INTERPOPULATIONAL AND INTRAPOPULATIONAL VARIATION IN TIME TO METAMORPHOSIS IN A FRESH-WATER COPEPOD

1. Adjacent populations of the copepod Cyclops scutifer Sars living in lakes in southern Norway exhibit remarkably different life cycles. A series of laboratory-common environment experiments were used to parti tion variance in one life cycle trait-time to metamorphosis - among an d within five populations of C. scutifer, to examine the extent to whi ch variation in this trait has a genetic basis and to test whether pop ulations are polymorphic for development rates. The experimental popul ations exhibit a variety of life cycles in the field and occupy enviro nments that represent a broad range of conditions. 2. Populations with different life cycles in the field continued to express differences i n time to metamorphosis in the laboratory, indicating a genetic contri bution to life cycle variation. Differences in developmental rates in the laboratory were smaller, however, than differences observed in the field, suggesting that environmental conditions also contribute to ob served life cycle variation. 3. In addition to interpopulation variati on, each population maintained substantial intrapopulation variation i n time to metamorphosis; differences between individuals from the same population were often as large as or larger than differences between populations. 4. Individual females in most populations produced highly variable offspring, and there often was little difference in time to metamorphosis among families within a population. 5. Cyclops scutifer exhibits a hierarchy of variation in time to metamorphosis, with a maj or portion of this variation expressed among siblings. Intrapopulation life history variation important to natural selection may be maintain ed by different processes among the major groups of freshwater zooplan kton.