Polystyrene microplastics impair skeletal development in zebrafish: Biphasic toxicity and comparison with marine medaka.

Microplastic pollution poses a growing threat to aquatic ecosystems, yet its impacts on skeletal development in fish remain poorly understood. This study investigated the toxic effects of polystyrene microplastics (PS MPs) on skeletal development in zebrafish and compared the findings with recent reports in marine medaka to identify conserved and species-specific responses. In zebrafish, PS MPs accumulated primarily in the intestine, gill, and liver, leading to spinal deformities and cartilage matrix degradation. Exposure induced biphasic (14 vs. 28 days), time-dependent toxicity: short-term (14 days) stimulated osteogenic activity and locomotion, whereas prolonged exposure (28 days) suppressed cartilage differentiation and reduced swimming performance. Transcriptomic and qPCR analyses revealed dysregulation of key osteogenic and chondrogenic genes. To evaluate cross-species relevance, we also compare our zebrafish data with those from the marine medaka, which do not have osteocytes-a key structural difference that may influence toxic susceptibility. Consistent with marine medaka, both models show PS MP accumulation in gastrointestinal and respiratory tissues, biphasic behavioral changes, and disruption of conserved skeletogenic pathways (e.g., Runx2-Sp7 axis). However, differences in bone cell biology and in the manifestation of skeletal lesions highlight model-specific toxicological nuances. These findings underscore the dual-phase toxicity of PS MPs on fish skeletal systems and emphasize the value of multi-species comparisons for comprehensive ecological risk assessment of microplastics in aquatic environments.