Plasticity of growth rate and metabolism inDaphnia magnapopulations from different thermal habitats

Rapid evolution of unimodal but not of linear thermal performance curves in Daphnia magna

Species may cope with warming through both rapid evolutionary and plastic responses. While thermal performance curves (TPCs), reflecting thermal plasticity, are considered powerful tools to understand the impact of warming on ectotherms, their rapid evolution has been rarely studied for multiple traits. We capitalized on a 2-year experimental evolution trial in outdoor mesocosms that were kept at ambient temperatures or heated 4°C above ambient, by testing in a follow-up common-garden experiment, for rapid evolution of the TPCs for multiple key traits of the water flea Daphnia magna. The heat-selected Daphnia showed evolutionary shifts of the unimodal TPCs for survival, fecundity at first clutch and intrinsic population growth rate toward higher optimum temperatures, and a less pronounced downward curvature indicating a better ability to keep fitness high across a range of high temperatures. We detected no evolution of the linear TPCs for somatic growth, mass and development rate, and for the traits related to energy gain (ingestion rate) and costs (metabolic rate). As a result, also the relative thermal slope of energy gain versus energy costs did not vary. These results suggest the overall (rather than per capita) top-down impact of D. magna may increase under rapid thermal evolution.

A common well pad does not imply common toxicity: Assessing the acute and chronic toxicity of flowback and produced waters from four Montney Formation wells on the same well pad to the freshwater invertebrate Daphnia magna

Large stores of previously inaccessible hydrocarbons have become available due to the development of hydraulic fracturing technologies. During the hydraulic fracturing process, a mixture of water and proprietary additives is injected into geologic formations to release trapped hydrocarbons. After fracturing, injected water and fluid from the target formation return to the surface as flowback and produced water (FPW), a potentially toxic byproduct of hydraulic fracturing activities. FPW is a complex mixture that contains chemical additives present in the initial injection fluid as well as salts, metals, and a variety of organic compounds. As a result, FPW composition can be highly variable across wells from different geological formations, methods of fracturing and well development, and well age. The present study sought to determine if FPW sourced from four wells (O, P, U, V) located on the same well pad within the Montney Formation have similar levels of acute and chronic toxicity to the freshwater invertebrate, Daphnia magna. Minimal differences in the estimated 48 h LC50 concentrations were observed among the studied wells. Long-term, 21 d exposures to ≤2% FPW revealed differences in the level of lethality between wells, including complete mortality in daphnids exposed to 2% well O by day 9. No sublethal effects were observed as a result of exposure to FPW from wells P, U or V; however, a large impairment of reproductive traits and molting behaviour were detected after exposure to 0.75% well O FPW. These results indicate that FPW sourced from wells on the same well pad cannot be considered the same in terms of chemical composition or toxicity, an important distinction to make for risk assessment practices.

Aerobic scope does matter in the temperature-size rule, but only under optimal conditions

We united theoretical predictions of the factors responsible for the evolutionary significance of the temperature-size rule (TSR). We assumed that (i) the TSR is a response to temperature-dependent oxic conditions, (ii) body size decrease is a consequence of cell shrinkage in response to hypoxia, (iii) this response enables organisms to maintain a wide scope for aerobic performance, and (iv) it prevents a decrease in fitness. We examined three clones of the rotifer Lecane inermis exposed to three experimental regimes: mild hypoxia, severe hypoxia driven by too high of a temperature, and severe hypoxia driven by an inadequate oxygen concentration. We compared the following traits in normoxia- and hypoxia-exposed rotifers: nuclear size (a proxy for cell size), body size, specific dynamic action (SDA, a proxy of aerobic metabolism) and two fitness measures, the population growth rate and eggs/female ratio. The results showed that (i) under mildly hypoxic conditions, our causative reasoning was correct, except that one of the clones decreased in body size without a decrease in nuclear size, and (ii) in more stressful environments, rotifers exhibited clone- and condition-specific responses, which were equally successful in terms of fitness levels. Our results indicate the importance of the testing conditions. The important conclusions were that (i) a body size decrease at higher temperatures enabled the maintenance of a wide aerobic scope under clone-specific, thermally optimal conditions, and (ii) this response was not the only option to prevent fitness reduction under hypoxia.

