Effects of different regimes of low temperature on egg hatching of Dactylogyrus vastator (Monogenea: Dactylogyridae).

The development of dactylogyrids is dependent on water temperature, and their eggs fail to hatch below 5 °C. In the field, however, mean abundance of Dactylogyrus species increases and reaches a high level in winter, which suggests that infective oncomiracidia hatch from eggs in winter. Therefore, the effect of low water temperature on in vitro egg hatching of D. vastator was determined in laboratory. D. vastator hatching success was 65.3%, 62.7%, 42.6% and 22.3% when eggs were firstly incubated for 0, 7, 14 and 21 days at 5 °C and then consecutively maintained for 15 days at 20 °C. When eggs were directly incubated at 5 °C, eggs failed to hatch within one month. However, hatching success was 69.8% and 66.7%, respectively, when maintained at 5 °C after 12 and 24 h incubation at 20 °C. The results suggested that egg incubation for more than 1 week at 5 °C had significant impacts on hatching success of D. vastator subsequently maintained at 20 °C. But low temperature (5 °C) had no effect on hatching success when eggs were firstly exposed to room temperature (20 °C) for one day.