1. Leaf litter breakdown by shredders in the field is affected by leaf toughness, nutritional value and the presence of secondary compounds such as polyphenols. However, experiments involving the use of single fungal strains have not supported the assumption that leaf parameters determine food selection by shredders perhaps because of a failure to test for high consumption prior to isolation of fungal strains, overrepresentation of hyphomycetes or the potential effects of accompanying bacteria. In this study, we used bacteria-free, actively growing fungi and oomycetes isolated from conditioned leaf litter for which a shredder had already shown high consumption rates. 2. Black alder (Alnus glutinosa) leaf litter was exposed to the littoral zone of Lake Constance in autumn, and subsamples were analysed for leaf parameters and consumption by Gammarus roeselii under standard conditions at regular intervals. On dates with a high consumption rate of the exposed leaves, 14 single strains of fungi and oomycetes were isolated, freed of bacteria and grown on autoclaved leaves. 3. Six of eight measured leaf parameters of exposed leaves were significantly correlated with Gammarus consumption rates, with high colinearity among leaf parameters hampering the identification of causal relations between leaf parameters and feeding activity. 4. When single strains of fungi and oomycetes were grown on autoclaved leaf litter, toughness of colonised leaves was always lower than in the control and the content of protein, N and P were increased. There were pronounced strain-specific effects on leaf parameters. Consumption rates also differed significantly, with nine of fourteen isolates consumed at higher rates than controls and none proving to be a deterrent. Protein and polyphenol content were significantly correlated with consumption rates. Oomycetecolonised leaves were consumed at similar rates but were of lower food quality than fungicolonised leaves. 5. We argue that direct strain-specific attractant or repellent effects of fungi and oomycetes on consumption by G. roeselii are not important. However, we found indirect strainspecific role operating via effects on leaf parameters.
The interaction of microbial colonization with leaf litter breakdown by the shredder Gammarus roeselii was studied in the littoral of large Lake Constance. In a first step we studied selective feeding of gammarids on leaf litter from three different treatments. Littoral exposed, tap water exposed, and autoc1aved leaves were offered to G. roeselii in standard food-choice assays under laboratory conditions. We found highly selective feeding on littoral exposed leaf litter indicating that microbial conditioning is 'affecting the shredders feeding behaviour. Neither C, N, and P nor protein nor ergosterol content were positively correlated with the relative consumption rates of littoral exposed leaves. For a more detailed study of the microbial conditioning of the leaves oomycetes and fungi were isolated from the littoral exposed leaves. Based on the sequence of internal transcribed spacer (ITS) regions of rDNA, we identified single strains of Fusarium sporotrichoides, Microdochium sp., Ascomycete sp., and Cylindrocladiella parva; two strains of Cylindrocarpon sp.; and three strains of Pythium sp. Subsequently, food choice assays were conducted using the isolated strains by offering autoclaved leaves and leaves colonized with a single strain. Three strains (c. parva, Cylindrocarpon sp. 94-2057 and Ascomycete sp.) were preferred by G. roeselii, while leaves with F. sporotrichoides were avoided. Leaves with each of the three oomycete isolates (Pythium sp.) were neither preferred nor rejected by G. roeselii. Our results suggest that the selective feeding behaviour of G. roeselii is affected by the relative abundance of specific fungal strains. Thus, whether microbial conditioning acts as a repellent or as an attractor for shredders depends on the microbial community on the leaves.
The strong invasive freshwater mysid Limnomysis benedeni, a detritivorous-herbivorous feeder, has a preference for small food particles, but also feeds on leaf litter. Here, we tested whether leaf litter consumption by L. benedeni depends on the tree species and leaf conditioning (two types of physical and biological leaf conditioning). At the physical leaf conditioning, L. benedeni was fed with shortly leached or extensively leached leaves of five tree species in laboratory food assays. The mysid consumed shortly leached leaves of Copper Beech, Lombardy Poplar, Common Oak, and especially White Willow, and did not feed on shortly leached Black Alder leaves. The consumption of extensively leached leaves by L. benedeni did not depend on the tree species. Overall, 74% of the variation of the leaf consumption by L. benedeni was explained by the significant interaction of the factors carbon content and polyphenol content of the leaves, caused the feeding strategy of L. benedeni. For the biological leaf conditioning, the mysids consumed to a high degree naturally conditioned leaves, followed by leaves colonized by one of three fungi, but oomycete-colonized leaf litter and autoclaved leaves were consumed at similar low levels. Our results indicate that L. benedeni feeds on different types of conditioned leaves to different extents, and therefore may affect leaf litter degradation in many invaded freshwaters.
We investigated the effect of methanol and methanol/methylene chloride extracts of the oomycete Pythium sp. JN 1-b and of the fungi Ascomycete sp. PVSo8, Fusarium sporotrichoides, and Cylindrocarpon sp. 94-2057 on the food preference of Gammarus roeselii. The preference for leaf discs coated with these extracts compared to uncoated leaf discs was tested in food-choice assays. Methanol extracts of all strains repelled G. roeselii, and the effect of the extract concentration on relative consumption was strain specific. The repellent effect of these extracts, especially of extracts of Cylindrocarpon sp., decreased when the fungi were grown on leaf extract medium as opposed to synthetic medium containing sucrose. None of the methanol/methylene chloride extracts affected the food preference of the gammarid. We conclude that biologically active compounds were extracted from fungi and an oomycete were soluble in methanol but not in methanol/methylene chloride. Only repellent activity was observed with the extracts, and relative ratios of repellents and attractants might determine the consumption of fungi by G. roeselii.
1. The freshwater leech, Erpobdella octoculata, is a generalist predator feeding on prey organisms such as Tubifex spp., Chironomus spp. and Asellus aquaticus. Using different experimental designs, we studied the use of foraging kairomones by the leeches for prey location. 2. Leeches were attracted to living as well as to freshly killed larvae of Chironomus sp., to Tubifex sp., and to A. aquaticus offered in Petri dishes covered with gauze. The leeches also reacted to an extract of macerated Tubifex sp. presented in agar. 3. Using ion exchange chromatography, the presence of histidine and glutamic acid was demonstrated in water contaminated by living larvae of Chironomus sp. and Tubifex sp. Agar blocks containing a synthetic mixture of these compounds at concentrations above 5 mg mL )1 were attractive to the leeches. 4. Choice tests showed that leeches preferred chemical signals from Tubifex sp. over larvae of Chironomus sp. No difference was found between chemicals from Tubifex sp. and A. aquaticus, and A. aquaticus and larvae of Chironomus sp. 5. The results demonstrate that E. octoculata uses specific foraging kairomones in searching for prey and indicate that amino acids serve as foraging kairomones.
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