Embryonic Toxicity: Systematic Review of Short-Term Exposure Effects of Rare Earth Elements in Danio rerio (Zebrafish)

Rare earth elements (REEs) are widely used in industries due to their unique chemical and physical properties, but are not found in concentrated or pure forms in nature. Their extensive use, long and large-scale mining and poor e-waste management have led to increasing environmental contamination, particularly in aquatic ecosystems. REEs are considered emerging pollutants, and their exact ecosystem impacts are inconclusive. Hence, a systematic review was carried out to comprehend the biological effects of REEs using the early stage of the model organism, zebrafish (Danio rerio). The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) strategy was used to conduct a comprehensive literature review. Databases including ResearchGate, PubMed, Google Scholar, and Science Direct were searched for peer-reviewed research articles between 2000 and 2025. The published articles were collected, screened, and reviewed on the effects of REEs in the embryonic stages of zebrafish. Only nine articles became eligible and were analysed for effects, including hatching rate, mortality, malformations, body length, swimming speed, and coagulation on zebrafish embryos exposed to different concentrations of REEs. One study showed significant effects on heart rate, while significant effects of malformations have been investigated by four studies upon exposure to REEs. Significant effects on swimming speed were observed in one study and five articles reported significant effects on hatching rate and time. Three studies investigated the survival and mortality of zebrafish embryos, but only one article showed a significant effect. Three studies investigated the significant effect on body length of zebrafish embryos, and one study reported on the significant effect on coagulation. Based on the systematic review, it can be concluded that acute exposure to REEs in zebrafish embryos caused a significant decrease in heart rate, body length, swimming speed, hatching rate, survival and a significant increment of malformations, hatching time, mortality, and coagulated embryos. Therefore, based on the review, it can be concluded that acute exposure to REEs caused significant toxic impacts on early life stages of embryos. However, a significant knowledge gap exists, as only selected growth effects have been studied in 14 selected REEs using the zebrafish embryonic model.