Juvenile growth rate as a measure of fitness in Daphnia

1. An empirical relationship is established between the somatic juvenile growth rate (g) and the instantaneous rate of increase (r) in Daphnia magna in order to test if g can be used as a measure of fitness. Tested variables influencing g include food concentration, temperature, clonal differences and maternal effects. 2. The juvenile growth rate g was calculated for fixed time periods of 2 to 6 days (g(f)) or for the 'physiological' juvenile period to the deposition of the first clutch (g(j)), which varied with environmental conditions. Both g(f)and g(j) resulted in linear relationships between g and r when food concentration was the only variable. If the other variables were included then only g(j) resulted in a good fit. 3. Although the relationship may be slightly curvilinear, g(j) is a robust predictor of r. The ratio r: g(j) is not affected by juvenile size and size at maturation. Hence g(j), can be used as a relative measure of fitness in Daphnia. This can save considerable time and effort when determining reaction norms in life-table experiments with many genotypes.