Exposure to microplastics decreases swimming competence in larval zebrafish (Danio rerio)

Microplastics have been frequently detected in both marine and freshwater ecosystems. Their impact on aquatic organisms has raised much concern. This study investigated the impact of microplastics on zebrafish embryos and larvae, with a special focus on their swimming competence. The zebrafish embryos were exposed to microplastics starting from 4 h post fertilization. Microplastics first adhered to the embryo chorion, then entered the stomach and intestinal tract of the larvae later. In the free swimming test, exposure to 1000 μg/L (around 1.91 × 10⁷ particles/L) of microplastics led to a significant decrease in both swimming distance and speed of zebrafish larvae under the dark condition by 3.2% and 3.5% respectively. In the alternating light-to-dark photoperiod stimulation assay, exposure to 100 and 1000 μg/L (around 1.91 × 10⁶ and 1.91 × 10⁷ particles/L) of microplastics caused a 4.6% and 2.6% decrease in swimming distance, and reduced the active speed by 4.9% and 2.8%, possibly as a result of inhibited dark avoidance in treated zebrafish larvae. At the molecular level, exposure to microplastics induced upregulated expression of inflammation (il1b) and oxidative stress (cat) related genes. This study demonstrates that exposure to microplastics significantly decreases larvae swimming competence, which may have significant impacts on its population fitness in the aquatic environment and further ecological consequences.