Zooplankton 259 



The total production of Cyclops eggs was estimated from the duration 

 of embryonic development, the numbers of gravid females, and the 

 number of eggs per female at each sampling (Table 6-2). An estimate of 

 the maximum development time came from measurements in v^-hich gravid 

 females were placed in glass tubes 2.5 x 20 cm and held in the pond. 

 Unfortunately, the hatching success of broods of eggs other than the first 

 brood was low and an accurate development time could not be measured. 

 The measured development time of 1 week at approximately IO°C is 

 conservative and agrees with the literature. 



Despite the large number of eggs produced by Cyclops each year 

 (Table 6-2), only a few nauplii survive. For example, from the 126.6 eggs 

 liter"' produced in 1971, only 10 nauplii liter"' were found in the pond 

 during August. The loss could have been caused by sampling error, by 

 invertebrate predation, or by failure to hatch. In fact, hatching success was 

 poor in experiments. Gravid females (n = 20) with about 60 eggs in each of 

 the two egg sacs were collected from the pond and placed in glass tubes. 

 They produced a mean of approximately 40 nauplii from the first brood 

 and about 4 and 10 nauplii, respectively, from the two later broods. 



Egg production of the fairyshrimp was calculated (Table 6-2) but 

 because the embryos enter diapause, the hatching rates were not 

 experimentally measured. It is known that animals in the field almost 

 always have fewer eggs than their potential maximum. To measure this 

 maximum, Kangas (1972) confined both species in food-enriched cultures 

 at 5.5°C and found a mean brood size for Branchinecta of 22.5 and for 

 Polyartemiella of 14.3 eggs per female. In the same year (1969), the pond- 

 grown animals had brood sizes of 6.7 for Branchinecta and 9.3 for 

 Polyartemiella. In a pond adjacent to Pond C in 1969 and 1973, the brood 

 size was 9.0 and 14.9 for Branchinecta and 4.2 and 1 1 .8 for Polyartemiella. 

 In other words, it appears that the egg productivity varied from year to 

 year. Kangas also found that there was a 4- or 5-day interval from the start 

 of one brood to the beginning of the next. 



In the Barrow ponds, both species of Daphnia produce a single brood 

 of young before shifting to the production of resting eggs. This single 

 brood of developing embryos, visible through the translucent carapace of 

 the mother, may be as numerous as 20 in D. pulex and 12 to 14 in D. 

 middendorffiana. These are field measurements only and are not the 

 maximum brood sizes reported for these species. In contrast to these 

 broods of young, only two resting eggs are produced at a time. 



In Pond C, where the only Daphnia present is D. middendorffiana, 

 there was an increase both in abundance (Figure 6-5) and in the mean 

 brood size (Figure 6-6) from 1971 to 1973. At the same time the mean size 

 of the adults also increased. These data are summarized in Table 6-3 along 

 with the computed coefficients for the equation Y = aX'' where Y is the 

 number of embryos in the brood and X is the length of the adult. The 

 estimated values, based on least squares regression, are 12.0 and 0.01 for a 



