Development of monogeneans is usually water temperature dependent [1, 12, 19]. Hatching times are inversely related to water temperature [10, 21]. Hatching success increases with the rise of water temperature [18, 24]. Eggs fail to hatch at extremely low water temperature [10, 24]. In the present study, eggs of D. vastator also failed to hatch when directly incubated at the water temperature of 5 °C. However, the eggs exposed to low temperature hatched when subsequently maintained at 20 °C, and the hatching success decreased with the duration of cold exposure (7-21 days at 5 °C).
These results suggested that a short period of cold shock (within one week) had limited impact on egg viability, whereas long-term exposure to low temperature had serious impacts on embryonic development. Eggs may be arrested at low temperature but retained high viability; their development resumed and hatching occurred after the period normally required at this higher temperature . Eggs of Pseudodactylogyrus bini did not hatched when incubated for 10 days at 5 °C, but hatched with 75.5% hatching success after being transferred to room temperature . The eggs of Heterobothrium okamotoi did not hatch after 23 days of incubation at 10 °C, but high hatching rates were detected when transferred to 15 °C . The eggs of Protopolystoma orientalis and P. xenopodis apparently retained a relatively high viability when exposed to 5 °C for a short period (18 h) and then incubated at 25 °C . But all eggs of P. xenopodis were found to have died after incubation for 3 months at 10 and 12 °C . More than three weeks of incubation at 5 °C had significant impacts on hatching success of Diplectanum aequans eggs before incubation at room temperature . Our results revealed that 2 weeks of exposure to 5 °C appeared to have significant impacts on the hatching success of. D. vastator, and the effects of low temperature on egg viability were dependent on exposure time.
Although viability of eggs of D. vastator was related to exposure time to low temperature, hatching success was hardly affected at low temperature after short-term incubation at room temperature. This result suggested that eggs of D. vastator laid in late autumn could hatch in winter. Generally, monogeneans survived winter mainly as adults on host or eggs in the water (Bychowsky 1957). As a rule, reproduction does not take place during winter for freshwater monogeneans . When directly incubated at low temperature, eggs did not hatch for Diplectanum aequans , Pseudodactylogyrus bini , Heterobothrium okamoto , Protopolystoma orientalis and P. xenopodis [24, 27].
In the present study, however, a high hatching success was detected when eggs incubated at 5 °C after 12-24 h incubation at 20 °C. This result indicated that short exposure to moderate temperatures activated embryonic development of the eggs of D. vastator, and the developing eggs eliminated the effects of low temperature and continued hatching. The eggs of Entobdellu soleue are originally colourless inside the reproductive adults, but become darker in the uterus or after laying . The change in colour of the E. soleue eggs corresponds with the hardening of the egg shell . Hatching success was significantly higher at day 35 than at day 6 when treated with freshwater and formalin since the harder egg shell was less flexible and more impermeable . Therefore, the eggshell of monogeneans had the capability to protect the developing embryo and unhatched larva from detrimental osmotic effects . Based on this result, some eggs of D. vastator laid in late autumn were supposed to hatch during winter.
Seasonal occurrence of D. vastator also provided some evidences for eggs hatching in winter. In the field investigation, mean abundance of D. vastator in C. auratus increased in winter and reached a high level in early spring (unpublished data). In addition, the host goldfish tends to shoal at the bottom of water bodies, where the water temperature is higher than 5 °C. So it is possible that the hatched oncomiracidia have the infective ability to find hosts and locomote to the gills of goldfish in winter.