Sodium arsenite modified burrowing behavior of earthworm species Metaphire californica and Eisenia fetida in a farm soil

Arsenic (As) is a widely distributed contaminant in soil and is toxic to plants, animals and humans. Many studies have explored the effects of As on various physiological and biochemical processes in soil fauna. However, there is no report on the effect of sodium arsenite on the behavior of soil fauna such as earthworms. In this study, two earthworm species, Metaphire californica and Eisenia fetida, were exposed to sodium arsenite in farm soils at concentrations of 5, 10, 20, 40 and 80 mg As kg(-1) soil for 7 days to investigate the effect on burrowing behavior. Earthworm burrowing behavior responded significantly to both arsenite concentration and exposure time. Both species displayed an initially similar burrowing rate in farm soil with no added As. With added As, M. californica displayed an 'escape response' of increased burrowing activity while E. fetida displayed an 'exposure avoidance' response of decreased burrowing activity and increased burrow reuse. Data are consistent with an early, concentration dependent sensory response, followed by addition of a physiological response component as As bioaccumulates in earthworm tissues with time. Any bioassay using earthworms to detect As contamination would need to be designed with awareness of the complex behavioral response.