Survival and divergence of Arctic endemic cod species during 50,000 years of climate change.

Despite significant climate variability in the Northern Hemisphere ~50,000-12,000 years ago, the potential for vertebrates to periodically thrive in the glacial Arctic Ocean has been largely overlooked because ecosystem productivity was assumed to be too low. Here, we analyze fish otoliths preserved in deep-sea sediments and find that two Arctic endemic cod species, ice cod (Arctogadus glacialis) and polar cod (Boreogadus saida), lived at 87-89 °N ~ 48,000-35,000 years ago and reappeared at the onset of the Holocene ~12,000 years ago. A. glacialis also occurred north of Greenland at 84-85 °N ~ 20,000 years ago, during the peak of the Last Glacial Maximum, suggesting the presence of refugia that enabled survival and divergence from B. saida under the harshest climate conditions. These findings suggest that the glacial Arctic Ocean was not a barren ice desert but rather a breeding ground for the evolution of cold adaptation in marine fish.