Experiments on feeding and growth of the amphipod Gammarus pulex (L.) related to its distribution in the River Duddon

Summary Individuals of Gammarus pulex were kept at 15°C for periods of 23–70 days on diets comprised of elm or oak leaves, Tricladium and Clavariopsis (Hyphomycete fungi), Molinia (grass), Zygogonium (alga), Nardia (liverwort). Both the mean interval between moults (m i ), and daily increments in body wet weight, were related to diet. The lowest value for m i was 14·6 days on a diet of naturally decaying elm and oak leaves. The largest weight gains also occurred on this diet; the average daily gain in weight (D w ) = 130·8 μg/day, the mean specific gain in weight (G w ) = 1·69 μg %/day. On green Molinia and fungi diets D w ranged from nil to 60·7 μg/day, and m i = 17·4–18·4 days was significantly (P<0·01) longer than m i on leaf diets. Survival and growth were poor on brown Molinia. Nardia did not support growth or survival. A mixed diet of Tricladium, Zygogonium, Nardia and decaying grasses was not sufficient to promote an increase in body weight, and m i = 21·8 days. This diet represents the commonly available food materials in Mosedale Beck, an acid stream in the upper Duddon catchment. Survival and growth of G. pulex on fungi and leaf diets in media containing low concentrations of potassium ions are also described and discussed briefly in relation to the distribution of G. pulex in the upper catchment of the R. Duddon.