Exposure to Cadmium-Phenanthrene Mixtures Elicits Complex Toxic Responses in the Freshwater Tubificid Oligochaete, Ilyodrilus templetoni Kurt A. Gust, 1 John W. Fleeger 1 1 Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803 Received: 29 March 2005 /Accepted: 15 August 2005 Abstract. The joint toxicity of metal–hydrocarbon mixtures in sediments was investigated using cadmium (Cd) and phenan- threne (Phen) as model contaminants. Sediment bioassays were utilized to quantify effects of individual and combined contaminants in the bulk-deposit feeding oligochaete Ilyodri- lus templetoni. Combined contaminants elicited antagonistic lethal effects and independent responses for feeding rate (measured as sediment ingestion). The 10-d LC 50 for Cd alone was 1375 mg kg )1 (95% C.I. 1340–1412), whereas Phen elicited no mortality even when loaded to sediment saturation. The presence of Phen decreased Cd lethality, increasing the LC 50 of Cd by as much as 40%. Regression analyses indicated that Phen was nearly 10 times more potent than Cd in eliciting feeding rate reductions. Exposure to Cd–Phen mixtures re- sulted in feeding rate reductions equivalent to those caused by Phen alone. The marked reduction in sediment ingestion in- duced by the co-pollutant Phen reduced exposure to Cd via ingestion. We suggest that this Phen-induced reduction in Cd exposure decreased Cd bioaccumulation and subsequent lethality. More generally, we suggest that even if the toxico- logical effects among dissimilarly acting chemicals (including metals and hydrocarbons) are independent, contaminant mix- tures may elicit unexpected interactive effects facilitated by modifying exposure. Environmental pollution often occurs as a mixture of various classes of chemical constituents (Kennicutt et al. 1996). The toxicity of contaminant mixtures to organisms in natural environments is complex and is related to environment-spe- cific bioavailability, organismal exposure, biodynamics (bio- accumulation versus elimination rates), and mixture toxicology. Regulators often use data from single-contaminant toxicity tests to estimate maximum allowable concentrations of individual chemicals within a given environment. In areas where contaminant mixtures are present, environmental risk is typically determined assuming concentration-additive toxicity (USEPA 2000b), which is defined as toxicity proportional to the summed toxicities of each individual contaminant con- centration (Cassee et al. 1998). Concentration-additive toxicity has been demonstrated to be an effective predictor of the overall toxicity of combined chemicals that have similar tox- icological effects and/or those having related quantitative structure–activity relationships (Altenberger et al. 2004; Van Leeuwen et al. 1996). Many organic contaminants fit this relationship well (Faust et al. 2000; Broderius et al. 1995). Contaminants with dissimilar modes of toxic action, on the other hand, are generally hypothesized to elicit independent joint toxicity, which occurs when the contaminants in a mix- ture have no influence on one otherÕs toxic effects, thus resulting in less than concentration-additive toxicity (Alten- berger et al. 2004; Faust et al. 2000; Cassee et al. 1998). Because 95% of all toxicity studies investigate single compounds (Yang 1994), the assumptions that concentration- additive or independent toxicity are representative of con- taminant mixtures (especially independence among dissimilar classes of contaminants) remain largely untested. Because metals and hydrocarbons are generally dissimilar in terms of their molecular structure and modes of toxic action, they are hypothesized to elicit independent toxic effects. However, a literature review of studies investigating metal–hydrocarbon mixtures (Gust 2005a) suggests that effects can be additive, independent, or interactive (synergistic or antagonistic). Twenty-six of 31 studies suggest that toxicity is interactive, with a compelling trend toward synergism (22 of 31; Gust 2005a). This is of concern because mixtures of heavy metals and polynuclear aromatic-hydrocarbons (PAHs) may be pre- valent in benthic and wetland sediments as a result of urban- ization and industrial contamination (Callender and Rice 2000, Van Metre et al. 2000, Sanger et al. 1999a, 1999b) which may lead to complex and deleterious environmental effects. Of additional concern, recent studies indicate that even if dissimilar chemicals elicit independent toxicological effects, contaminant mixtures may produce unexpected interactive effects facilitated via changes in environmental exposure. This may be considered an ‘‘apparent’’ interactive effect (Nor- wood et al. 2003). For example, Gust (2006) observed that Cd–Phen mixtures elicited synergistic lethal effects in the amphipod Hyalella azteca in sediment exposures, but not in Correspondence to: Kurt A. Gust; email: kurt.a.gust@erdc.usace. army.mil Arch. Environ. Contam. Toxicol. 51, 54–60 (2006) DOI: 10.1007/s00244-005-1075-7