrafish Larvae and Daphnia magna Neonates Using a Phenomics Approach with Multiple Locomotor Activity Endpoints

Heavy metals (HMs) and rare earth elements (REEs) persist as environmental contaminants that accumulate in aquatic ecosystems and pose risks to aquatic organisms. Although the toxicity of HMs is well-established, the ecological and biological impacts of REEs remain comparatively underexplored, despite their expanding industrial use. In this study, zebrafish (Danio rerio) larvae and Daphnia magna neonates were employed as complementary aquatic models to comparatively assess the toxicological effects of multiple metal groups, leveraging their rapid development and suitability for high-throughput behavioral analysis. Both species were exposed to 17 HMs and 16 REEs across concentration ranges defined by species-specific guidelines to determine LC50 values, followed by 24 h behavioral assessments at sublethal concentrations, and employing optimized, high-throughput protocols. Thirteen behavioral endpoints were analyzed to comprehensively characterize locomotor activity, spatial exploration, and movement complexity. Multivariate behavioral patterns were evaluated using principal component analysis and heatmap clustering. The results revealed pronounced species- and metal-specific toxicity profiles, with several REEs inducing behavioral effects comparable to or exceeding those of certain HMs. Notably, behavioral responses differed between zebrafish larvae and Daphnia magna for the same compounds, highlighting species-specific sensitivity and underscoring the value of multi-species assessment for environmental risk evaluation.