Disentangling shape and size in a population of unusually large Threespine Stickleback (Gasterosteus aculeatus Linnaeus, 1758) from Vancouver Island, British Columbia

Sarita Lake, British Columbia houses a distinctive population of threespine stickleback (Gastrosteus aculeatus Linnaeus, 1758) with a phenotype characterized by unusually large individuals relative to nearby conspecifics. We tested the hypothesis that members of this population are not isometrically larger but rather exhibit variation in allometric trajectories that reflect changes in developmental timing impacting the developmental-genetic architecture of the phenotype. We used 3D geometric morphometrics to characterize the size and shape of skulls, pectoral girdles and pelvic girdles from a sample of individuals from nearby freshwater and marine populations and compare them to a sample from Sarita Lake. We showed that individuals from the Sarita Lake population are larger in each body region compared to most other populations examined. Further, these individuals have dorsally expanded skulls and relatively robust pelvic armour. We also showed that the relationship between size and shape is differently structured among body regions and is heavily influenced by non-uniform sexually mediated variation across populations sampled. Our results reflect complex underlying developmental trajectories, and we suggest that the larger average size observed in the Sarita Lake population may be driven by a combination of a limnetic trophic niche and additional selective pressures in Sarita Lake.