334 M. Butler etal. 



TABLE 7-7 Computation of the Area Mixed by Normal Foraging of Lepi- 

 durus arcticus in Each Square Meter of Pond A, 1973 



day " ^ . Other experiments indicate that the depth of plowing was about 0.5 

 cm. Thus, the tadpole shrimp are extremely important in aerating and 

 mixing the sediment. 



Control of Density 



Lepidurus is an "r" selected species which maximizes its reproductive 

 output by reaching reproductive age quickly (within 30 days). In addition, 

 the reproductive output is maximized by having all individuals in the 

 population reproductive and self-fertilizing (hermaphrodites). Also, the 

 population produces its eggs during mid-summer while food is readily 

 available. However, there is a lessened dependence upon food availability 

 during late summer because the eggs have been previously formed (unlike 

 Daphnias parthenogenic broods) and stored under the carapace. 



There is good survival of eggs over the winter but the exact number 

 produced is unknown. If the maximal density of the first naupliar stage 

 was 30.6 m "^ and the population was in a steady state from year to year, 

 then an adult survival of 1.5 animals m " would mean that each animal 

 had to deposit 20 eggs if they all were to hatch. We observed that by the 

 end of August the average animal carried 55 eggs. If the animals deposited 

 eggs from mid-August through to the end of September at 2 to 4 eggs 

 once every 3 to 4 days, then egg deposition would have been between 45 

 and 67 eggs m ^ Thus, were the population in steady state, then half the 

 egg production could have been lost,assuming all surviving eggs hatched. 

 Unfortunately, our observations always end at the end of August, so we 

 can not say how long egg production continued into September. 



The greatest mortality we observed occurred between the time the 

 planktonic nauplii disappeared (5 July) and the young adults reappeared 

 on the surface of the sediments (3 August). In the laboratory all but two 

 animals raised for several molts died on the fifth molt for an unknown 

 reason. However, death in the laboratory incubators may not be analogous 

 to what occurred in the pond. It should be noted that by the time of the 



