This report investigates female sizes, egg sizes and egg hatching rates in relation to temperature for the near-shore calanoid copepod Acartia tonsa cultured at 6, 9, 14 and 24°C for several generations to achieve acclimatization. Inverse size relationships of eggs and females were revealed with increasing temperature. Eggs produced at 6°C were 85 ± 4 µm in diameter, but decreased to 80 ± 3 µm at 24°C. Female cephalothorax length was 840 ± 52 and 692 ± 39 µm at 9 and 24°C, respectively. Parallel hatching experiments were performed between non-acclimatized and acclimatized cultures across a range of temperatures reflecting natural conditions in Danish waters. A greater fraction of eggs enter quiescence as temperature declines. Eggs were able to hatch at temperatures as low as 1.5°C. Final egg hatching success increased with temperature. Acclimatization of the copepods resulted in a lower maximum hatching rate, but a higher final hatching success at 9 and 6°C compared with egg hatching from non-acclimatized copepods. Eggs develop significantly better at low temperatures when compared with females in culture. This adaptation suggests that the observed annual population fluctuations in, for example, Danish waters can be explained as follows: the population of copepods in the pelagic virtually die out during the autumn–winter months; the population the following year emerges from resting eggs from the sediment and is produced by the pelagic copepods the previous year(s).