Abstract
Marine organisms generally develop in one of two ways: biphasic, with distinct adult and larval morphology, and direct development, in which larvae look like adults. The mode of development is thought to significantly influence dispersal, with direct developers having much lower dispersal potential. While dispersal and population connectivity is relatively well understood for biphasic species, comparatively little is known about direct developers. In this study, we use a panel of 8,020 SNPs to investigate population structure and gene flow for a direct developing species, the New Zealand endemic marine isopod Isocladus armatus. We find evidence that on a small spatial scale (20 kms), gene flow between locations is extremely high and suggestive of an island model of migration. However, over larger spatial scales (600km), populations exhibit a strong pattern of isolation-by-distance. The intersection of our sampling range by two well-known biogeographic barriers (the East Cape and the Cook Strait) provides an opportunity to understand how such barriers influence dispersal in direct developers. We find that I. armatus has high migration rates across these barriers. However, we find evidence of a north-south population genetic break occurring between Māhia and Wellington, although there are no obvious biogeographic barriers between these locations. This study suggests that biogeographic barriers may affect migration in direct development species in unexpected ways.
