Behavioral and morphological endpoints: as an early response to sublethal malathion intoxication in the freshwater fish, Labeo rohita.

A short-term definitive test by the static renewal bioassay method was conducted to determine the acute toxicity (LC(50)) of commercial-grade organophosphate insecticide, malathion (50% EC) on the freshwater fish, Labeo rohita. Carp fingerlings were exposed to different concentrations of malathion for 96 hours. The acute toxicity (LC(50)) of malathion was found to be 4.5 microg/L. One tenth (1/10, 0.45 microg/L) of the acute toxicity value was selected as the sublethal concentration for subacute studies. The fish were exposed to sublethal concentration for 1, 5, 10, and 15 days and allowed to recover in toxicant-free medium for 15 days. Behavioral responses and morphological deformities were studied in the experimental tenures. Fish in toxic media exhibited irregular, random, circular swimming movements, hyperexcitability, loss of equilibrium, and sinking to the bottom. Caudal bending was the prime morphological malformation. The behavioral and morphological deformities were due to inhibition of acetylcholinesterase (AChE) activity. Inhibition of AChE activity results in excess accumulation of acetylcholine in cholinergic synapses, leading to hyperstimulation and cessation of neuronal transmission (i.e., paralysis). The carp were found under stress, but mortality was insignificant at the sublethal concentration tested. Impaired behavioral responses and morphological deformities were observed during recovery. This may be a consequence due to inhibition of brain and muscular AChE by malaoxon, via the biotransformation of sequestered malathion.