(A) The embryonic development in the egg from zygote to hatching (over 11–12 days) is depicted in the white area of the life cycle and consists of six main stages: cleavage, blastula, gastrula, trochophore, veliger and metamorphosis (Source data from Ivashkin et al., 2015). The grey area of the life cycle depicts growth and development after hatching. Although L. stagnalis is a simultaneous hermaphrodite, the male reproductive organs are functional before the female ones (Koene and Ter Maat, 2004): specimens reach male and female maturation on average at an age of 30 and 60 days, respectively (based on Koene, 2010). (B) In the wild, generations only partly overlap, as depicted by the two dotted growth curves (top; based on Nakadera et al., 2015). Individuals that are born during spring and summer, overwinter as adults (light grey dotted line) after which they overlap with the adult generation of the next year (black dotted line). The external conditions such as light and temperature (middle), which strongly influence when egg laying occurs (bottom), are depicted for the situation in a typical temperate zone.