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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 1062

John J. Gilbert; Differential effects of Anabaena affinis on Cladocerans and rotifers: Mechanisms and implications. Ecology 71(5):1727-1740, 1990

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Preliminary observations of plankton community structure in Star Lake, Vermont, over several years indicated that blooms of the filamentous cyanobacterium Anabaena affinis were associated with low densities of Daphnia pulex and high densities of rotifers. A series of laboratory experiments was conducted to analyze this pattern. A. affinis was isolated from Star Lake and cultured in a defined medium. Suspended filaments, averaging 322 um in length, variably affected the population growth rates (r(m)) of seven cladocerans (six species, one with two strains) and five rotifers on their algal (Cryptomonas) of ciliate food. High filament concentrations (1-2x10E4 cells/mL, ~155-310 filaments/mL) affected none of the rotifers, intermediate concentrations (5x10E3 cells/mL, ~78 filaments/mL) inhibited four of the seven cladocerans, and low concentrations (10E3 cells/mL, ~16 filaments/mL) inhibited the three most sensitive cladocerans. The inhibition of some moderately sensitive cladocerans increased greatly in the second generation of exposure. Cladoceran susceptibility was positively and significantly correlated with maximal adult body length. A competition experiment showed that the presence of A. affinis filaments prevented the suppression of a rotifer by a large cladoceran and led to dominance by the rotifer. Such differential sensitivities of cladocerans and rotifers to cyanobacterial filaments should increase the relative importance of rotifers in zooplankton communities. Experiments indicated that a sensitive strain of Daphnia pulex was inhibited by ingesting an A. affinis endotoxin. Starved Daphnia lived longer than Daphnia fed only A. affinis filaments, and extracts of A. affinis were just as inhibitory as filament suspensions at comparable concentrations. Sonically disrupted filaments were no less inhibitory than whole filaments, indicating that the inhibition could not be attributed to mechanical interference with feeding. Filtrates of 1-d-old filament suspensions were inactive, suggesting that an exotoxin was not being excreted. The pattern of different susceptibilities of the cladocerans and rotifers to A. affinis filaments is at least partially due to different sensitivities to the endotoxin, possibly reinforced by a similar pattern of different tendencies to ingest the filaments. The Star Lake strain of D. pulex was unaffected by the highest tested concentration of A. affinis filaments (5x10E3 cells/mL), indicating a co-evolved resistance to the toxin, possibly a counter to an antiherbivore defense.