Interactive effects of extreme temperature and a widespread coastal metal contaminant reduce the fitness of a common tropical copepod across generations

Tropical coastal areas are increasingly exposed to temperature extremes from marine heatwaves and contaminants from anthropogenic activities. The interactive effects of these environmental changes on marine life are understudied. We investigated the direct and cross-generational effects of copper (Cu) on F0 and F1 generations of the common tropical copepod Pseudodiaptomus annandalei under extreme temperatures (30 and 34 °C). In F0, Cu exposure reduced survival and nauplii production; these patterns were more pronounced at 34 °C and in females. F0 Copepods produced more faecal pellets at 34 °C than 30 °C, indicating a higher energetic demand. In F1, the number of F1 adults was lower in CuF0 and at 34 °C. Cu-exposed F0 produced larger adult F1, while exposure to 34 °C resulted in smaller adult F1. Our results show that tropical copepods are highly vulnerable to the interactive effects of contaminants and extreme temperatures.

Intraspecific variation in resource use is not explained by population persistence or seasonality

Populations of generalist grazers often contain genotypes with "powerful" and "efficient" strategies. Powerful genotypes grow rapidly on rich-quality resources, but slowly on poorer-quality ones, while efficient genotypes grow relatively better on poorer resources but cannot exploit richer resources as well. Via a "power-efficiency" trade-off, variation in resource quality could maintain genetic diversity. To evaluate this mechanism, we sampled six populations of the freshwater cladoceran Daphnia pulicaria. In persisting (year-round) populations, Daphnia consume resources that vary in quality, whereas in non-persisting (spring-only) populations, Daphnia primarily encounter rich-quality resources. We hypothesized that non-persisting populations harbor no efficient clones (hence should show lower growth on poor-quality resources). Although individuals from non-persisting populations remained smaller than individuals from persisting populations, no evidence arose for a trade-off between powerful and efficient strategies. In fact, growth rates on the two diets were positively correlated (instead of negatively, as predicted). Furthermore, in the persisting populations, we predicted that clonal selection from spring to summer should shift the distribution of genotypes from powerful (specialists on richer spring resources) to efficient (poorer, summer resources). Genetic composition of populations shifted from spring to summer, but not toward more efficient genotypes. Therefore, in these lakes, maintenance of variation among genotypes must stem from more complicated factors than population persistence patterns or seasonal shifts in resource quality alone.

Physiological endpoints in daphnid acute toxicity tests

Daphnids are freshwater crustaceans used in toxicity tests. Although lethality and immobilisation are the most commonly used endpoints in those tests, more sensitive parameters are required for determination of sublethal acute effects of toxicants. The use of various physiological endpoints in daphnids is considered as a low-cost and simple alternative that meets the 3R's rule (Replacement, Reduction, Refinement) criteria. However, currently there is no review-based evaluation of their applicability in toxicity testing. This paper presents the results on the most commonly determined physiological parameters of Daphnia in ecotoxicological studies and human drug testing, such as feeding activity, thoracic limb movement, heart rate, cardiac area, respiratory activity, compound eye, mandible movements and post-abdominal claw contractions. Furthermore, their applicability as promising endpoints in the assessment of water quality or drug testing is discussed.

The temperature-size rule in Daphnia magna across different genetic lines and ontogenetic stages: Multiple patterns and mechanisms

