Size and reproduction in Daphnia magna (Crustacea: Cladocera)

S ummary. In natural populations parthenogenetic egg production in Daphnia magna Straus is directly related to size. Previous workers have shown that the number of parthenogenetic eggs produced during the life of a female increases in the early broods and decreases in later broods. Egg production and size are positively correlated in the first adult instar, both under constant conditions and with varying food. The size of the young liberated varies with maternal age, the third brood consisting of young of a larger average size than earlier or later broods. The initial size of the young liberated influences the instar in which maturity is reached. The probable effects of maternal age on growth, maturation and longevity of the young are discussed.