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要旨

The effect of three different Scenedesmus food concentrations (0.04, 0.2 and 1 mg C l-1) on maternal investment was studied in two cladoceran species of similar size, Daphnia pulicaria and D. hyalina. It was observed that as food concentration decreased (between 1 and 0.2 mg C l-1), there was an increase in size, protein content, lipid content, carbon and mass of the egg, while. at the same time, the clutch size of the females became smaller. Such an increase in 'per offspring investment' was reflected in an increase in body length, body carbon and body mass of neonates as the food available for females decreased. However, in D. pulicaria this tendency was not maintained down at 0.04 mg C l-1 in which there was a decrease of the egg characteristics mentioned above. Although, there are not available all the egg and neonate parameters of D. hyalina at 0.04 mg C l-1, the body length of the neonates was larger than at 0.2 and 1 mg C l-1. These results show that, as food diminishes, these two cladoceran species are able to respond by decreasing clutch size, but increasing the size of egg, thereby increasing the probability of neonate survival. This tendency is probably maintained until the food concentration is too low and the females have to reduce the energy allocated for reproduction.