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Abstract

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