FOOD SELECTION IN 3 LEAF-SHREDDING STREAM INVERTEBRATES

Leaf disks (Betula papyrifera) were conditioned for two weeks by six s pecies of aquatic hyphomycetes. Mass losses of the leaves were determi ned, and their concentrations of protein (extracted at pH 7, 10 and 12 .8), phenolics (Folin-Ciccalteu and BSA-precipitation), lipids, and er gosterol (as indicator of fungal biomass) were measured. Enzymatic act ivities of the culture filtrates against cellulose, xylan and pectin w ere estimated. Gammarus tigrinis, Pycnopsyche guttifer and Tipula calo ptera were given a choice of the six leaf/fungus combinations. G. tigr inus and P. guttifer consistently preferred some combinations over oth ers; T. caloptera appeared to feed randomly. There were no significant correlations between consumption and any of the measured characterist ics of leaf disks. With G. tigrinus and P, guttifer, the sequence of p reference could be reproduced by extracting mycelia with non-polar sol vents and applying the extracts to unconditioned leaf disks. Consumpti on of extract-coated disks was lower than consumption of conditioned d isks. Numbers of endosymbiotic gut bacteria increased from G. tigrinus to P. guttifer to T. caloptera; diet diversity showed the opposite tr end.