Abstract

The objective of this study was to determine the relative importance of photoperiod and temperature upon the induction of diapausing egg production in a single species of copepod found in habitats that differ in permanence. Field studies of four populations of Diaptomus leptopus carried out from 1995 to 1997 reveal quite different seasonal patterns in the production of subitaneous and diapausing eggs; these distinct patterns are similar between years for three of the four ponds studied. To determine the possible roles of temperature and photoperiod on the induction of diapausing eggs, I performed a common-garden experiment in which mating pairs from each of the four populations were exposed to different temperature and photoperiod regimes: long-day (16:8h L/D) 25$\sp\circ$C, long-day 10$\sp\circ$C, short-day (8:16 L/D) 25$\sp\circ$C and short-day 10$\sp\circ$C to determine if there are differences in the onset of diapausing egg production between and within the populations in each treatment. The populations of the occasionally temporary environments Hill and Stoneycroft exhibited a decrease in the onset of diapause under cold conditions and the overall timing of onset of diapause egg production in the experimental populations differed from that of the field populations giving indirect evidence that each population are responding to the available cues as predictors of environmental change in different ways.