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

The variation in life-history patterns that allow closely related species t o 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 graciloi des) inhabiting three lakes in Northern Germany. Diapause has been hypothes ized in both theoretical and empirical studies to be important in the coexi stence of competing species. We found no diapause for E. gracilis whereas w e documented two distinct periods of diapause in the life cycle of E. graci loides. In the latter species, diapausing eggs were produced in all three l akes during autumn, however, relative investment in diapausing eggs differe d between lakes. Diapausing egg production was delayed or fewer females swi tched to making diapausing eggs in the more productive lakes relative to th e less productive systems. In the sediments of all three lakes, viable diap ausing eggs were found buried from the sediment-water interface down severa l centimeters in each lake, suggesting the presence of a long-lived egg ban k. 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 lipid s, 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 th ese congeners, and the role that the life-history differences observed may play in their coexistence.