Effect of temperature and feeding frequency on asexual reproduction and polyp growth of the scyphozoan Cyanea nozakii Kishinouye

When jellyfish aggregate in great abundance and form large swarms,or 'blooms',they often cause significant environmental and economic impacts.Cyanea nozakii Kishinouye is one of the jellyfish species causing problems in the waters of China.C.nozakii has a life cycle consisting of a planktonic sexually-reproducing medusa and a benthic asexually-reproducing polyp.Polyps produce young medusae(ephyrae) through strobilation.Increasing medusa populations may reflect the increasing success of asexual polyp reproduction.Environmental factors(temperature,salinity,light,food,etc.) significantly affect the population size of both polyps and medusae.Therefore,studying the effects of environmental factors on the asexual reproduction of C.nozakii is important for exploring the reproduction potential of this jellyfish and to forecast jellyfish blooms.In this study,polyps of C.nozakii,from Liaodong Bay,Bohai Sea,were tested in six temperature gradients(7.5℃,11℃,14.5℃,18.5,21.5 ℃and 25℃) and nine temperature(14.5℃,18℃ and 21.5 ℃) and feeding frequency(1/2d、1/8d and 1/16d) combinations in laboratory experiments.The influences of the two environmental factors and their interactions with asexual reproduction and polyp growth were observed.The results showed that polyps did not produce podocysts at low temperatures(7.5—14.5 ℃).The percentage of polyps that produced podocysts and the number of podocysts produced by each polyp increased with increasing temperature at warm temperatures(18—25 ℃).Podocysts germinated only in the 21.5 ℃ and 25 ℃ groups,but the difference was not significant(P0.05,n=3).Optimum temperatures for podocyst reproduction were from 18 ℃ to 25 ℃.Strobilation occurred only at 21.5 ℃ and 25 ℃.At 25 ℃,strobilation occurred earlier,the strobilation rate was always higher,the polyps strobilated more times,and the time between strobilations was shorter than that at 21.5 ℃.The results of the temperature and feeding frequency combinations showed that the maximum percentage of polyps that produced podocysts and the highest number of podocysts produced by each polyp occurred at the 21.5 ℃ and 1time/2d feeding frequency combination.Podocyst reproduction(P0.01,n=45) and strobilation rates(P0.05,n=45)differed significantly with temperature,feeding frequency,and their interaction.Strobilation times differed significantly with temperature(P0.05,n=45).The polyp survival rate was 100% in all temperatures(7.5—25 ℃).The relative growth rate and the specific growth rate of polyps increased with increasing temperature.The results of the combination of temperature and feeding frequency showed that polyp growth differed significantly with temperature,feeding frequency,and their interaction(P0.01,n=45).Strobilation times and ephyra primary diameter increased with larger polyp diameter.The cumulative number of ephyrae released by polyps at the combination of 25 ℃ and 1time/2d was about 293 times that released at 21.5 ℃ and 1time/8d,suggesting that temperature and nutritional conditions strongly affect ephyra numbers for C.nozakii.Temperature,feeding frequency and their interaction significantly affected the potential production of polyps and medusa,indicating that both increases in water temperature and increases in zooplankton prey abundance because of eutrophication and declining fisheries may encourage prominent blooms of C.nozakii medusae.