ADDISON, J.A.; Dalhousie University: Circumpolar phylogeography of the green sea urchin (Strongylocentrotus droebachiensis).

The green sea urchin Strongylocentrotus droebachiensis is a circumpolar species found subtidally at high latitudes. These urchins are thought to have colonized the north Atlantic from the north Pacific shortly after the Bering Seaway opened 3-3.5 Mya. Glaciation events during the Pleistocene may have extirpated populations of sea urchins and other obligate rocky species in the northwest Atlantic. The last glacial maximum 20,000 years ago is thought to be especially severe. Both species distributions and molecular evidence suggest that some populations recently extirpated in the northwest Atlantic were recolonized by refugial populations from the northeast Atlantic. However, molecular evidence for green sea urchins suggests trans-Arctic exchange between the Pacific and northwest Atlantic as recently as 90,000 years ago. To further resolve the phylogeographic relationship among green sea urchin populations I sequenced a 418 bp segment of the mitochondrial gene cytochrome c oxidase subunit I (COI) in 183 individuals collected from the northeast Pacific and both sides of the north Atlantic. Preliminary analysis of gene trees indicates a recent unidirectional migration of sea urchins across the north Atlantic from the east to the west. However, these migration events were not likely responsible for the recolonization of the northwest Atlantic following the last glacial maximum. The majority of the haplotypes detected in the northwest Atlantic appear to be descended from a Pacific ancestor. These results suggest that northwest Atlantic populations of sea urchins were either recolonized from the Pacific or survived in local refugia during the last glacial maximum.