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.