Adaptive divergence of seasonal heart plasticity between Canadian and Spanish pumpkinseed sunfish populations

Laboratory experiments suggest that reversible changes in the heart ventricle phenotype of fish accompany acclimation to temperature change to maintain cardiac function, but related work in fish living in natural conditions is scant. We investigated seasonal variation in heart ventricular mass and collagen content in pumpkinseed sunfish (Lepomis gibbosus) living in outdoor ponds where they experienced high seasonality conditions. Additionally, we compared populations adapted to high and low seasonality to evaluate potential divergence in seasonal heart plasticity. Heart ventricular mass decreased in the summer compared to colder seasons only in populations adapted to high seasonality. The absence of seasonal variation in ventricular mass in sunfish adapted to low seasonality was not due to changes in foraging activity, suggesting a loss of ventricle size plasticity due to either costs of plasticity or relaxed selection. Seasonal variation in ventricle collagen content also occurred, with the highest collagen content in summer regardless of population adaptation to high or low seasonality. Only the proportion of thick collagen fibres changed across seasons. We conclude that natural seasonal cues induce plastic responses in some functional heart traits and propose that these responses can rapidly diverge among populations under different seasonal regimes.