Ectotherms tend to grow faster, but reach a smaller size when reared under warmer conditions. This temperature-size rule (TSR) is a widespread phenomenon. Despite the generality of this pattern, no general explanation has been found. We therefore tested the relative importance of two proposed mechanisms for the TSR: (1) a stronger increase in development rate relative to growth rate at higher temperatures, which would cause a smaller size at maturity, and (2) resource limitation placing stronger constraints on growth in large individuals at higher temperatures, which would cause problems with attaining a large size in warm conditions. We raised Daphnia magna at eight temperatures to assess their size at maturity, asymptotic size, and size of their offspring. We used three clonal lines that differed in asymptotic size and growth rate. A resource allocation model was developed and fitted to our empirical data to explore the effect of both mechanisms for the TSR. The genetic lines of D. magna showed different temperature dependence of growth and development rates resulting in different responses for size at maturity. Also, at warm temperatures, growth was constrained in large, but not in small individuals. The resource allocation model could fit these empirical data well. Based on our empirical results and model explorations, the TSR of D. magna at maturity is best explained by a stronger increase in development rate relative to growth rate at high temperature, and the TSR at asymptotic size is best explained by a size-dependent and temperature-dependent constraint on growth, although resource limitation could also affect size at maturity. In conclusion, the TSR can take different forms for offspring size, size at maturity, and asymptotic size and each form can arise from its own mechanism, which could be an essential step toward finding a solution to this century-old puzzle.

Sensitivity of a tropical micro-crustacean (Daphnia lumholtzi) to trace metals tested in natural water of the Mekong River

Metal contamination is one of the major issues to the environment worldwide, yet it is poorly known how exposure to metals affects tropical species. We assessed the sensitivity of a tropical micro-crustacean Daphnia lumholtzi to three trace metals: copper (Cu), zinc (Zn) and nickel (Ni). Both, acute and chronic toxicity tests were conducted with metals dissolved in in situ water collected from two sites in the lower part of the Mekong River. In the acute toxicity test, D. lumholtzi neonates were exposed to Cu (3-30μgL^-1), Zn (50-540μgL^-1) or Ni (46-2356μgL^-1) for 48h. The values of median lethal concentrations (48h-LC₅₀) were 11.57-16.67μg Cu L^-1, 179.3-280.9μg Zn L^-1, and 1026-1516μg Ni L^-1. In the chronic toxicity test, animals were exposed to Cu (3 and 4μgL^-1), Zn (50 and 56μgL^-1), and Ni (six concentrations from 5 to 302μgL^-1) for 21days. The concentrations of 4μg Cu L^-1 and 6μg Ni L^-1 enhanced the body length of D. lumholtzi but 46μg Ni L^-1 and 50μg Zn L^-1 resulted in a strong mortality, reduced the body length, postponed the maturation, and lowered the fecundity. The results tentatively suggest that D. lumholtzi showed a higher sensitivity to metals than related species in the temperate region. The results underscore the importance of including the local species in ecological risk assessment in important tropical ecosystems such as the Mekong River to arrive at a better conservational and management plan and regulatory policy to protect freshwater biodiversity from metal contamination.

Toxicity of two fungicides in Daphnia: is it always temperature-dependent?

The joint effect of increasing temperature and pollution on aquatic organisms is important to understand and predict, as a combination of stressors might be more noxious when compared to their individual effects. Our goal was to determine the sensitivity of a model organism (Daphnia spp.) to contaminants at increasing temperatures, allowing prior acclimation of the organisms to the different temperatures. Prior to exposure, two Daphnia genotypes (Daphnia longispina species complex) were acclimated to three temperatures (17, 20, and 23 °C). Afterwards, a crossed design was established using different exposure temperatures and a range of concentrations of two common fungicides (tebuconazole and copper). Daphnia life history parameters were analysed in each temperature × toxicant combination for 21 days. Temperature was the most influencing factor: Daphnia reproduced later and had lower fecundity at 17 °C than at 20 and 23 °C. Both copper and tebuconazole also significantly reduced the fecundity and survival of Daphnia at environmentally-relevant concentrations. Temperature-dependence was found for both toxicants, but the response pattern was endpoint- and genotype-specific. The combination of contaminant and high temperature often had severe effects on survival. However, unlike some literature on the subject, our results do not support the theory that increasing temperatures consistently foment increasing reproductive toxicity. The absence of a clear temperature-dependent toxicity pattern may result from the previous acclimation to the temperature regime. However, a proper framework is lacking to compare such studies and to avoid misleading conclusions for climate change scenarios.

