Life-history variation in the coexisting freshwater copepods Eudiaptomus gracilis and Eudiaptomus graciloides

The variation in life-history patterns that allow closely related species to co-exist has been an important theme in ecology for decades. We examined intra- and interspecific variation in a key life-history trait - diapause - for two congeneric copepods (Eudiaptomus gracilis and Eudiaptomus graciloides) inhabiting three lakes in Northern Germany. Diapause has been hypothesized in both theoretical and empirical studies to be important in the coexistence of competing species. We found no diapause for E. gracilis whereas we documented two distinct periods of diapause in the life cycle of E. graciloides. In the latter species, diapausing eggs were produced in all three lakes during autumn, however, relative investment in diapausing eggs differed between lakes. Diapausing egg production was delayed or fewer females switched to making diapausing eggs in the more productive lakes relative to the less productive systems. In the sediments of all three lakes, viable diapausing eggs were found buried from the sediment-water interface down several centimeters in each lake, suggesting the presence of a long-lived egg bank. In addition, detailed population studies carried out on both species in one lake (Schohsee) revealed a second period of diapause in E. graciloides. After the diapausing eggs are produced in autumn, females accumulate lipids, reduce feeding and apparently over-winter in a state of active diapause. The prevalence of diapause in E. graciloides and lack of it in E. gracilis raises questions regarding the factors that shape the life-histories of these congeners, and the role that the life-history differences observed may play in their coexistence.