Growth, development and body size of field and laboratory populations of Daphnia ambigua [temperature, nutrition, dry weight, eggs, embryos, juvenile instar, adult instar, reproduction, longevity]

The mean carapace lengths of first instar and primiparous females were 356 +/- 27 mu-m (SD, n = 1225) and 721 +/- 62 mu-m (n = 346), respectively. Females maturing in an early instar were smaller than those maturing later. Duration of juvenile and adults instars decreased at higher temperatures. Juveniles molted more frequently and had a shorter instar duration than adults. The rates of embryonic and postembryonic development increased and the duration of development decreased at higher temperatures. The duration of development in the laboratory was shorter than that determined at comparable temperatures in the field. The duration of embryonic development was a function of temperature, whereas the duration of postembryonic development was a function of both temperature and nutrition. The mean longevity of laboratory animals at 10 deg C was 102 d (17 instars) for females and 78 d (13 instars) for males. The mean longevity of field D. ambigua tended to decrease with increase of temperature. At comparable temperatures, the much shorter life-span of field Daphnia compared to laboratory animals was attributed to lower nutrition in the field. Growth rate was comparatively rapid in young animals and progressively decreased as animals aged. Within the range of temperatures (5-30 deg C) studied, growth rates generally increased with increasing temperature. The growth rate of field D. ambigua was affected by both temperature and food. The lower growth rates of field animals compared to laboratory animals probably resulted from lower food availability in the field. For laboratory animals, the mean number of young produced per brood tended to increase progressively during the early stage of adult life and decreased toward the end of the life-span. High temperature increased the frequency of reproduction, high food concentration increased the number of young per brood and the total number of young produced during the life-span. Female Daphnia grown in the field lived a significantly shorter time, had smaller clutch sizes, and produced fewer broods and fewer total young than laboratory females. Although reproduction is affected by temperature, light, food, oxygen, and other factors, food probably was the main factor which regulated the number of young produced in the field.