Ontogenetic changes in digestive enzyme activities of a common water flea

Although the nutritional demands of animals are known to differ between immature and mature stages and between neonates from different clutches in various organisms, little evidence has been accumulated to examine this possibility in crustacean zooplankters. To this end, we examined the five digestive enzyme activities related to assimilations of carbohydrates, lipids, amino acids and phosphorus in the neonates, immature and mature individuals of panarctic Daphnia pulex. We found that all the enzyme activities were higher at lower food levels, indicating that the activities increased with decreasing supplies of the nutritional substances relative to the demands. As expected, these enzyme activities changed ontogenetically. Beta-glucosidase activities increased with age, especially at a high food level, while the opposite pattern was found for lipase activities. A decreasing trend with age was also found in the activities of arginine and alanine amino-peptidase and alkaline phosphatase at a low food level. These results suggest that juvenile individuals require more lipids, amino acids and phosphorus relative to the requirements of adult individuals, while the opposite pattern was true for carbohydrates. We also measured the enzyme activities of the neonates born from the 1^st and 2^nd clutches of the maternal individuals. The results showed that most digestive enzyme activities differed significantly between the neonates from the first and second clutches depending on the food levels. The present study confirmed that the nutritional demands differ not only between mature and immature individuals in Daphnia pulex and suggests that the mature individuals invest nutrients and energy reserves differently to neonates of the different clutches depending on their nutritional conditions.