Comparison of Rates of Killing of Parthenogenetic and Sexual Forms of Daphnia magna at Higher Temperatures.

Abstract

In a recent paper 1 the temperature characteristics for the rate of killing of 2 species of cladocerans were reported. One of these species, Moina macrocopa, yielded a value of the temperature characteristic equal to about 108,500 calories, for the temperature interval of 42° to 47° C. The second species, Daphnia pulex, yielded a slightly higher value, 119,500 calories, for the temperature interval 32° to 37° C. The animals used in these experiments were adult parthenogenetic females. The present paper presents data for the parthenogenetic females of a 3rd species, Daphnia magna, and an account of a comparison of this parthenogenetic form with the sexual females and the adult males of the same species.

The procedure used in determining the rate of killing in the present case was the same as that described in the paper just referred to. Curves of the velocities of killing at each of several lethal temperatures were determined separately for the 3 forms used. The parthenogenetic females were reared from young females placed in plenty of food. The males were reared as young from parthenogenetic females which previously had been somewhat crowded together in rich food; the sexual females were obtained by crowding females together in weak food. The rate of killing was taken as the reciprocal of the time necessary to kill 50% of the animals (Table I). To determine the temperature characteristic, the logarithms of these rates were plotted against the reciprocals of the absolute temperatures (Fig. 1) and the value of the constant μ calculated from the slope of the line drawn through the plotted points according to the equation,

where K₁ and K₂ are proportional to velocity constants at the respective temperature T₁ and T₂, R the gas constant, and μ the critical thermal increment or temperature characteristic. 2

The graphs show that the thermal increment for the partheno-genetic and sexual females is practically the same, with μ equal to 187,700 calories. The plotted line gives a better fit for the parthenogenetic females than for the sexual females. This may be due in part to the fact that fewer animals were used in determining the velocities of killing in the latter form, but probably is due to the greater difficulty in getting animals of equivalent ages in the case of the ephippial females.