The water content, apoptosis, and proliferation of the brain in marine medaka affected by seawater acidification

Abstract A possible explanation for ocean acidification‐induced changes in fish behavior is a systemic effect on the nervous system. Three biological barriers at the blood–brain interface effectively separate the brain from the body fluids. It is not known whether fish brain regions in contact with these barriers are affected by acidification. Here, we studied structural changes in medaka ( Oryzias melastigma ) brain regions contacting cerebrospinal fluid (CSF) after short‐term (7 days) CO 2 exposure. The brain water content decreased significantly and the superficial structure of the pia mater was changed, but there was no obvious damage to the internal structures of the brain after seawater acidification. Seawater acidification also led to an increase in apoptosis and a decrease in the number of proliferative cells in brain areas contacting CSF. These results indicate that the structure of CSF‐contacting brain regions in medaka was affected by seawater acidification, and the brain responded to seawater acidification stress by increasing apoptosis and reducing proliferation.