A choice chamber experiment on the selection of algae as food and substrata by Nais elinguis (Oligochaeta: Naididae)

SUMMARY. A choice chamber experiment was performed in the laboratory to quantify the selection of benthic algae as food and substrata by Nais elinguis. Most worms actively chose filamentous algae and/or diatoms as substrata whereas unicellular or colonial chiorophycean algae were discriminated against. Diatoms were selected in preference to latex beads but the worms could not discriminate between living filaments and nylon monofilaments. A thigmotactic response to filaments and a chemotactic response to diatoms may be implicated. The worms ingested unicellular algae at a greater rate than colonial and filamentous algae. The rates at which the algae were ingested were negatively correlated with the lengths of the algae. A filament 200 μm long was the largest item ingested, representing about 63% of the length of the pharynx of the worm. Ivlev's electlvity indices were positive when the rates at which the algae were ingested exceeded about 20 individuals h‐1. The index was negatively correlated with the lengths of the algae, but the time periods spent by the worms in association with the algae were unrelated to the electivity index. The data confirmed previous observations on the selection of algae as food and substrata by Nais elinguis in a natural habitat.