A REPRODUCTIVE-RESTING STAGE IN AN HARPACTICOID COPEPOD, AND THE SIGNIFICANCE OF GENETICALLY BASED DIFFERENCES AMONG POPULATIONS

Dormancy is an important life-history strategy which allows copepods t o increase their fitness by delaying growth and reproduction until har sh environmental conditions have ameliorated. For marine species, the primary strategies identified to date include the production of dorman t eggs by shallow-water species, and copepodite overwintering in deep- water species. Herein, we describe a third strategy in which fertilize d adult females enter a ''reproductive-resting'' stage during the late fall that allows them to overwinter and provide a first source of spr ing naupliar recruitment. This strategy has been observed in the estua rine copepod Coullana canadensis, but may also occur in other species. Laboratory studies indicate that daylength and temperature are the en vironmental cues that induce the developing female copepodite to switc h between active reproduction and reproductive-resting stage. In Maine populations, daylengths equal to 14 h induce > 90% of the females to reduce development rate and accumulate lipid before maturation and mat ing. The resulting females, however, do not develop ova regardless of food level. A similar reproductive-resting stage is triggered at dayle ngths < 14 h in animals collected from Maryland. Transition from repro ductive-resting stage to active ova production may be triggered in bot h populations by increased photoperiod and/or dramatically increased t emperature. Cross breeding experiments indicate that the daylength tri ggered switch to reproductive-resting is under tight genetic control. Daylength likely serves as a critical cue for all populations in diffe rentiating between the onset of harsh (i.e., winter) and favorable (i. e., spring) environmental conditions. At these times water temperature s 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 environm ental conditions are conducive to population growth.