Characterizing phenotypic diversity in marine populations of the threespine stickleback.

The threespine stickleback (Gasterosteus aculeatus) is an important model for studying the evolution of vertebrate morphology. Sticklebacks inhabit freshwater, brackish, and marine northern hemisphere waters. Anadromous and marine populations (hereafter marine) are assumed to have remained unchanged morphologically from ancestral marine sticklebacks, despite marine environments varying on regional and local scales. Recent studies suggest that genetic and phenotypic structure exists in marine populations, yet the scale of this variation, and its ecological causes remain unclear. Our goal was to assess morphological trait variation in marine stickleback populations around Southern British Columbia (BC) and determine if oceanographic and habitat characteristics were associated with this variation. Between May-July 2019, we sampled 534 sticklebacks from 15 sites around Vancouver Island, a region characterized by a large diversity of oceanographic and habitat features. We characterized trait variation using two-dimensional (2D) geometric morphometric analysis, comparing individuals between oceanographic regions and habitats. We focused on head and body shape. We found that marine sticklebacks varied morphologically among and between regions and habitats, but the variation did not appear to be related to environmental variation. Sexual dimorphism was the largest source of variation, but oceanographic and habitat variables influenced differences between sexes. We concluded that marine sticklebacks offer abundant opportunities for expanding our knowledge of drivers of morphology.