Reproductive biology of the arctic collembolan Hypogastrura tullbergi

Terrestrial organisms of the Arctic are faced with strong climatic fluctuations. Predictable seasonality with cold/long winters and short/cool summers are combined with unpredictable between and within year variation. This indicates that various selection pressures act on the reproductive strategies of the populations. The arctic collembolan Hypogastrura tullbergi reproduces in a short period following snow melt. Hatching occurs in late summer, the animals grow to adult size within their second summer and reproduce for the first time in the beginning of their third summer. We performed several experiments to determine the reproductive investment and proximate mechanism that regulate timing and duration of reproduction. We found that H. tullbergi entered a reproductive diapause when reared at constant temperature, a diapause that was terminated by a cold exposure (winter). Surprisingly, cold exposure of small juveniles may also prevent development of a reproductive diapause in adults. Thus, the life-cycle normally spanning 2 yr can potentially be reduced to one year if the overwintering juveniles reach maturity before the end of the reproductive period in the field. After termination of the diapause, the animals reproduced up to 3 times during a period of 6 weeks at 15°C. This reproductive period was considerably longer (measured in degree days) than the one observed in the field. Our results suggested that temperature quiescence, i.e. the inability to reproduce under a certain temperature threshold, may adjust the termination of the reproductive period with current temperature before a new diapause occurs in late summer. The cost of reproduction was low and suggests that it can be adaptive to spread reproduction over more than one year. The results are discussed in relation to the arctic climate and strategies favoured by unpredictable and predictable (seasonal) variations in the environment. The present study forms part of a larger investigation on population dynamics and life history strategies of H. tullbergi from the Arctic.