Zebrafish embryos/larvae develop externally and are optically transparent. The bioaccumulation of microplastics in fish at early life stages is readily assessed with fluorescently labeled microbeads.
Xu, C., Guo, H., Wang, R., Li, T., Gu, L., Sun, L. Accumulation and Distribution of Fluorescent Microplastics in the Early Life Stages of Zebrafish. J. Vis. Exp. (173), e62117, doi:10.3791/62117 (2021).