Hormetic effects of heavy metals in aquatic snails: is a little bit of pollution good?

Hormesis is the term to describe a stimulatory effects associated with a low dose of a potentially toxic substance or stress. We had anecdotal evidence of hormetic effects in some of our previous experiments concerning the influence of heavy metals on aquatic snail growth and recruitment. We therefore repeated a version of an earlier experiment but this time we expanded our low-dose treatments and increased our sample size. We also explored if metals had a hormetic effect on algae periphyton. We raised snails in outdoor mini-ecosystems containing lead, zinc, and cadmium-contaminated soil from an Environmental Protection Agency Superfund site in the Silver Valley of northern Idaho. The snails came from two sites. One population (Physella columbiana) has evolved for 120 years in the presence of heavy metals and one (Lymnaea palustris) has not. We found that P. columbiana exhibited hormesis with snails exposed to small amounts of metals exhibiting more reproduction and growth than snails not exposed to metals. Naturally occurring Oscillatoria algae also exhibited a hormetic effect of heavy metals but L. palustris did not display hormesis. Large doses negatively impacted all three species. Overall the levels of cadmium, lead, and zinc measured in the tissues of the snails were inversely correlated to the number of snails recruited into the tub populations. Only in comparisons of the lowest metal treatment to the control treatment is a positive effect detected. Indirect effects on competing species of snails, periphyton, and also fishermen, may be less favorable.