THE RESPONSES OF MOINA MICRURA IN AQUACULTURE LAKES TO HIGH CONCENTRATIONS OF CHEMICAL CUES FROM PLANKTIVOROUS FISH:A LIFE TABLE DEMOGRAPHIC STUDY

A life table demographic study was performed to explore the responses of Moina micrura in aquaculture lakes to high concentrations of chemical cues from planktivorous fish,and the underlying mechanism accounting for the dominance of M.micrura under planktivorous fish predation was also discussed.In aquatic systems,the non-visual animals usually assess predation risk through chemical communication.During the experiment,the test cladoceran M.micrura Kurz,1894 was isolated from Lake Donghu,a subtropical eutrophic lake densely stocked with planktivorous fish.The species was kept in an environmental chamber at 25℃ and a 14h:10h photoperiod cycle,and cultured with filtered lake water(by 1.2 μm mesh net) for about one month.Neonates,less than 12h older with an average body size of 0.66 mm,were used for life table experiment.Seven bighead carps(a total weight of 1291 g) were collected from Lake Donghu and maintained in an aquarium with a diameter of 1 m.Prior to the experiment,the water in the aquarium was renewed with fresh lake water and the fishes were maintained for two days at a water depth of 0.3m.The conditioned water and the lake water were both filtered through a 1.2 μm mesh net and added to the beakers in a gradient of 10∶1,5∶1 and 1∶1,making a final volume of 50 mL.Meanwhile,the filtered lake water was used for control treatment.The culture water was renewed daily.Eight replicates were set for each treatment.The data were analyzed by ANOVA and Tukey-test was used to compare the differences between treatment means(α0.05).The averaged clutch size was 12.4,21.1 and 26.8 in the first three broods,respectively.The absolute growth rate(0.084-0.096 mm/d) and the maximum body size(1.30-1.45mm) showed no significant difference between treatments(F3,26=1.67,p=0.20;F3,26=1.69,p=0.19,respectively).Also,the treatments had no significant effect on the size at first reproduction(0.82-0.83 mm)(F3,26=0.32,p=0.81) and the size of neonates in the first brood(0.52-0.54 mm)(F3,26=1.38,p=0.27).Likewise,no difference was found in total offspring and mean clutch size in the four treatments(F3,26=2.78,p=0.06;F3,26=0.85,p=0.48,respectively).The averages for total offspring and mean clutch size were 1.5 and 22.9,respectively.The generation time(3.82-4.11d) and rm(1.22-1.24/d) both showed a narrow range between treatments.These results suggest that high concentrations of fish cues have no significant effects on M.micrura inhabiting in lakes with dense planktivorous fish.M.micrura may have accustomed to the cues from their long-term coexistence with planktivorous fish.However,total offspring,mean clutch size and intrinsic rate of population increase were much larger when compared to those values reported in other studies,suggesting that M.micrura may have already evolved a strong resistance to planktivorous fish predation through life history changes.In addition,decreased generation time,age and size at first reproduction may be also responsible for their resistance to planktivorous fish predation.