Growth, Size and Reproduction in Daphnia (Crustacea: Cladocera)*

Factors influencing the growth of Daphnia are summarized; it is shown that lack of oxygen inhibits the growth of Daphnia magna. Synchronization of egg laying by females in samples from natural populations has been found on several occasions. The pre‐adult growth of Daphnia has been studied by rearing 207 specimens belonging to eight species in standard conditions. The greatest growth increment does not always occur at the end of the adolescent instar; it may occur at the end of the pre‐adolescent instar or more rarely even earlier. Initial size influences the instar in which maturity is reached; as the initial size increases the animals tend to become mature in earlier instars. In single samples from natural populations a direct correlation has been found between the size of the females and the number of eggs carried in their brood pouches. This applies to several species of Daphnia and certainother Cladocera (excluding the subfamily Chydorinae). Synchronous fluctuations in the size of parthenogenetic females and the number of eggs in their brood pouches have been found in a population of D. magna. Factors influencing the size of eggs and young are analysed. Within a species the following factors are operative: egg number in relation to the size of the mother, the age of the mother, inherited racial or clonal characteristics, available food and temperature. The size of the ephippial eggs of D. magna varies with the size of the female producing them. The ephippial eggs may be bigger or smaller than the parthenogenetic eggs. The eggs of thirty‐three species of Cladocera have been measured. The larger species produce eggs which are slightly larger in relation to maternal size than those of the smaller species. A mature female of D. magna can assimilate enough material during each instar to produce eggs with a dry weight at least equal to that of her body after the eggs have been laid. The dry weight of the parthenogenetic egg of D. magna diminishes by 16 to 25 per cent during embryonic development.