Fig. 1. Comparison of thermotolerance between P. aurelia syngens. Each syngen, cultured at 25°C, was incubated at 33°C for 30 min, and then the incubation temperature was raised stepwise by 1°C and kept constant for about 10 min. At each temperature, swimming cells were counted and the temperature at which no cells were swimming was determined as the killing temperature. Experiments were conducted three times.