| Cyanobacteria have evolved into dominant groups of phytoplankton in freshwater lakes with the global warming and lake eutrophication.The interaction mechanism between M.aeruginosa(as a common cyanobacterial species)and cladoceran Daphnia has become a research hotspot in ecologists.Many researches have shown that Daphnia species can adapt to the adverse environmental factors such as M.aeruginosa and fish predation,which result in the phenotypic plasticity of life cycle characteristics and morphology of Daphnia.In the study,the resting eggs of D.galeata in different sedimentary layers(representing different ages)from Lake Chaohu(Anhui province)are selected.The effects of M.aeruginosa,fish kairomone and their combination on the reproduction and helmet morphology of D.galeata are studied.The different adaptation mechanisms of D.galeata to these two environmental factors are revealed.Moreover,whether the coexistence of fish kairomone and M.aeruginosa aggravate the toxic effect of toxic M.aeruginosa on Daphnia species is also discussed.This study can provide the reference for cyanobacterial bloom control,fishery cultivation and environmental assessment in eutrophic lakes.The results are as follows:1.Compared with the control treatments,the survival rates of six D.galeata clones were significantly lower than those in the treatments containing M.aeruginosa,and the survival rates of D.galeata in higher M.aeruginosa concentration was significantly lower than that in lower M.aeruginosa concentration.The body lengths of D.galeata in the treatments containing M.aeruginosa in the time at first pregancy and the end of the experiment were significantly smaller than those in the control treatments.The maximum body length at maturity(1.44±0.09 mm)of D.galeata appeared in the M0 treatment of clone 6,whereas the minimum value(1.00±0.0 mm)appeared in the M10 treatment of clone 1.The helmet lengths of D.galeata in the treatments containing M.aeruginosa were all significantly smaller than those in the control treatments,except for that the M0 treatment of clone 6.In the treatmentscontaining M.aeruginosa,the time at first pregancy of D.galeata were significantly higher than those in the control treatments,and its maximum value(220.50 h)appeared in the M10 treatment of clone 1.M.aeruginosa,clone and their combination had significant effects on the time at first pregancy of D.galeata(P<0.05).With the increase of M.aeruginosa concentration,both the egg number at the pregancy and the number of offspring at the brood of D.galeata decreased significantly,and there were differences among clones.The number of eggs at first pregancy and the number of offspring at first reproduction of six D.galeata clones in the control treatments were significantly higher than those in the treatments containing M.aeruginosa.No eggs in both M2 and M10 treatments of clone 5 were observed,and no offspring were also produced.The maximum eggs number at the pregancy(12.5±2.2 ind./per female)of D.galeata appeared in the M0 trreatment of clone 4,and the maximum eggs number at first pregancy(6.4±1.5 ind./per female)occurred in the M0 treatment of clone 6.The eggs of D.galeata clones 1-3 located in the surface sedimentary layers could develop normally in the M2 treatment,but all of them disintegrated in the M10 treatment.All eggs of the D.galeata clones 4-6 in the lower sedimentary layers were dissolved in the M2 treatment,and no individuals in the M10 treatment survived until the time at first pregancy.With the increase of M.aeruginosa concentration,the intrinsic growth rate of six D.galeata clones showed a decreasing trend.2.In the presence of fish kairomone,the survival rates of six D.galeata clones in the early period of the experiment was slightly higher than that in the control treatment(F0),while the survival rates of D.galeata in the later period of the experiment was significantly lower than that in the control treatment.At the end of the experiment,all D.galeata individuals of clone 3 and clone 5 in the F0 treatment died.The body lengths of D.galeata increased with the increase of instar number in the presence of fish kairomone,and the body lengths of clones 4-6 located in the lower sedimentary layers were significantly longer than those of clones 1-3 in the surface sedimentary layers.In the presence of fish kairomone,the body lengths of the other five clones were slightly longer than those in the control treatments except clone 2,mainly depending on D.galeata clone and fish kairomone concentration.The helmet lengths of clone 1 and clone 3 in the control treatment were longer than those in the present of fish kairomone,whereas helmet lengths of the other 