Found across North America and Eurasia, Daphnia species mostly reproduce by parthenogenesis. In the spring and summer, a female D. magna will let her all-female offspring partially mature in her abdominal brood pouch. After several molts, these females will produce their own offspring by parthenogenesis. The onset of winter, drought, or other environmental stress will trigger some of D. magna's developing parthenogenetic offspring to become males, who then mate with the parthenogenetic females. This method of sexual reproduction produces eggs with an extra shell layer that lets them survive the stressful period.