Ecological and Physiological Differentiation Among Low‐Artic Clones of Daphnia Pulex

A distributional survey of obligately parthenogenetic clonal groups belonging to the Daphnia pulex complex was conducted on rock bluff and tundra ponds near Churchill, Manitoba. Clonal distributions were spatially heterogeneous. The clonal patterns of microgeographic heterogeneity suggested a strong association of Daphnia clonal type with salinity/conductivity gradients in pond habitats that varied in their proximity to Hudson Bay. Acute salinity tolerance experiments showed significant differences in survivorship of six clonal isolates at salinities encountered in nature. Clones from high—salinity ponds had greater survivorship at high salinities than clones from low—salinity ponds. There were no significant differences in clonal survivorship at low salinities. Clonal differences were also found in hatching success of ephippial eggs at different salinities. Laboratory measurements of hemolymph osmolality from these clones indicated significant differences in oscmoregulatory capacity. Coupled with morphological and electrophoretic data, the ecophysiological differences among D. pulex clones from Churchill suggest strongly that this Daphnia complex consists of physiologically different "ecotypes." The ecological and evolutionary implications of these data are discussed.