Recalibrating the molecular clock for Arctic marine invertebrates based on DNA barcodes

Divergence times for species assemblages of Arctic marine invertebrates have often been estimated using a standard rate (1.4%/MY) of molecular evolution calibrated using a single sister pair of tropical crustaceans. Because rates of molecular evolution vary among taxa and environments, it is essential to obtain clock calibrations from northern lineages. The recurrent opening and closure of the Bering Strait provide an exceptional opportunity for clock calibration. Here, we apply the iterative calibration approach to investigate patterns of molecular divergence among lineages of northern marine molluscs and arthropods using publicly available sequences of the cytochrome c oxidase subunit I (COI) gene and compare these results with previous estimates of trans-Bering divergences for echinoderms and polychaetes. The wide range of Kimura two-parameter (K2P) divergences among 73 trans-Bering sister pairs (0.12%-16.89%) supports multiple pulses of migration through the Strait. Overall, the results indicate a rate of K2P divergence of 3.2%/MY in molluscs, 5%-5.2%/MY in arthropods, and 3.5%-4.7%/MY in polychaetes. While these rates are considerably higher than the often-adopted 1.4%/MY rate, they are similar to calibrations (3%-5%/ MY) in several other studies of marine invertebrates. This upward revision in rates means there is a need both to reevaluate the evolutionary history of marine lineages and to reexamine the impact of prior climatic changes upon the diversification of marine life.