Abstract
This work investigates whether the scale-up to multi-animal exposures that is commonly applied in genomics studies provides equivalent toxicity outcomes to single-animal experiments of standard Daphnia magna toxicity assays. Specifically, we tested the null hypothesis that intraspecific interactions (ISI) among D. magna have neither effect on the life history strategies of this species, nor impact toxicological outcomes in exposure experiments with Cu and Pb. The results show that ISI significantly increased mortality of D. magna in both Cu and Pb exposure experiments, decreasing 14 day LC50 s and 95 % confidence intervals from 14.5 (10.9–148.3) to 8.4 (8.2–8.7) µg Cu/L and from 232 (156–4810) to 68 (63–73) µg Pb/L. Additionally, ISI potentiated Pb impacts on reproduction eliciting a nearly 10-fold decrease in the no-observed effect concentration (from 236 to 25 µg/L). As an indication of environmental relevance, the effects of ISI on both mortality and reproduction in Pb exposures were sustained at both high and low food rations. Furthermore, even with a single pair of Daphnia, ISI significantly increased (p < 0.05) neonate production in control conditions, demonstrating that ISI can affect life history strategy. Given these results we reject the null hypothesis and conclude that results from scale-up assays cannot be directly applied to observations from single-animal assessments in D. magna. We postulate that D. magna senses chemical signatures of conspecifics which elicits changes in life history strategies that ultimately increase susceptibility to metal toxicity.Electronic supplementary materialThe online version of this article (doi:10.1007/s10646-016-1667-1) contains supplementary material, which is available to authorized users.
Highlights
Daphnia magna is an important model species in ecotoxicology for which standard assays have been developed for use in regulatory toxicity assessment (ASTM 2012; USEPA 2002)
We tested the null hypothesis that intraspecific interactions (ISI) among D. magna have neither effect on the life history strategies of this species, nor impact toxicological outcomes in exposure experiments with Cu and Pb
We reject the null hypothesis that ISI among D. magna neither impacts its life history strategies, nor affects toxicity assessment measures in Cu and Pb exposure experiments. These results, in addition to those of the published ‘‘Daphniaconditioned water’’ experiments refute the assumption that experiment scaling has no effect on toxicity outcomes in D. magna
Summary
Daphnia magna is an important model species in ecotoxicology for which standard assays have been developed for use in regulatory toxicity assessment (ASTM 2012; USEPA 2002). Over the past 10 years, Daphnia spp. have become increasingly utilized as genomic model organisms (Colbourne et al 2011) and used in toxicogenomic investigations to determine molecular and mechanistic effects of contaminant exposures (Ananthasubramaniam et al 2015). In order to meet minimum mRNA requirements for toxicogenomics methods, a common practice has been to scale up exposures to include 10–100 s of Daphnia per exposure replicate. This type of scale-up procedure has been applied in a number of toxicogenomics studies with D. magna We have found no published studies that have explicitly tested this critical assumption for D. magna in ecotoxicological exposures, in context with genomics investigations
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