Do not despise the little ones: population-level responses of common gobies to seasonality along latitudinal cline show biased vulnerability of southern populations

This study investigated multi- molecular and trait responses of natural populations of the intertidal goby Pomatoschistus microps to seasonal changes, distributed along a latitudinal cline. Fish, as well as environmental data were collected in situ from three locations with different climatic regimes along a north-south gradient in the North Atlantic Ocean during spring and summer seasons. Primary metabolomes and stress biomarkers were analysed in muscle tissue from immature, male and female individuals, together with a qualitative assessment of histopathological traits and fish size. Results show inter-population differences in ability to respond to seasonal changes, with Northwestern populations exhibiting high metabolic plasticity while Southern populations remained stable. These differences in response strategy reflect variable populational capacity to optimize their molecular and phenotypic traits to track environmental variation depending on basal stress and energy levels. Macromolecular damage was generally higher in the non-plastic Southern population and was negatively correlated with body size. However, tissue health among populations or seasons did not present major variations. Metabolic adjustments to seasonal changes involved the remodelling of energy metabolism, favouring amino acids and carbohydrates as primary cellular fuels. There were also population-unique alterations to metabolic pathways, evidencing local-specific responses. Oxidative stress responses did not differ between males and females, however, immature individuals displayed greater macromolecular damage in proteins and lipids, suggesting differential performance of antioxidant systems along development. We conclude that the differential capacity of populations to respond to seasonal changes results in a spatially-biased pattern of vulnerability, with Southern populations being more vulnerable to environmental changes that occur during natural warming periods.