Heavy metals can influence earthworm behavior and physiology which could drastically impact the soil functioning. To test this hypothesis, under laboratory conditions, the cast production and burrow systems(burrow length and patterns), macropores formed and surface migration of three ecologically different earthworm species (the epigeic Eudrilus eugeniea, aneic Lampito maurutii and the endogeic Pontoscolex corethrururs) in natural spiked soil with different concentrations of heavy metals (chromium Cr(VI) and zinc (Zn)) were performed. The amount of cast produced in all the three species varied with increasing concentrations of heavy metals which is inversely proportional to incidence of surface migration observed. Macropores formed increased significantly (P <0.05) in Zn-spiked soils and reduced significantly (P <0.05) upon chromium exposure. The burrow patterns of three earthworm species varied: E.eugeniea being an epigeic exhibited only horizontal movements which showed statistically significant (P<0.05) change in its burrow length at the lower concentration of 10 ppm Cr (VI) and 300ppm of Zn, whereas L. maurutii made more continuous, less branched and more vertical burrows and exhibited statistically significant (P <0.05) change at 10ppm of Cr. However upon Zn exposure significant (P <0.05) change was evident in 100ppm of Zn onwards. Similar pattern was also evident in P. corethrurus. Some conspicuous changes in L. maurutii and P. corethrurus burrow systems upon exposure to chromium were they made a smaller and narrower burrow system, at concentration above 20ppm of Cr and 400ppm of Zn. As a consequence, the continuity of the burrow systems made by both species was altered following heavy metal exposure which could be considered as a potential tool for evaluation of heavy metal toxicity.
Earthworms, burrowing, cast production, surface migration, macropores, heavy metal toxicity
