Evidence of Countergradient Variation in Growth of Spotted Gars from Core and Peripheral Populations

Abstract

Peripheral populations occupy the edge of a species’ range and may exhibit adaptations to potentially “harsher” marginal environments compared with core populations. The peripheral population of Spotted Gar Lepisosteus oculatus in the Great Lakes basin represents the northern edge of the species’ range and is completely disjunct from the core Mississippi River basin population. Age‐0 Spotted Gars from the peripheral population experience a growing season approximately half that of the core population but reach similar sizes by winter, suggesting potential for countergradient variation in growth, i.e. an evolutionary response to an environmental gradient such as latitude to compensate for the usual phenotypic effect of that gradient. In this study we used two common garden experiments to investigate potential countergradient variation in growth of young‐of‐year Spotted Gars from peripheral populations in comparison with those from core populations. Our first experiment showed that in a common environment under temperatures within the first growing season (22–24°C), Spotted Gars from the peripheral population had significantly higher growth rates than those from the core population. Final Spotted Gar weight–length ratio was also higher in the peripheral versus core population. In our second experiment, under three temperature treatments (16, 23, and 30°C), maximum growth occurred at the highest temperature, whereas growth ceased at the lowest temperature for both populations. These results suggest that important genetic and physiological differences could exist between the two population groups, consistent with countergradient variation. Our findings indicate that countergradient growth variation can occur even in relatively slowly evolving fishes, such as gars (family Lepisosteidae), and that protection of peripheral populations should be a key component of fish conservation planning.