Genetic and environmental components of phenotypic variation in body shape among populations of Atlantic cod (Gadus morhua L.)

A common-garden experiment was conducted on larvae to test for genetic differences in body shape among populations of Atlantic cod (Gadus morhua). Offspring from four north-west Atlantic regions were reared from hatching to postmetamorphosis at two temperatures (7 ± 1°C and 11 ± 1°C) and two food levels (1500 and 4500 prey L-1). Body shape differed between populations and treatments. Population differences were greatest between south-west Scotian Shelf cod and those further north; the former were characterized by a deeper body, larger head, and longer caudal peduncle than cod from the other populations. Significant differences were also observed between two putative populations on the south-west Scotian Shelf, suggesting genetic divergence between spawning aggregations at small spatial scales (< 100 km). Temperature and food supply also influenced body shape, with the effect of the former being more pronounced. Individuals reared at the higher temperature or food level had a deeper body and a larger head than those reared at the lower temperature or food supply. Phenotypic responses to changes in the rearing environment also differed among populations, indicating genetic differences in phenotypic plasticity. Differences between populations in morphology and in phenotypic plasticity suggest genetic divergence at both large (> 1000 km) and small (< 100 km) spatial scales. The genetic differences at large spatial scales counteracted the expected effects of temperature differences in the wild, suggesting countergradient variation in morphology among these populations.