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Paraquat (PQ) is a widely used herbicide; however, it has been linked to various diseases, including an increased risk of developing Parkinsonism. To study this, invertebrates such as ascidians have been used. They have a simple nervous system and are considered an emerging model for the study of neurodegenerative diseases. Here, we investigated the effects of PQ in the brain of the ascidian Styela plicata. We performed behavioral tests, assessed morphology, and monitored oxidative stress and the expression of tyrosine hydroxylase (TH) and caspase-3 (Casp 3) using immunofluorescence. In the presence of PQ, siphon movement was reduced and cortical thickness was increased. In addition, an observed increase in ROS was detected, along with attenuated SOD and CAT activities and increased LPO production. Interestingly, PQ downregulated the expression of TH and upregulated that of Casp 3. We conclude that PQ-induced oxidative stress leads to the death of catecholaminergic neurons in the S. plicata brain. [Display omitted] • Paraquat (PQ) slows down ascidian siphon movements; • PQ-induced brain degeneration in ascidians; • PQ increased oxidative stress in ascidian brain; • PQ downregulated the synthesis of tyrosine hydroxylase, an enzyme involved in dopamine synthesis; [ABSTRACT FROM AUTHOR]