A Re-Evaluation of Chironomid Deformities as an Environmental Stress Response: Avoiding Survivorship Bias and Testing Noncontaminant Biological Factors

Larval deformities have been observed in chironomids, and are thought to be associated with aquatic contaminant exposure. However, in laboratory assays, deformities have not been linked with contaminants in the absence of potential confounding variables including mortality, which introduces a survivorship bias. There is also a paucity of data on noncontaminant causes. In addition, power analyses are rarely undertaken, meaning that effect sizes detectable are usually uncertain. We therefore aimed to clarify factors associated with deformities, by running survivorship bias–free (i.e., sublethal) assays, assessing contaminant (copper and imidacloprid) and noncontaminant (malnutrition) stressors, and considering natural biological (metamorphosis) factors in Chironomus tepperi. We included a posteriori power analyses for all tests. Our assays found no significant association between tested factors and deformity rate. Power analyses indicated that the stressor experiment had moderate power to detect deformity effects. The metamorphosis assay had relatively lower power (due to an unexpectedly high control deformity rate), highlighting the importance of power tests in these types of evaluations. These results, in conjunction with others recently published, raise doubts as to the causal effects of environmental stressors on deformity incidence. By avoiding survivorship bias, and by testing noncontaminant factors and statistical power, we present a more holistic methodology, to resolve ongoing uncertainty in this area. We also discuss possible future directions for chironomid deformity research, and concerns regarding survivorship bias in ecotoxicology.