4 clones in the higher fish kairomone concentrations grew longer than those in the F0 treatment.Themaximum helmet length(0.21±0.01 mm)of D.galeata appeared in the F20 treatment of clone 2.Fish kairomone,clone and their combination had significant effects on helmet length at first pregancy of D.galeata(P<0.05).The time at first pregancy of clones 1-3 were significantly longer than those of clones 4-6 in the F0-F20 treatments,but the time at first pregancy of clone 4 was higher than those of the other 5 clones in the F50 treatment.The maximum time at first pregancy(111.1±23.1 h)of D.galeata appeared in clone 3,whereas the minimum value(66.6±6.0 h)occurred in the F10 and F20 treatments of clone 4.The eggs number at the pregancy and the offspring number at the brood of D.galeata in the other five clones in the higher fish kairomone concentrations were bigger than those in the control treatment except clone 2,and their maximum values(21.0±3.3 ind./per female and 21.0±0.0 ind./per female)were both found in the F10 treatment of clone 5.The eggs number at first pregancy of six D.galeata clones were bigger in the F50 treatment than those in the control treatment,and both maximum eggs number at first pregancy(7.4±1.3 ind./per female)and the maximum offspring number at first reproduction(6.5±1.6 ind./per female)of D.galeata appeared in the F50 treatment of clone 1.Fish kairomone,clone and their combination had significant effects on the eggs number at first pregancy of D.galeata(P<0.05).Compared with the control treatment,the eggs of D.galeata in the treatments containing fish kairomone showed disintegration of eggs.With the increase of fish kairomone concentration,the intrinsic growth rate of six D.galeata clones decreased.3.The combined effects of M.aeruginosa and fish kairomone on the growth,reproduction and survival of D.galeata were complex,and there were significant differences between clones.In the five treatments of M.aeruginosa and fish kairomone,the survival rates of the other five D.galeata clones except clone 5 were higher than those in the control treatment(M0+F0).The body lengths of six D.galeata clones in the M10+F10 and M10+F50 treatments were significantly smaller than those in other treatments.The body lengths in the control treatment(M0+F0)were significantly longer than those in the other 4 treatments except clones 2 and 3.In the later period of the experiment,the helmet lengths of the other five clones in the control treatment was longer than those in the other 4 treatments except clone 6,and the maximum value(0.1±0.03 mm)appeared in the M10+F50 treatment of clone 6.The times at first pregancy of six D.galeata clones in the M10+F10 treatment were significantly higher than those in the other four treatments,and its maximum value(128.5±26.1 h)was found in the M10+F10 treatment of clone 2.The effects of M.aeruginosa + fish kairomone and clone on the helmet lengths of D.galeata at first pregancy were all significant(P<0.05),but their combination had no significant effects.Under the combination of lower M.aeruginosa concentration and higer fish kairomones(M2+F10,M2+F50),the eggs number at the pregancy,the number of offspring and the intrinsic growth rate of six D.galeata clones were higher than those in higher M.aeruginosa concentration and higer fish kairomones(M10+F10,M10+F50).The eggs number at first pregnancy and the number of offspring at first reproduction of clone 4 in the M2+F10 treatment were bigger than those in the control treatments,whereas the eggs number at first pregancy and the number of offspring at first reproduction of other five D.galeata clones in the other treatments were lower than those in the control treatments.The maximum offspring number(17.3±5.0ind./per female)and the maximum intrinsic growth rate(0.40)of D.galeata were found in the control treatment(M0+F0)of clone 1.M.aeruginosa + fish kairomone,clone and their combination had significant effects on the eggs number at the first pregancy and the number of offspring at first reproduction of D.galeata(P<0.05).In summary,M.aeruginosa had a significant negative effects on the life cycle parameters of D.galeata,and the negative effects were enhanced with the increase of M.aeruginosa concentrations.The response of D.galeata from Lake Chaohu to fish kairomone was characterized by such phenotypes as decreasing helmet length,increasing body length,increasing eggs number at the pregancy and offspring number,and earlier time at maturity.When these two disadvantageous factors worked together,fish kairomone could mitigate the inhibitory effect of M.aeruginosa on D.galeata in some extent. |
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