title: Daphnia magna's sense of competition: intra-specific interactions (ISI) alter life history strategies and increase metals toxicity.
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.
authors.affiliation: Environmental Laboratory, US Army, Engineer Research and Development Center, Vicksburg, MS, USA. kurt.a.gust@usace.army.mil.
authors.forename: Kurt A
authors.initials: KA
authors.lastname: Gust
authors.affiliation: Environmental Laboratory, US Army, Engineer Research and Development Center, Vicksburg, MS, USA.
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Kennedy
authors.affiliation: Environmental Laboratory, US Army, Engineer Research and Development Center, Vicksburg, MS, USA.
authors.forename: Nicolas L
authors.initials: NL
authors.lastname: Melby
authors.affiliation: Environmental Laboratory, US Army, Engineer Research and Development Center, Vicksburg, MS, USA.
authors.forename: Mitchell S
authors.initials: MS
authors.lastname: Wilbanks
authors.affiliation: Environmental Laboratory, US Army, Engineer Research and Development Center, Vicksburg, MS, USA.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Laird
authors.affiliation: SpecPro Technical Services, San Antonio, TX, USA.
authors.forename: Barbara
authors.initials: B
authors.lastname: Meeks
authors.affiliation: Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA, USA.
authors.forename: Erik B
authors.initials: EB
authors.lastname: Muller
authors.affiliation: Department of Ecology, Evolution & Marine Biology, University of California, Santa Barbara, Santa Barbara, CA, USA.
authors.forename: Roger M
authors.initials: RM
authors.lastname: Nisbet
authors.affiliation: Environmental Laboratory, US Army, Engineer Research and Development Center, Vicksburg, MS, USA.
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Perkins
mesh_terms: D000818:Animals; D001681:Biological Assay; D003621:Daphnia / Q000187:drug effects* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D000071421:Life History Traits; D008670:Metals / Q000633:toxicity*; D014874:Water Pollutants, Chemical / Q000633:toxicity*
chemical_list: D008670:Metals; D014874:Water Pollutants, Chemical
keywords: Daphnia; Ecotoxicology; Intra-specific interactions; Metals toxicity; Standard toxicity assays