Sensitivity of the freshwater tropical oligochaete, Branchiura sowerbyi (Beddard, 1892) to the grey list metal, Zinc

Abstract

Although the freshwater oligochaete, Branchiura sowerbyi Beddard, 1892 (Oligochaeta: Naididae: Rhyacodrilinae) has been used as an indicator organism to monitor aquatic pollution, the data on the toxic effects of the grey list metal, zinc on this worm is remained scanty. The present study was undertaken to determine the sensitivity of the worm, B. sowerbyi to zinc on the basis of 24, 48, 72 and 96h lethal concentrations (LC_{1, 5, 10, 15, 50, 85, 90, 95, 99}) and their behavioural responses. The 96h LC₅₀ value of zinc with 95% confidence limit to the worm was recorded as 45.48 (41.32-49.33) mg/l. The rate of mortality of the worm was significantly increased (p<0.05) with increasing concentrations and time of exposure (24, 48, 72 and 96h). The study further focussed on estimation of the toxicity factor at different time scale and possible safe level of zinc to the worm to strengthen the base line data that could be used to set up local water quality criteria (WQC) for the toxicant. The worms showed behavioural changes like clumping tendency, movement and mucous secretion with increasing concentration of the toxicant and the progress of time. The findings can be used in determination of ecological risk assessment for the worm to zinc toxicity as well as to understand its mode of action in the light of their ethological changes.

Keywords: Zinc, Branchiura sowerbyi, acute toxicity, toxicity factor, safe level, behavioural responses