Single-nucleus transcriptomics links telencephalic cell signatures to ocean acidification effects on social behaviour in a coral reef fish

Mutualistic interactions are fundamental to the health and stability of reef ecosystems. While elevated CO2 has been implicated in impairments in interaction behaviours, how this environmental change alters cellular states and intercellular communication in the brain remains unresolved. Here, by analysing the single-nucleus RNA profile of ~66,000 telencephalic nuclei from a keystone mutualist, the cleaner wrasse ( Labroides dimidiatus ), interacting under ambient or elevated CO2 conditions, we identify highly cell type specific neural responses to elevated CO2. We highlight a substantial depletion in Oligodendrocyte Precursor Cells (OPCs), notably linked to oligodendrocyte myelination and neural structure maintenance. Elevated CO2 also elicited targeted transcriptional shifts across discrete cell types, characterised by widespread glutamatergic signalling and altered co-expression networks associated with pH regulation, energetic metabolism, and circadian rhythm, thereby compromising the cleaner's physiologic equilibrium. Additionally, cell-cell communication was altered under elevated CO2, including remodelling in SEMA4 and PTPR signalling pathways within radial glial and OPCs, respectively, revealing network signalling rewiring in response to the changed environment. These findings uncover the high-resolution neural substrates vulnerable to ocean acidification, providing mechanistic insight into how climate-driven environmental changes alter brain function and destabilise complex mutualistic ecological interactions.