Effects of Temperature, Salinity and Chlorine Treatment on Egg Hatching of the Monogenean Neoheterobothrium hirame Infecting Japanese Flounder

Effects of temperature, salinity and chlorine treatment on egg hatching of the monogenean parasite, Neoheterobothrium hirame, of the Japanese flounder, Paralichthys olivaceus, were examined. More than 85%of the eggs developed and hatched between 10 and 25°C, but at 30°C hatching rates were lower than 5%. They hatched similarly in a wide range of salinities from 1/3-to full-strength seawater (salinity : 11-33‰). In distilled water and double-strength seawater, they did not hatch at all. In 1.5-strength seawater, a decrease of hatching rates was observed. Eggs treated with sodium hypochlorite (available chlorine concentrations below 50ppm for 15min) showed hatching rates similar to those in the control. Even an extraordinary chlorine treatment (available chlorine concentration 100 ppm for 15 min) had a limited effect on the hatching rates.