Effect of light and temperature on hatching in fasciola hepatica (Trematoda: Fasciolidae)

Hatch rate of Fasciola hepatica eggs allowed to develop in the dark at 24°C and 28°C and then illuminated was very low, but marked hatching occurred following illumination at 20°C of eggs previously developed in the dark at 20°C, 24°C or 28°C. Eggs developed in the dark under constant temperature failed to hatch, while eggs subjected to temperature changes of 4°C or more sometimes hatched even in the absence of light. With eggs subjected to temperature changes, hatching was invariably higher upon lowering than upon raising of the temperature, and under illumination than in the dark. Fastest development of the eggs occurred at 24°C-28°C, but maximal hatching was at l6°C-20°C. There were no marked differences in the hatching at various wavelengths of the visiblelight spectrum or at different light intensities. These findings suggest that: (a) temperatures above 20°C are not conducive to hatching; (b)while light stimulates greater hatching, temperature changes, especially lowering of the temperature, are the main stimulus for massive hatching, and (c) wavelength of the light is of little significance to the hatching.