Progressive acclimation alters interaction between salinity and temperature in experimental Daphnia populations

Environmental stressors rarely act in isolation, giving rise to interacting environmental change scenarios. However, the impacts of such interactions on natural populations must consider the ability of organisms to adapt to environmental changes. The phenotypic adaptability of a Daphnia galeata clone to temperature rise and salinisation was investigated in this study, by evaluating its halotolerance at two different temperatures, along a short multigenerational acclimation scenario. Daphniids were acclimated to different temperatures (20°C and 25°C) and salinities (0gL(-1) and 1gL(-1), using NaCl as a proxy) in a fully crossed design. The objective was to understand whether acclimation to environmental stress (combinations of temperature and salinity) influenced the response to the latter exposure to these stressors. We hypothesize that acclimation to different temperature×salinity regimes should elicit an acclimation response of daphniids to saline stress or its interaction with temperature. Acute (survival time) and chronic (juvenile growth) halotolerance measures were obtained at discrete timings along the acclimation period (generations F1, F3 and F9). Overall, exposure temperature was the main determinant of the acute and chronic toxicity of NaCl: daphniid sensitivity (measured as the decrease of survival time or juvenile growth) was consistently higher at the highest temperature, irrespective of background conditions. However, this temperature-dependent effect was nullified after nine generations, but only when animals had been acclimated to both stressors (high salinity and high temperature). Such complex interaction scenarios should be taken in consideration in risk assessment practices.

Adaptive phenotypic plasticity and local adaptation for temperature tolerance in freshwater zooplankton

Many organisms have geographical distributions extending from the tropics to near polar regions or can experience up to 30°C temperature variation within the lifespan of an individual. Two forms of evolutionary adaptation to such wide ranges in ambient temperatures are frequently discussed: local adaptation and phenotypic plasticity. The freshwater planktonic crustacean Daphnia magna, whose range extends from South Africa to near arctic sites, shows strong phenotypic and genotypic variation in response to temperature. In this study, we use D. magna clones from 22 populations (one clone per population) ranging from latitude 0° (Kenya) to 66° North (White Sea) to explore the contributions of phenotypic plasticity and local adaptation to high temperature tolerance. Temperature tolerance was studied as knockout time (time until immobilization, T(imm)) at 37°C in clones acclimatized to either 20°C or 28°C. Acclimatization to 28°C strongly increased T(imm), testifying to adaptive phenotypic plasticity. At the same time, Timm significantly correlated with average high temperature at the clones' sites of origin, suggesting local adaptation. As earlier studies have found that haemoglobin expression contributes to temperature tolerance, we also quantified haemoglobin concentration in experimental animals and found that both acclimatization temperature (AccT) and temperature at the site of origin are positively correlated with haemoglobin concentration. Furthermore, Daphnia from warmer climates upregulate haemoglobin much more strongly in response to AccT, suggesting local adaptation for plasticity in haemoglobin expression. Our results show that both local adaptation and phenotypic plasticity contribute to temperature tolerance, and elucidate a possible role of haemoglobin in mediating these effects that differs along a cold-warm gradient.

Evolutionary and plastic responses of freshwater invertebrates to climate change: realized patterns and future potential

We integrated the evidence for evolutionary and plastic trait changes in situ in response to climate change in freshwater invertebrates (aquatic insects and zooplankton). The synthesis on the trait changes in response to the expected reductions in hydroperiod and increases in salinity indicated little evidence for adaptive, plastic, and genetic trait changes and for local adaptation. With respect to responses to temperature, there are many studies on temporal trait changes in phenology and body size in the wild that are believed to be driven by temperature increases, but there is a general lack of rigorous demonstration whether these trait changes are genetically based, adaptive, and causally driven by climate change. Current proof for genetic trait changes under climate change in freshwater invertebrates stems from a limited set of common garden experiments replicated in time. Experimental thermal evolution experiments and common garden warming experiments associated with space-for-time substitutions along latitudinal gradients indicate that besides genetic changes, also phenotypic plasticity and evolution of plasticity are likely to contribute to the observed phenotypic changes under climate change in aquatic invertebrates. Apart from plastic and genetic thermal adjustments, also genetic photoperiod adjustments are widespread and may even dominate the observed phenological shifts.