Snakes as indicators of environmental contamination: relation of detoxifying enzymes and pesticide residues to species occurrence in three aquatic ecosystems

Abstract

Pesticide detoxifying enzymes and residues of p,p'-DDE and PCB were measured in snakes of two genera collected in three aquatic ecosystems in central Texas. Snake genera were Agkistrodon (water moccasins and copperheads) and Natrix (water snakes). One collecting site had heavy exposure to insecticides, one to PCBs and one to neither. Objectives were to correlate pesticide residues and detoxifying enzymes with species occurrence in different ecosystems, to evaluate the usefulness of snakes as indicators of environmental contamination, and to establish a biochemical basis for the differential distribution of snakes between ecosystems. Agkistrodon spp. had several times the NADPH-dependent oxidative detoxifying activity of Natrix spp. Within Natrix, oxidase activity was significantly higher in erythrogaster and fasciata than in rhombifera. Alkyltransferase activity was higher in A. piscivorus (water moccasins) than in any other species. There were no consistent differences between sexes in levels of detoxifying enzymes and enzyme activity was similar in snakes of each species from all collecting sites, indicating no induction had occurred as a result of heavy exposure to insecticides or PCBs. DDE residues were highest in snakes collected adjacent to a cotton agro-ecosystem. PCB residues occurred only in snakes collected near a heavily-traveled highway. Residues in A. piscivorus were only half those in Natrix spp. from the same site, indicating the former is more efficient as disposing of residues. The data indicate that snakes with high levels of detoxifying enzymes, particularly microsomal oxidases, are more likely to occur in a contaminated ecosystem than snakes with less active detoxifying enzymes and that residues are lower in the better detoxifiers. Snakes are apparently excellent indicators of environmental contaminated by insecticides and other persistent chemicals.