Aiken, S.C. – Environmental concerns over selenium toxicity in wildlife have re-emerged in recent years and become a topic of substantial debate. Currently, the U.S. Environmental Protection Agency (EPA) is considering a new and controversial tissue residue regulatory standard for selenium, a very different approach than more traditional water quality standards. However, for such approaches to be effective, a clear understanding of how selenium accumulates in different organisms, and in different tissues, is required.
As little is known about the accumulation and effects of selenium in reptiles, scientists at the University of Georgia Savannah River Ecology Laboratory developed a simplified laboratory food chain to study these processes. Unlike some other contaminants that may primarily be accumulated by exposure in air or water, mercury and selenium are ingested primarily in the diet.
Understanding how contaminants travel through the food web and where contaminant burdens are likely to be found within the subject’s body is ultimately critical for assessing health risks, according to the authors. Researcher William Hopkins and others fed commercial feed laden with selenium to crickets that were then fed to juvenile fence lizards for 98 days. The researchers chose selenium as the contaminant because of its current focus within the regulatory community, propensity to accumulate in food webs, and its well-known ability to cause birth defects in wildlife. Other lizards were fed uncontaminated crickets.
Fence lizards were chosen for this study because they are one of the most common lizards found in the United States and because their entire life cycle is manageable in the laboratory. Also, a great deal is already known about their ecology, physiology, performance and life history.
The researchers found that female lizards fed the contaminated crickets accumulated the most selenium, especially in the reproductive organs. In fact, they approached the highest thresholds for reproductive toxicity in egg-laying vertebrates. This is the first study to document trophic transfer of any contaminant in a lizard using a laboratory-based food chain.
“One interesting finding in the study is the difference in how females and males handled the same toxic burden,” said Hopkins. “Females stored much of it in their reproductive organs, regardless of the total amount ingested, but males did not store like amounts in their testes.”
He said some of this difference can be attributed to the greater mass of the female reproductive organs, but the remaining discrepancy between sexes is independent of mass and remains a mystery.
“We believe that females may sequester disproportionately large quantities of selenium in their gonads due to specific proteins which either require selenium or have a higher-than-normal affinity for the element,” Hopkins continued. “Such disproportional allocation of selenium to the female reproductive organs has important implications for their reproductive success because the element is so lethal to developing embryos.”
Though this study shows high levels of accumulation in female lizards, the researchers observed no consistent effect of dietary treatment on survival or other, less lethal parameters. However, Hopkins said the simplified food chain has proven to be an ecologically relevant method of exposing lizards to selenium and will be used in future studies on maternal transfer and embryonic malformations. Such studies will help determine whether currently proposed regulatory criteria are protective of reptiles and amphibians, which is particularly important since these populations have been found to be declining worldwide.