Most studies of cadmium toxicity have focused on sublethal effects and there has been little research into the mechanism of its toxicity. The aims of this work were to assess the toxicity of cadmium to earthworm Eisenia foetida using contact and avoidance tests and to examine their effects on behavior, morphology and histopathology to explain the cadmium toxicity. The first contact of the toxic substance with the earthworm is with the cuticle. The LC50 for cadmium was 19 µg/cm2. Earthworms avoided cadmium in artificial soil but this was not dependent on cadmium concentration. Cadmium toxicity initially caused abnormal behavioral/physiological responses, including curling, coiling, excessive mucus secretion, sluggish movements and, finally, loss of locomotion. Histological changes included destruction of cuticle, severe lesions and destruction of body wall, bleeding, and autotomy. Cadmium was found in the chloragogenous tissue. Together, these observations suggest a toxicity mechanism that involves disruption of the sensory system, anomalous behavior, loss of locomotion, and severe lesions, leading to death.
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