Abstract
Copepods of the genus Acartia dominate zooplankton assemblages in northwestern Atlantic estuaries, many of which originated after the last glacial maximum 10,000-18,000 years ago. Acartia hudsonica occurs, at least seasonally, in estuaries from Chesapeake Bay to Labrador/Newfoundland. We sequenced the mitochondrial gene Cytochrome B (CytB) of 75 individuals of A. hudsonica from 26 estuaries from New Jersey to Maine, covering four biogeographic regions, and 11 individuals of Acartia tonsa from four of these estuaries in the southern part of the sampling range. A. hudsonica exhibited exceptionally high intraspecific DNA sequence variation. Uncorrected p-distances between sequences ranged from 0.3 to 31%. Five highly divergent sequence groups differed in frequencies across populations and biogeographic regions. One sequence group dominated northern localities, and two sequence groups were found at intermediate to high frequencies in two southern biogeographic regions. Ages of the sequence groups were estimated to be 11, 13, 30, and 37 million years, by applying a molecular clock calibrated by divergence in Alpheus snapping shrimps across the Isthmus of Panama. These ages were compared with independent biogeographic paleoceanographic data, and may have coincided with periods of global climate change over the past 40 MY.