Macrobenthos 323 



of larval mortality preclude a very close coupling between primary 

 production and secondary production of midge larvae. 



TADPOLE SHRIMP* 



Introduction and Life History 



The tadpole shrimp, Lepidurus arcticus (Order Notostraca, 

 Crustacea), only inhabits bodies of water with no fish. In temperate 

 regions, it is limited to ephemeral ponds; in the Arctic it inhabits ponds as 

 well as any lakes shallow enough to freeze to the bottom. 



In the Barrow ponds, studied by Kallendorf (1974), the tadpole 

 shrimp overwinter in the egg stage (33±0.5 ng C) and grow to maturity (10 

 to 12 mg C) in 60 days. Like the fairyshrimp, there is but one generation 

 per year. Eggs hatched in the ponds sometime in the third week in June 

 1972, 17 days after the onset of runoff. The smallest naupliar instar 

 contained 1.96±10.2 Mg C. At this time, there were 2.6 animals m"^ in 

 Pond C and 14.6 in Pond A. By 2 July, more eggs had hatched and Pond A 

 contained 31 animals m~^ These early stages, the first four instars, were 

 planktonic but about the same time as the molt to the fifth instar (about 2 

 July) the animals moved into the upper layers of the sediment and became 

 extremely difficult to sample (Table 7-4). When they reappeared at the 

 surface of the sediments on 3 August, they had grown from 2.6 mm to 17 

 mm in length (Figure 7-11) and their density had dropped to 1.4 to 1.6 

 m~'. Their density remained about the same during August and the 

 animals reached 25.6 mm in length. Length was related to dry weight by a 

 log-log regression where log dry weight (mg) = 2.71 (log length in 

 mm) —2.593. For this regression, r = 0.91, n=47, and the regression was 

 highly significant. 



In the laboratory, growth was nearly continuous, with a molt every 48 

 to 60 hours at a constant temperature. Over 60 days, the animals will 

 undergo 24 to 30 molts. 



Egg production began in mid-August when organisms longer than 

 17.3 mm deposited eggs in their ovisacs. At this stage, animals are 

 hermaphrodites and possess self-fertilizing ovotestes (Arnold 1967, 

 Longhurst 1955). Adult animals carried 29 to 78 developing eggs beneath 

 the carapace on the dorsal side (this is 13 to 35% of the total body weight). 

 One to three eggs at a time are carried externally in each of the paired 

 ovisacs on the 11th postcephalic appendage. These eggs (final diameter 

 0.71 mm) harden and turn orange in the 3 days they are held before 

 release. The modal number of eggs in each ovisac was one; thus each 

 animal could deposit about 20 eggs in the 30 days after sexual maturity. 



* M.C. MiUer. 



