Exposure to three levels of CO2-induced freshwater acidification results in multi-tissue transcriptional and behavioural responses in medaka (Oryzias latipes)

Elevated atmospheric carbon dioxide (CO2) can acidify aquatic environments, which several studies have linked to molecular and behavioural responses in fish. However, few studies on CO2-induced acidification have focused on freshwater fishes and the potential for fish to overcome acute responses. Our study investigated the effects of acidification over a 20 d exposure on medaka (Oryzias latipes) behaviour and mRNA transcripts sampled from the brain and gill. Adult medaka were exposed to three levels of acidification (6.1, 5.9, and 5.8 pH), observing behaviours on days 0, 5, 10, 15, and 20. Brains were sampled on days 0, 5, 10, and 20 for pH treatments 6.1 and 5.8 to monitor relative abundance of mRNA of genes linked to GABA synthesis, reward-seeking behaviour, neurogenesis, feeding behaviour, circadian rhythm, and water and ion transport. Gill tissue was sampled in a similar fashion to explore mRNA abundance of genes related to ion transporters and acid-base regulation. Exposure to pH 5.8 altered some individual and group behaviours, including increased vertical activity and changes in zone preference on day-5. Sustained gill transcript responses occurred by day-5 of exposure, showing decreased relative mRNA abundance at 6.1 pH but increased abundance at 5.8 pH. Brain transcripts showed more delayed and selective adjustments. Our findings suggest that CO2-induced freshwater acidification causes some immediate behavioural responses in medaka, possibly linked to distinct, tissue-specific transcriptional pathways.