Mechanisms of the inhibitory effect of the cyanobacterium Microcystis aeruginosa on Daphnia galeata's ingestion rate

The possible effects of five Microcystis aeruginosa strains (two unicellula r and three colony forming) on the filtering and feeding behavior of Daphni a galeata were investigated to clarify the mechanisms of the inhibitory eff ect on the ingestion rate of the animals. Of the tested cyanobacterial stra ins, three inhibited Daphnia's food ingestion. This was primarily caused by affecting the frequency and food transport efficiency of the maxillary con tractions. As a result, Daphnia's swallowing rate, and in one case the amou nt of food contained in the swallowed boluses, were lower than in suspensio ns of well-ingestible food sources. In contrast, there is no indication tha t Microcystis can influence thoracic appendage beat or rejection rate. It i s likely that colony-forming and unicellular strains can affect the ingesti on process due to different factors. The mucilage of the colony-forming Mic rocystis strains HUB-W75 and HUB-W334 possibly produced a mechanical hindra nce of the maxillules, which resulted in an abnormal movement and consequen tly in the inhibitory effects described above. This was related to the suga r composition of the mucilage polysaccharides, but not to the cellular micr ocystin content or occurrence of accompanying bacteria. The mucilage-lackin g strain HUB 5-2-4, however, must contain another factor which interfered w ith the maxillules' movement in an unknown way.