Effects of lead and arsenic on medial giant fibre conduction velocity in Aporrectodea caliginosa : A dissertation submitted in partial fulfilment for a Post Graduate Diploma in Applied Science at Lincoln University

Abstract

Three experiments were carried out to examine the effects of the heavy metals Pb and As on the medial giant fibre conduction velocity in the earthworm Aporrectodea caliginosa. In experiment 1, a modified contact filter paper test, individual immature earthworms were exposed to a dose of 0-15 mgPb/kg bw or 0-40 mgAs/kg bw. Non-invasive electrophysiological recordings were made at 0, 3, 6, 9, 24, and 48h. Results indicated dose dependent reductions in conduction velocity (CV) of the medial giant nerve fibre (MGF) on exposure of A. caliginosa to As. No such effect was seen on exposure to Pb. Soil studies using both juvenile and immature exposed to similar Pb and As doses in soil confirmed these observations. A decrease in growth of earthworms exposed to As in soil was also dose dependent and this could be used as an ecotoxicological test endpoint. The potential for MGF CV and earthworm growth as biomarkers of soil heavy metal pollution is discussed. A mathematical calculation of expressing toxicant exposure for earthworm ecotoxicity tests as mg/kg bw and its relationship to both filter paper concentration and soil concentrations is described.

It is concluded that As has a sublethal neurotoxic effect on earthworm MGFs. The data indicate that the onset of as dose dependent effects on MGF CV occurs earlier than the changes in earthworm growth .