SUSCEPTIBILITY OF PLANKTONIC ROTIFERS TO A TOXIC STRAIN OF ANABAENA-FLOS-AQUAE

Reproduction in Asplanchna girodi, Brachiounus calyciflorus, Keratella cochlearis, and Synchaeta pectinata fed Cryptomonas at 19-degrees-C was inhibited by the presence of a strain of Anabaena flos-aquae (IC-1) producing the neurotoxic alkaloid anatoxin-a. The most susceptible species, B. calyciflorus, was suppressed at an Anabaena dry mass concentration of 0.5 mug ml-1; the others were suppressed at a concentration of 4 mug ml-1. Reproduction of all species at 19-degrees-C was inhibited by anatoxin-a; the most sensitive species (S. pectinata) was inhibited at a concentration of 0.2 mug ml-1, and the least sensitive species (B. calyciflorus), was inhibited at a concentration of 5 mug ml-1 but not one of 2 mug ml-1. In contrast, A. girodi and two clones of B. calyciflorus were not inhibited by filtrates of very dense Anabaena suspensions (1:1 and 1:3 dilutions of 1-d-old, 80 mug ml-1 suspensions); this showed that the Anabaena did not release extracellular toxin, and thus that it could only inhibit rotifers that ingested it. B. calyciflorus probably was the most susceptible to the cyanobacterium, even though it was the least sensitive to the soluble toxin, because it ingested the filaments most efficiently. Its relatively low sensitivity to the toxin may reflect an evolutionary response to its greater tendency to ingest toxic cyanobacteria. The effect of a toxic cyanobacterium on the structure of a freshwater zooplankton community should depend on the size and morphology of its colonies. Large colonies should be more readily ingested by daphniids than rotifers, and hence more likely to inhibit competitively dominant daphniids. Small, amorphous colonies or short, thin, nonmucilage-coated filaments should be ingested by, and thus inhibit, some rotifers as well as daphniids.