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Three Daphnia species of different body sizes were grown on a single food resource in steady—state conditions. They differed in food threshold concentrations necessary for body growth and reproduction. The largest species, D. pulicaria, had the lowest thresholds and the smallest, D. cucullata, had the highest thresholds, while D. hyalina was intermediate. This implies competitive superiority of the largest species and inferiority of the smallest one. The sequence, however, was reversed in the presence of blue—green algal filaments, when the thresholds became lowest for D. cucullata and highest for D. pulicaria. Thus the inhibitory effect of filaments is body—size dependent. High densities of filaments that mechanically interfere in food collection may cause shifts in competitive ability among congeneric cladocerans. This may be the reason why, in Europe, D. pulicaria is most successful in ultraoligotrophic lakes while D. cucullata is most successful in eutrophic lakes.

We observed a shift in maternal investment per offspring in clonal cultures of twoDaphnia species. Mothers grown at high food levels produced large clutches of smaller eggs but their offspring could not survive long under starvation conditions. Genetically identical mothers grown at low food levels produced small cultches of larger eggs, and their offspring, albeit low in numbers, were able to survive long periods of starvation. Our data show thatDaphnia mothers are capable of assessing food level and use this information in adjusting their fractional peroffspring allocation of reproductive resources.

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