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 to increase their fitness by delaying growth and reproduction until harsh environmental conditions have ameliorated. For marine species, the primary strategies identified to date include the production of dormant eggs by shallow-water species, and copepodite overwintering in deep-water species. Herein, we describe a third strategy in which fertilized adult females enter a "reproductiveresting" stage: during the late fall that allows them to overwinter and provide a first source of spring naupliar recruitment. This strategy has been observed in the estuarine copepod Coullana canadensis, but may also occur in other species. Laboratory studies indicate that 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 14 h induce >90% of the females to reduce development rate and accumulaKe lipid before maturation and mating. The resulting females, however, do not develop ova re:gardless of food level. A similar reproductive-resting stage is triggered at daylengths < 14 h 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. Cross breeding experiments indicate that 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 the onset of harsh (i.e., winter) and favorable (i.e., 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. Resting and diapause stages are common life-history attributes among planktonic copepods, particularly the Calanoida and Cyclopoida (e.g., Labidocera aestiva, Acartia hudsonica and Mesocyclops edax; Marcus, 1979; Sullivan and McManus, 1986; Wyngaard, 1988), and likely are adaptive responses to avoid harsh environmental conditions such as desiccation, low food availability, and high predation pressure (Hairston and Olds, 1987). A seasonal pattern in adult encystment has been reported in one species of marine and several species of freshwater Harpacticoida, but in general, little is known about diapause or resting strategies in this order of copepods and benthic species in general (Coull and Grant, 1981; Williams-Howze and Coull, 1992; reviewed in Williams-Howze, 1992). Cou/lana canadensis [i.e., Scotto/ana canadensis (Willey); Por, 1984] is a brackish-water, harpacticoid copepod which occurs over a wide geographic range on the east coast of North America (Willey, 1923; Coull, 1972; Lonsdale and Levinton, 1985), and populations are interfertile from at least Maine to South Carolina (Lonsdale et al., 1988). Cou/lana also was collected on both the east and west coasts of Florida, but they are reproductively isolated from all other northern populations (Lonsdale et al., 1988). Coullana is usually seasonally restricted throughout its entire range, with highest naupliar densities occurring in early spring to early summer months, although egg production appears to be continuous throughout much of the summer (e.g., Maryland and Maine; Lonsdale, 1981b, this study). In Chesapeake Bay, for example, spring naupliar densities of C. cana-