| Reproduction of Daphnia pulex was studied by a series of experiments in laboratory. The effect of ecological factors on the reproduction of Daphnia pulex and the accumulative hatching rate of resting eggs are included in this paper.1. The type of diet, concentration of diet, constant temperature and excretion produced by mother females have no effect on the induce of sexual reproduction of single Daphnia pulex cultured in previous four broods, but the fecundity has changed. The number of offspring at per brood increase with the increase of concentration of chlorella from 0. 1 to 4. OX 106cells/ml, and the number of eggs ranges from 3.33 ind. to 21.33 ind. in the first brood. The fecundity is restricted when the chlorella concentration surpasses 6. OX 106cells/ml and will be generally down when the temperature is raised. From 15"C to 30癈, total fecundity is from 128. 34 ind. to 64. 67 ind.The shocking of temperature can induce single Daphnia pulex from parthenogenesis to sexual reproduction switch. When mature females are stimulated at 4癈, male offspring appear in the second and the fourth brood, the rate is 32.33% and 13.50%. In addition, male offspring appear in the third and the fourth brood shocked at 30癈, the rate is 29. 5%and 13.33%. The great fluctuation of temperature induces the reproduction switch significantly. Photoperiod shows effect on the reproduction switch and the fecundity is coming down when photoperiod declines. Excretory products induce increase of offspring slightly in the previous four broods when they singly grow, but have no effect on the sex reproduction.All the above ecological factors can induce appearance of sexual reproduction in group Daphnia pulex. With the increase of chlorella concentration from 0.1 to 4. OX 106cells/ml, the fecundity ranges from 13. 00 ind . to 75. 67 ind. per female in lOdays, the male rate and the resting eggs increase at first, then drop. Hunger and stimulus of temperature prompt the reproduction switch such as reducing fecundity, increasing resting eggs and the male rate. The female produces 0.47 resting eggs shocked at 15癈, the male rate is 6.70%. The fluctuating of temperature in big range shows marked effect on the induce of reproduction.With the increase of the population density, the male rate and the number of resting eggs are rising, but the fecundity decreases. The threshold of population density can induce reproduction switch is 0. 15ind/ml. Groups cultured show great effect on the sexual switch when add some excretory product than ever.2. The experiment of resting eggs indicated that the factors affect accumulative hatching rate.Resting eggs would not be hatched if they were stored in aeration condition. Hatching fluids, temperature and quality of resting eggsaffect hatching rate. Aeration water is the best for hatching resting eggs, while mixed water is better than the filtered water at 25℃ within 10 days. Light is essential for the hatch of resting eggs, while the length is not important. Preserved in water at low temperature, resting eggs can be hatched best, and they can be hatched better at low temperature than at normal temperature stored dryly. The preserved time has significant effect on hatching rate. The resting eggs can be hatched better stored from 21 to 35 days at -4℃, the range of its hatching rate is from 28.33% to 38.33% in 10 days. With prolong of time, the hatching rate increases constantly until it reaches a certain degree. The accumulative hatching rate of resting eggs is 41. 33%or 48. 33% stored in 14 or 21days at -4℃. |
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