Rare earth elements (REEs) are increasingly important for modern technologies, from electronics to renewable energy, leading to rising environmental inputs and their recognition as emerging contaminants. Yet, our understanding of the ecological implications of REEs in marine ecosystems remains limited. This presentation will provide a brief overview of current knowledge on the ecotoxicological effects of REEs in marine organisms, integrating evidence from laboratory and field studies across multiple trophic levels. Although the majority of studies focus on bivalves, planktonic species at the base of the food web, where impacts could propagate through the ecosystem, remain largely unstudied. We will report novel experimental investigations on the marine branchiopod crustacean Artemia franciscana, a well-established model for preliminary toxicity screening and a key component of marine food webs, including seabirds and fish. Early life stages were exposed to Ce, Nd, Gd, and La for mortality assessment (48 h; 0.1–500 µg/L). Cerium is typically present at the highest concentration in seawater relative to other REEs, followed by Nd and La, while Gd shows increasing anthropogenic inputs because of its uses in medicine, with persistent Gd anomalies now commonly observed in coastal waters near urban areas. Additional assessments focused on Ce and Gd, assessing their effects on behavioural endpoints in juvenile and adult stages, specifically avoidance responses, swimming speed, and social interaction, in order to capture potential sublethal effects. Avoidance responses were tested in a six-compartment setup (0–600 µg/L), while movement and social spacing were quantified by tracking 10 adults for 3 min at five concentrations (0.1–500 µg/L), with movement speed, exploration area, and social spacing analysed using AnimalTA software. Even though A. franciscana is tolerant to a variety of environmental stressors, results revealed sublethal yet potentially ecologically relevant effects that may influence population dynamics, warranting further research on more sensitive planktonic species. Overall, this study contributes to the understanding of potential ecological implications of REEs in marine systems and highlights the need for further research to better assess their environmental relevance and long-term impacts.