A bacteriological investigation to elucidate the feeding biology of Nais variabilis (Oligochaeta: Naididae)

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Abstract
SUMMARY. The estimated average number of bacteria in the gut of laboratory‐cultured Nais variabilis was 100.4 × 104. This compares with values of 6.7 × 104 and 32.8 × 104, respectively, for N. variabilis and N. barbata obtained from a river.Comparison of the viability of heterotrophic bacteria in the food and gut of N. variabilis indicated a decline in viability of up to 99.6% during the passage of bacteria through the gut. The estimated gut retention time was 39 ± 8 min at 18°C and therefore loss of viability is unlikely to be due to long retention in the gut. Loss of viability is, however, a prerequisite of bacterial digestion by the worm and provides circumstantial evidence that ingested bacteria are used as food.No conclusive evidence was obtained indicating (a) preferential ingestion of certain kinds of bacteria, (b) that the gut environment was selectively hostile to the particular kinds of bacteria studied or (c) that worms had a specialized gut micro‐flora.