A reproductive-resting stage in an harpacticoid copepod, and the significance of genetically based differences among populations

Describes a strategy in which fertilized adult females enter a "reproductive-resting' stage during the late fall that allows them to overwinter and provide a first source of spring naupliar recruitment. This strategy has beeen observed in the estuarine copepod Coullana canadensis. Daylength and temperature are the environmental cues that induce the developing female copepodite to switch between active reproduction and reproductive-resting stage. In Maine populations, daylengths equal to 14h induce >90% of the females to reduce development rate and accumulate lipid before maturation and mating. The resulting females, however, do not develop ova regardless of food level. A similar reproductive-resting stage is triggered at daylengths <14h in animals collected from Maryland. Transition from reproductive-resting stage to active ova production may be triggered in both populations by increased photoperiod and/or dramatically increased temperature. The daylength-triggered switch to reproductive-resting is under tight genetic control. Daylength likely serves as a critical cue for all populations in differentiating between onset of harsh (winter) and favourable (spring) environmental conditions. At these times water temperatures are similar, but daylengths are different. Population differences in the daylength necessary to trigger the reproductive-resting strategy likely reflect latitudinal variation in the period over which environmental conditions are conducive to population growth. -from Authors