Cryptic intercontinental hybridization in Daphnia (Crustacea): The ghost of introductions past

The dearth of known cladoceran introductions is paradoxical, as their parthenogenetic lifestyle and desiccation-resistant resting eggs make them excellent candidates for invaders. Unfortunately, the biogeography and systematics of the group are poorly studied so natural species distributions and the importance of invasions are difficult to assess. In this study we conducted an allozyme analysis comparing populations of the common lacustrine cladoceran Daphnia galeata from North America and Europe. Four populations from the lower Laurentian Great Lakes were genetically intermediate between North American and European populations. This geographic pattern of genetic variation suggests that the introduction of European D. galeata has gone unnoticed, and that extensive hybridization with native Daphnia has resulted. North American and European populations were not only genetically distinct but mated non-randomly under sympatry, supporting a taxonomic split at the species level. Our study suggests that insights into evolutionary and ecological processes and conservation biology may be lost when genetic markers are ignored in the biogeographical study of morphologically conserved groups.