Cadmium bioconcentration in an aquatic oligochaete Limnodrilus hoffmeisteri: a semistatic bioassay evaluation

Direct uptake of an element from water by an aquatic organism and its retention, leading to a higher concentration than in its surrounding water, is an environmental concern known as bioconcentration. It occurs when an organism absorbs a substance faster than it is eliminated by various metabolic processes, such as catabolism and excretion. A bioassay experiment was performed to determine the 96-hour LC₅₀ of cadmium chloride (CdCl₂), a heavy-metal salt. For experimentation, pre-acclimated Limnodrilus hoffmeisteri worms were divided into eleven treatment groups (T₁ through T₁₁) and a control group (C), each of 50 number of individuals and were exposed to 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.25, 2.5, 2.75 and 3.0 mg/L cadmium concentrations respectively for 96 h under semi-static condition in a closed-circuit flow-through system with complete renewal of water with the respective relevant cadmium concentration after 48 h. Post-exposure cadmium content per unit amount of tissue samples and of residual water samples was measured with atomic absorption spectrophotometry (AAS). Probit analysis revealed the 96-hour LC₅₀ value of cadmium against L. hoffmeisteri as 1.002 mg/L. The outcome of this study established a low BCF for the experimental species for cadmium, indicating a positive correlation between BCF and exposure concentration and demonstrating the inherent bioconcentration nature of cadmium in L. hoffmeisteri. A gradual declining trend in cadmium content of the whole body was observed at exposure to a cadmium concentration higher than 2.0 mg/L. Concurrent autotomy of the caudal part of the organism has also been observed at this exposure level. Cadmium content of the autotomized tail has been estimated significantly (P < 0.05) higher. It is concluded that autotomization is a defensive mechanism evolved by L. hoffmeisteri to reduce the body's heavy metal load and, hence, toxicity, thereby serving as an indicator species.