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Abstract

Daphnia can suppress ciliates and rotifers through predation and interference competition, but it is not known whether this produces any direct benefit to Daphnia. We conducted survivorship and cohort lifetable experiments to determine whether Daphnia can utilize ciliates and rotifers as food. Three species of oligotrich ciliates (Halteria grandinella, Strobilidium gyrans and Strobilidium velox) and one rotifer (Keratella cochlearis) were used. Lifetable experiments were conducted with a basal level of algae (Cryptomonas sp.), plus either ciliates or rotifers, while survivorship experiments had only the rotifers or ciliates. Densities of 30 H.grandinella ml-1, 50 S.gyrans ml-1 and 15 S. velox ml-1 enhanced Daphnia pulex's population growth rate 35-50% over controls with only algae. Ten S.gyrans ml-1 did not produce a significant change in Daphnia's growth rate. Densities of 100 and 300 K.cochlearis l-1 increased Daphnia population growth rates by 11 and 10%, respectively. Both 10 and 50 S.gyrans ml-1 enhanced Daphnia's survivorship compared to starved controls, but neither 100 nor 300 K.cochlearis l-1 enhanced its survivorship. The amount of enhancement of Daphnia growth rates by rotifers and ciliates is roughly proportional to the death rates imposed by Daphnia. The death rate imposed by Daphnia on rotifers is a function of both algal density and Daphnia size. Per unit biomass, neither ciliates nor Keratella appear to be as nutritious for Daphnia as is Cryptomonas.