A new life history pattern for Asellus aquaticus is described from Lake Østensjøvatn in Oslo. Overwintering specimens (generation A) breed in spring. The next generation B matures, breeds and dies in first summer, giving rise to the overwintering generation C, which breeds next spring and dies. The death of generation B in its first summer could be shown by a study of oostegit development in the females. The end of the breeding season in autumn is thus accomplished by a complete replacement of generations. This pattern can be a) light stimulated, or b) a pre-determined cycle, uninfluenced by environmental factors. Breeding starts in spring when water temperature reaches about 7–8°C. Estimated incubation time was 23–35 days, corresponding to about 300 degree-days above 4°C. No growth took place from November till March–April. Growth started when water temperature reached 3–5°C. Growth increments per degree-day above 3°C were the same for the three generations which were followed, in spite of their different life spans and different temperatures. The total number of degree-days above 3°C available to the generations seemed to regulate the maximum length of the specimens in the respective generations. Comparison with life histories of the species in other lakes suggests that the different patterns described reflect an evolutionary adaptation to major environmental factors.
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