Predation on Daphnia pulex by Lepidurus arcticus

The tadpole shrimp Lepidurus arcticus frequently occurs in Greenland ponds and shallow lakes with a soft bottom. Literature describes it as mainly a s cavenger, feeding on the sediment. Previous observations of its behaviour s uggest, however, that large specimens can catch Daphnia pulex when swimming in the water. Experiments were set up to test how efficiently Lepidurus hu nts Daphnia and if a functional response between predator and prey exists. It was also tested if temperature and Lepidurus size played a significant r ole for the predation rate. Lepidurus and Daphnia were sampled from ponds a t Zackenberg in northeast Greenland (74 degrees N, 21 degrees E) in August 1998 and were placed in small containers (0.5 l) at desired temperatures. L epidurus was capable of consuming 5-15 Daphnia per hour at ambient temperat ures (5-20 degreesC) and changes in the temperature had apparently no signi ficant effects on the predation rates. There was, however, a clear differen ce in feeding activity between size groups, the rates of larger Lepidurus ( > 12.5 mm) being two to three times greater than that of smaller specimens (8-10 mm). For both size classes, the predation rates rose with increasing prey concentrations and the saturation levels were far above natural prey d ensities. Small-sized Daphnia (1.6 mm) were removed at significant faster r ates (t-test, p <0.05) than larger ones (2.4 mm) by Lepidurus when offered in combination but not when offered separately. Although the recorded preda tion rates were biased due to the manipulated conditions (e.g. increased en counter rates), Lepidurus appears to be an active and efficient predator on planktonic prey, and its presence in arctic lakes and ponds may consequent ly have a significant impact on the structure of the planktonic food web. T he calculated energetic advantages of plankton as supplement to a benthic d iet seemed low (similar to5% of body weight per day).