Nonetheless, PXR activity in rats and zebrafish has been reported upon nanoparticle exposure, that the PCL oligomers might perturb lipid metabolism. The activation of NRF2 suggests that the PCL extracts contained products capable of causing oxidative stress. The oxidative stress response is highly conserved in vertebrates. Embryonic development involves precisely regulated changes in cellular redox balance, and thus, developing embryos are susceptible to chemicals that disrupt redox homeostasis. Numerous studies report evidence of oxidative stress from exposure to plastic particles, 101,102 although the exact components triggering this response are not well understood. From our results and these arguments, the likely source for the observed acute toxicity of developing zebrafish embryos caused by passive, water-borne exposure to macroscopic PCL was its potentially rapid release of nanoplastics and oligomers within 24 h and thereafter.