A MATURATION SIZE THRESHOLD AND PHENOTYPIC PLASTICITY OF AGE AND SIZE AT MATURITY IN DAPHNIA-MAGNA

The combined effects of body-length at birth and juvenile growth on clutch size, age and body-length at maturity were studied in two clones of Daphnia magna. Juvenile growth in 13 food treatments was estimated by the mean juvenile increment, calculated as the ratio of post-moult-length to pre-moult-length. averaged over all juvenile instars. Females which were born smaller or grew slower had on average more juvenile instars. less eggs in their first clutch and were smaller and older at maturity than females which were born larger or grew faster. As females reached maturity in either their 5th, 6th or 7th instar, clutch size. age and body-length at maturity varied discontinuously across growth conditions. These discontinuities depended on both the juvenile increment and the body-length at birth. In contrast, within groups of females with equal numbers of instars at maturity, clutch size, age and body-length at maturity varied continuously with length at birth and juvenile increment. A descriptive model based on a size threshold for maturation is used to analyse the underlying mechanism generating the phenotypic plasticity of age and body-length at maturity.