Invasions of fresh water by marine organisms have been of great interest to evolutionary biologists and paleontologists because they typically constitute major evolutionary transitions. Recent (< 200 years) invasions of fresh water by brackish or marine species offer an opportunity to understand mechanisms underlying these events, but pathways of invasion from salt water have not been confirmed using genetic data. This study employed mitochondrial DNA sequences (652 base pairs from the cytochrome oxidase I (COI) gene) to reconstruct the geographic and evolutionary history of freshwater invasion by the common estuarine and saltmarsh crustacean Eurytemora affinis (Copepoda; Poppe 1880). Phylogenetic analysis of populations from North America, Europe, and Asia revealed at least eight independent invasions of fresh water from genetically distinct lineages. At least five of these freshwater invasions most likely arose independently in different river drainages, recently from saltwater sources within each river drainage. An analysis of molecular variance (AMOVA) was performed at three geographic scales (among continents, among drainages, and within drainages) to assess the hierarchical distribution of genetic variance. Results indicated that 52% of the genetic variance was explained by differences among drainages, 43% by differences among continents, but only 5% by differences within drainages, thus supporting geographic patterns of invasions inferred from the phylogeny. Physiological experiments were performed to determine whether adults and larvae from saltwater populations could tolerate freshwater conditions. Transfer to zero salinity resulted in high mortalities, but with some survival to the second generation in one population. This study provides genetic evidence and physiological support for rapid transitions from a saline life history into fresh water, with repeated invasions on a global scale.
