332 



M. Butler etal. 



18 



« 



a. 

 o 

 o 



4> 



E 



3 

 Z 



15- 



12 - 



1 — I — I — I — I — I — I — I — I — 



■ Daphnia middendorffiana 

 □ Heterocope septentrionalis 

 Diaptomus bacillifer 



• (■•' Sediments) 



o (- Sediments) 

 Flatworm 



T— T 

 / 



/ 



10 15 



Number of Prey Available 



FIGURE 7-16. Number of 

 prey captured by Lepi- 

 durus arcticus adults at 

 differ en t densities of 

 available prey. 



ml day ~ ' (Table 6-8). At this rate of clearing, this Lepidurus nauplius 

 would ingest 0.043 Mg algal C day"^ and 0.089 ng bacterial C day"' at 

 typical concentrations of phytoplanktonic algae (Figure 5-2) and bacteria 

 (Hobbieand Rublee 1975). This total of 0.133 MgCday"' is only 0.2% of 

 the body weight but these animals respire 11% of their weight each day 

 (Table 7-5). From this indirect evidence, we conclude that the nauplii are 

 either predators or can use detritus. 



Exactly the same results with pre-adult forms, 12.8 mm long, led to 

 the conclusion that these too were possibly predators. In one experiment 

 an animal ingested 0.17 mg C hr'' of benthic detritus labeled with ^^P. 

 This equalled 0.1 % of the body weight day ' and so indicates that they are 

 predators. 



Adult tadpole shrimp preyed on zooplankters but their preferred prey 

 was flatworms. The observations were made in square containers, either 

 38.5 or 63.6 cm'^ in area. Each such experiment used one Lepidurus, one of 

 three prey densities, and a 48-hour incubation at 10°C. Saturation of the 

 feeding rate was seen only when Daphnia were added at a concentration of 

 40 liter '. Usually the feeding rate increased as the number of prey 

 increased (Figure 7-16). 



In another experiment, Daphnia labeled with ^^P were fed upon by 

 Lepidurus in a 500-ml vessel. Three different food levels were used (5, 10, 

 20 Daphnia) and the experiments were run for 48 hours. There was a 

 positive correlation between the Daphnia missing from the containers and 

 the cpm in the Lepidurus (r = 0.754, n = 6, NS). Assimilation was low, with 

 only 12.9% of the cpm in the missing Daphnia found in the Lepidurus. 



We conclude that Lepidurus at all life stages is a rather non-selective 

 predator that eats everything that moves. Its prey will vary from pond to 



