The effects of photoperiod and temperature on the induction and termination of reproductive resting stage in the freshwater amphipod Hyalella azteca (Saussure)

Three experiments, one on the induction of reproductive resting stage and two on its termination, were performed to determine the effects of various combinations of temperature and photoperiod on the induction and termination of reproduction in Hyallella azteca. These showed that only photoperiod determined whether reproduction was continued or discontinued but that temperature influenced the rate of all changes. The 12 L – 12 D photoperiod terminated reproduction for at least 4 months at temperatures between 12 and 25 °C in animals previously reproducing at a 16 L – 8 D photoperiod. The 12 L – 12 D photoperiod also induced reproduction at temperatures between 16 and 26 °C in animals previously held in a reproductive resting stage in dim light. In contrast, the 16 L – 8 D photoperiod induced and maintained reproduction consistently, and the 8 L – 16 D photoperiod halted reproduction and maintained a reproductive resting stage consistently. The induction of reproduction occurred faster at higher temperatures.It is believed that although photoperiod is the main cue in the induction and termination of reproduction, active reproduction takes place when environmental temperatures are 20 to 26 °C, since optimum reproduction and growth rates occur in this range. The adaptive advantage and the biogeographic variability of the photoperiodic response are discussed.