Centre for Biodiversity Dynamics (CBD), Department of Biology, Norwegian University of Science and Technology, Høgskoleringen 5, 7491 Trondheim, Norway.
Department of Biology, Norwegian University of Science and Technology, Høgskoleringen 5, 7491 Trondheim, Norway.
Sci Total Environ. 2021 Feb 10;755(Pt 2):143625. doi: 10.1016/j.scitotenv.2020.143625. Epub 2020 Nov 12.
Toxicity studies on freshwater organisms are commonly conducted by quantifying effects on asexual (clonal) reproductive rates in Daphnia, whereas studies of effects on sexual reproductive rates remain relatively rare. Sexual reproduction in Daphnia and the associated production of resting eggs allows them to survive unfavorable environmental conditions and is thus a crucial component of their long-term fitness. It also maintains genetic diversity within Daphnia populations and hence their potential for adaptation to new environmental conditions. This aspect of their biology may therefore be important to consider in toxicity studies. The aim of this study was to investigate for the first time how mercury (Hg) affects sexual versus asexual reproduction in Daphnia under varying environmental conditions. Specifically, we experimentally tested the interactive effects of Hg and temperature on the population dynamics of Daphnia magna. For this purpose, we exposed D. magna to environmentally relevant concentrations (0 μg/L, 0.5 μg/L and 2 μg/L) of Hg (in the form of mercury (II) chloride) found in stream water and measured biomass growth rate resulting from asexual reproduction, and resting egg production resulting from sexual reproduction. This was done at both 17 °C and 24 °C. Biomass growth rate did not vary across Hg treatments and depended mainly on temperature and population density. Density dependence of biomass growth rate was indeed more pronounced at 24 °C than at 17 °C, as resource limitation from intraspecific competition was further exacerbated by the rise in feeding rates with temperature. Density dependence of resting egg production was unaffected by Hg and temperature, but resting egg production was higher under Hg exposure at low temperature. These findings show that depending on environmental conditions, rates of sexual reproduction in D. magna may respond to metal exposure at lower concentrations than those impacting population growth during the asexual phase.
淡水生物的毒性研究通常通过量化对蚤类无性（克隆）繁殖率的影响来进行，而对有性繁殖率影响的研究仍然相对较少。蚤类的有性繁殖及其相关休眠卵的产生使它们能够在不利的环境条件下生存，因此是其长期适应能力的关键组成部分。它还维持了蚤类种群内的遗传多样性，从而提高了它们适应新环境条件的潜力。因此，在毒性研究中考虑它们生物学的这一方面可能很重要。本研究的目的是首次研究在不同环境条件下汞（Hg）如何影响蚤类的有性繁殖与无性繁殖。具体来说，我们通过实验测试了 Hg 和温度对大型蚤种群动态的相互作用影响。为此，我们将大型蚤暴露于环境相关浓度（0μg/L、0.5μg/L 和 2μg/L）的 Hg（以二价汞的形式）中，这些 Hg 存在于溪流水中，我们测量了由无性繁殖产生的生物量增长率，以及由有性繁殖产生的休眠卵产量。这项研究在 17°C 和 24°C 下进行。生物量增长率在 Hg 处理之间没有差异，主要取决于温度和种群密度。生物量增长率的密度依赖性在 24°C 时确实比 17°C 时更为明显，因为随着温度升高而导致的摄食率增加，进一步加剧了种内竞争造成的资源限制。休眠卵产量的密度依赖性不受 Hg 和温度的影响，但在低温下，Hg 暴露会增加休眠卵的产量。这些发现表明，根据环境条件的不同，大型蚤的有性繁殖率可能会对金属暴露的反应浓度低于无性繁殖阶段影响种群增长的浓度。
