Effect of solar UV radiation on earthworm (Metaphire posthuma)

Human health risks like damage to the eyes, immune system, and skin are known to be associated with increasing ultraviolet radiation (UVR) in the environment. In this study, we evaluated the phototoxic effects of UVR in sunlight and its possible mechanism of action by using earthworm as an alternative model because earthworm skin contains several biomolecules (tetraene and triene sterol) similar to human beings. We studied the generation of reactive oxygen species (ROS), photooxidation of lipids, and histopathological changes in earthworm integument. It was observed that UVR-exposed earthworm skin homogenate produced a significant amount of singlet oxygen ((1)O(2)), superoxide anion (O(2)(*)(-)), hydroxyl radicals ((*)OH), and photooxidation of lipids. The production of ROS and lipid peroxidation product was found dependent on the dose of solar UVR in earthworm integument. Histological anomalies such as thickening, vacuolation, and hypertrophy of epidermal cells were observed when the animals were exposed for 1 to 2h, while a higher exposure period (3h) caused degeneration of circular and longitudinal muscles. Continuous sunlight exposure for more than 3h was found lethal to worms. These observations suggested that the current level of UVR in sunlight may produce significant phototoxic effects in the earthworms probably via the generation of ROS (photodynamic action). Possible increases in UVR in view of ozone depletion may be more detrimental to the biomolecules in the worm's skin. The earthworm thus turned out as a simple, sensitive, and cost-effective test organism for the assessment of the hazardous potential of solar radiation and also for planning safety measures for human beings.