| Water eutrophication has become a major global environmental problem.The algae can reproduce quickly in eutrophic waters,especially some blue-algae species?such as Microcystis aeruginosa?forming blooms.M.aeruginosa blooms have frequently occurred in some lakes in the middle and lower reaches of the Yangtze River,China?such as Lake Chaohu and Lake Taihu?.In addition,in the sediments of eutrophic lakes,there are rich cladaceran ephippia?such as Daphnia ephippia?,which can really reflect the historical succession of the cladoceran community structure in lakes.Aristichthys nobilis is an endemic fish,which widely distributed in China.It mainly feeds on plankton and plays an important role in regulating water quality and maintaining the balance of aquatic ecosystems.In global eutrophication,the interaction mechanism between filter-feeding fishes,M.aeruginosa and cladocerans has become the focus of many researchers.At present,the studies on the single effects of fish or M.aeruginosa on the life history characteristics of cladocerans are more common,but the combined effects of A.nobilis kairomone and M.aeruginosa on the life history characteristics and body phenotype of Daphnia species have not been reported.In this study,Daphnia similoides sinensis in different sedimentary layers of Lake Chaohu?1-3 cm and 17-19 cm?is taken as the experimental animals,and the high-quality Scenedesmus obliquus is taken as the food,the effects of fish kairomone,M.aeruginosa and their combination on the reprodution and body phenotype of D.similoides sinensis are studied.The purpose of this study is to reveal the adaptive changes in the reproduction and body phenotype of Daphnia species in response to fish predation and toxic M.aeruginosa,to explore whether the presence of fish kairomone superimpose toxic effects of M.aeruginosa on D.similoides sinensis,and to provide a reference for further exploring the molecular mechanism.The results are as follows:1.In the presence of A.nobilis kairomone,the reproduction and body phenotypes of D.similoides sinensis changed significantly.Fish kairomone had a significant effect on the body lengths at maturity of D.similoides sinensis from different sedimentary layers of Lake Chaohu?P<0.01?.The maximum helmet length at maturity?0.14 mm?of D.similoides sinensis appeared in the F10 treatment of clone6,whereas the maximum value?only 0.11 mm?in the control treatments appeared in clone 4.By the end of the experiment(day 21th),except for clone 1 located in the surface sedimentary layers,the helmet lengths of the D.similoides sinensis were longer than those of the control treatments in the presence of fish kairomone.However,there were no significant differences in the helmet lengths among different fish kairomone concentrations.With the increase of instar number,the tail spine length of D.similoides sinensis increased.In the presence of fish kairomone,the tail spine lengths of six D.similoides sinensis clones were longer than those in the control treatment,and the fracture of tail spine appeared earlier.The maximum length of tail spine?0.80 mm?appeared in the F50 treatment of clone 1.Fish kairomone and sedimentary layer had significant effects on the tail spine lengths at maturity of D.similoides sinensis?P<0.01?.With the increase of instar number,the offspring number of D.similoides sinensis gradually increased.After reaching a peak,it slowly decreased or remained stable.In addition,the average offspring numberd of D.similoides sinensis located in the lower sedimentary layers?17-19 cm?in the treatments containing fish kairomone were significantly higher than those in the control treatments.Except for clone 4 in the 17-19 cm layer?about 78 h?,the average time to maturation of other D.similoides sinensis in the control treatments was?102±5?h.With the increase of fish kairomone concentrations,the time to maturation of D.similoides sinensis shortened significantly,with an average value of?90±10?h.2.M.aeruginosa had a significant inhibitory effect on the growth and reproduction of D.similoides sinenesis.With the increase of M.aeruginosa concentration,the body length and growth rate of D.similoides sinenesis located in different sedimentary layers?1-3 cm,17-19 cm?decreased.The maximum body length?2.20±0.20 mm?appeared in the M0 treatment,and they were 1.7±0.02 mm in the M2 treatment and 0.60±0.02 mm in the M10 treatment,respectively.M.aeruginosa had significant effects on the body length at maturity and the maximum body length of D.similoides sinensis?P<0.01?.Compared with the control treatments,the helmet lengths of D.similoides sinensis increased significantly at higher M.aeruginosa concentrations.With the increase of the instar number,the helmet lengths decreased slowly.The maximum value?0.13 mm?appeared in clone 1,whereas the minimum value?0.05 mm?occurred in clone 2.With the increase of M.aeruginosa concentration,the tail spine lengths of six D.similoides sinensis clones had an obvious decreasing trend.The number of eggs?the egg number at first pregancy,total egg number?,the number of offspring?the offspring number at first reproduction,total cumulative offspring number?and the survival rates of D.similoides sinensis decreased.The survival rates of D.similoides sinensis located in the upper sedimentary layers were significantly higher than those located in the lower sedimentary layers.3.With the increase of instar number,the body lengths of six D.similoides sinensis clones increased gradually.By the end of the experiment,The maximum body length?2.68 mm?in the control treatments appeared in clone 3,whereas the minimum value?2.52 mm?occurred in clone 5.In the presence of fish kairomone and M.aeruginosa together,the maximum body length?2.48 mm?appeared in the M2-F50 treatment of clone 3,whereas the minimum value?1.64 mm?appeared in the M10-F10 treatment of clone 5.The helmet lengths of six D.similoides sinensis clones were longer than those in the control treatments until the end of the experiment,and the maximum value?0.10 mm?appeared in the M10-F50 treatment of clone 6.There were significant differences between the experimental treatments and the control treatment in the presence of A.Nobilis kairomone and M.aeruginosa together?P<0.01?.In the presence of A.nobilis kairomone and M.aeruginosa,the tail spine lengths and no.offspring of D.similoides sinensis were significantly smaller than those in the control treatments?P<0.01?.The clone and the combination of fish kairomone and M.aeruginosa had all significant effects on the tail spine lengths and the number of offspring of D.similoides sinensis?P<0.01?.Under the same M.aeruginosa concentration,compared with the lower A.nobilis kairomone concentrations?M2-F10,M10-F10?,D.similoides sinensis under the higher A.Nobilis kairomone concentrations?M2-F50,M10-F50?had longer tail spine lengths,longer body length,larger number of offspring and larger helmet length.Moreover,compared with the control treatments,there are significant differences among the treatments with A.nobilis kairomone and M.aeruginosa?P<0.01?on helmet length,body length,tail spine lengths and the number of offspring of D.similoides sinensis,The survival rate of D.similoides sinensis in the presence of A.Nobilis kairomone and M.aeruginosa together was obviously higher than those in single factor.In a word,in the face of fish kairomone induction,D.similoides sinensis had obvious resistant responses?such as increasing helmet length,longer tail spine,more number of offspring,etc.?.M.aeruginosa had also obvious inhibition on the growth and reproduction of D.similoiedes sinensis,and with the increase of M.aeruginosa concentrations,this inhibitory effect was more significant?such as decreasing body length,shorter tail spine length,smaller number of offspring or even no eggs,etc.?.However,when D.similoides sinensis worked A.nobilis kairomone and M.aeruginosa together,the toxic M.aeruginosa was obviously alleviated by fish kairomone,and even they could coexist better.This study demonstrates the effects of fish pheromones,M.aeruginosa and their comination on the reproduction and body phenotype of D.similoides sinensis from an ecological point of view.Although fish predation and serious M.aeruginosa bloom together,there were still high-density populations of Daphnia in Lake Chaohu.It will provide a reference for further research on the structure and function of eutrophic lake ecosystem in the future. |
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