Daphnia magna’s sense of competition: intra-specific interactions (ISI) alter life history strategies and increase metals toxicity
Ecotoxicology
Daphnia magna's sense of competition: intra-specific interactions (ISI) alter life history strategies and increase metals toxicity
Kurt A. Gust 0 1 2 3 4
Alan J. Kennedy 0 1 2 3 4
Nicolas L. Melby 0 1 2 3 4
Mitchell S. Wilbanks 0 1 2 3 4
Jennifer Laird 0 1 2 3 4
Barbara Meeks 0 1 2 3 4
Erik B. Muller 0 1 2 3 4
Roger M. Nisbet 0 1 2 3 4
Edward J. Perkins 0 1 2 3 4
0 SpecPro Technical Services , San Antonio, TX , USA
1 Environmental Laboratory , US Army , Engineer Research and Development Center , Vicksburg, MS , USA
2 & Kurt A. Gust
3 Department of Ecology, Evolution & Marine Biology, University of California , Santa Barbara, Santa Barbara, CA , USA
4 Marine Science Institute, University of California , Santa Barbara, Santa Barbara, CA , USA
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) lg Cu/L and from 232 (156-4810) to 68 (63-73) lg Pb/L. Additionally, ISI potentiated Pb impacts on reproduction eliciting a nearly 10-fold decrease in the noobserved effect concentration (from 236 to 25 lg/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
Daphnia; Intra-specific interactions; Ecotoxicology; Metals toxicity; Standard toxicity assays
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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.
Introduction
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
(i.e.
Stanley et al. 2013; Campos et al. 2013; Garcia-Reyero
et al. 2009, 2012; Poynton et al. 2007; Shaw et al. 2007)
where the expression results were directly applied to
understand the results observed in standard-scale, single
animal exposures. In order to draw these inferences among
exposure methods, the authors have made the assumption
that exposure scaling has no effect on the outcome of the
test. Thus far, we have found no published studies that have
explicitly tested this critical assumption for D. magna in
ecotoxicological exposures, in context with genomics
investigations.
When scaling up a toxicity assay such as the standard D.
magna reproduction test
(ASTM 2012)
from a single
animal to multi-animal exposure experiment, it becomes
logistically challenging to quantify reproductive output.
Note that the cited ASTM method allows the
experimentalist discretion to test either signal or multiple organisms
within a single replicate chamber. Thus, many researchers
run scale-up toxicity assays in parallel, where the latter is
used to quantify reproduction and the former to examine
toxicogenomic effects. However, animals exposed in the
scale-up tests could potentially experience intra-specific
competition, whereas individuals in the standard assays
would not. The adverse effects of contaminant exposure on
survival, reproduction and/or population structure can be
exacerbated by intraspecific competition among Daphnia
(Knillmann et al. 2012; Foit et al. 2012; Liess and Foit
2010; Viaene et al. 2015)
. In this context, researchers are
often careful to scale up both exposure volume and food
per individual to help minimize the potential for
intraspecific competition among Daphnia in the
multi-animal exposures, thus minimizing confounding effects of
intraspecifi (...truncated)