Arctic copepod copper sensitivity and comparison with Antarctic and temperate copepods.

The ongoing global climate crisis increases temperatures in polar regions faster and with greater magnitude than elsewhere. The decline of Arctic sea ice opens up new passages, eventually leading to higher anthropogenic activities such as shipping, fishing, and mining. Climate change and anthropogenic activities will increase contaminant transport from temperate to Arctic regions. The shipping industry uses copper as an antifouling coating. Copper is an essential element but becomes toxic at excess concentrations, and its use may inadvertently affect non-target organisms such as copepods. Copper affects copepods by lowering reproductive output, prolonging developmental time, and causing increased mortality. As data on copper sensitivity of polar copepods at low temperatures are rare, we conducted onboard survival experiments with the Arctic region's most common copepod species (Calanus finmarchicus, C. glacialis, C. hyperboreus). Acute survival tests were done for up to 8 days on individuals in 70 ml bottles at 1 °C with nominal copper concentrations ranging from 3 to 480 µg L-1. We used a reduced General Unified Threshold model for Survival (GUTS) to analyse the data, and placed our results in the context of the few published copper sensitivity data of the Antarctic and temperate copepod species at low temperatures. The sensitivity of Cu exposure was similar between the three Calanus species. However, a model comparison suggests that the tested C. glacialis population is less sensitive than the other two species in our experiments. Compared to published data, the three Arctic species appear slightly less sensitive to copper compared to their Antarctic counterparts but more compared to their temperate ones. Our literature search revealed only a few available studies on the copper sensitivity of polar copepods. In the future, this species group will be exposed to more pollutants, which warrants more studies to predict potential risks, especially given possible interactions with environmental factors.

Drivers of copper sensitivity in copepods: A meta-analysis of LC50s.

Copper is both an essential trace element and a potent pesticide. The use of copper as an antifoulant has increased in the last decades in line with the expanding aquaculture and shipping industries. In aquatic environments, it also affects non-target taxa. One of which are copepods, which constitute the central link in the marine food web. Despite their ecological importance, there are no systematic reviews of the lethal concentration range and drivers of copper toxicity in this taxon. Here, we combined literature data from 31 peer-reviewed articles recording the Lethal Concentration 50 (LC50) for copper in copepods and the experiments' respective environmental, developmental, and taxonomic parameters. The LC50 is a traditional endpoint for toxicity testing used in standardized toxicity testing and many ecological studies. In total, we were able to extract 166 LC50 entries. The variability in the metadata allowed for a general analysis of the drivers of copper sensitivity in copepods. Using a generalized additive modeling approach, we find that temperature increases copper toxicity when above approximately 25℃. Counter to our expectations; salinity does not influence copper sensitivity across copepod species. Unsurprisingly, nauplii are more susceptible to copper exposure than adult copepods, and benthos-associated harpacticoids are less sensitive to copper than pelagic calanoids. Our final model can predict sensible specific-specific copper concentrations for future experiments, thus giving an informed analytical approach to range testing in future dose-response experiments. Our model can also potentially improve ecological risk assessment by accounting for environmental differences. The approach can be applied to other toxicants and taxa, which may reveal underlying patterns otherwise obscured by taxonomic and experimental variability.

Predation Risk Potentiates Toxicity of a Common Metal Contaminant in a Coastal Copepod.

To examine whether natural stressors like predation risk affect responses to anthropogenic contaminants, we exposed nauplii of the copepod Tigriopus brevicornis to chemical cues from fish (kairomones) and copper (Cu). We tested effects of these treatments, singly and combined, on copepod age and size at maturity, and development stage sensitivity, while controlling for effects of genetic heterogeneity (clutch identity). Predation risk, Cu and clutch identity interacted in their effect on development time. Predation risk alone had minor effects, but potentiated Cu toxicity in the combined treatment by doubling the delay in age at maturity, as compared to Cu exposure alone. This potentiating effect on developmental delay appeared already at the first copepodite stage. The specific strength of response varied among nauplii from different females' clutches. There were no differences in copepod size at maturity among treatments. We did, however, find an interaction between the effect of Cu and clutch identity on copepod growth. Our results demonstrate the importance of ecological interactions for potentiating the toxicity of environmental contaminants. We also demonstrate the need to consider genetic heterogeneity in ecotoxicology. Natural variation in stressor responses has implications for the interpretation of results from toxicological studies using single-clone or inbred culture populations.

Density-Dependent Metabolic Costs of Copper Exposure in a Coastal Copepod.

Traditional ecotoxicology methods involving copepods have focused on exposure of pooled individuals and averaged responses, but there is increasing awareness of the importance of individual variation. Many biological traits are density dependent, and decisions to use single-individual or pooled exposure may affect responses to anthropogenic stressors. We investigated how conspecific density as a biotic stressor affects behavioral and respiratory responses to copper (Cu) exposure in the coastal copepod Tigriopus brevicornis. Adults were incubated at densities of 1, 2, or 4 individuals per replicate in 3.2 mL of exposure medium (23 µg Cu L-1 or control). Our results show an interaction of Cu exposure and density on respiration. The Cu exposure increased respiration, but this effect diminished with increasing density. We also found reduced swimming activity with increasing density. We propose 2 nonexclusive alternative explanations for the density-dependent respiratory increase of Cu exposure: 1) a behavioral stress response to low conspecific density, or 2) increased Cu exposure due to increased swimming activity. We emphasize the importance of considering density-dependency in responses when designing and interpreting ecotoxicology studies. Environ Toxicol Chem 2021;40:2538-2546. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The Hidden Dimension: Context-Dependent Expression of Repeatable Behavior in Copepods.

In ecotoxicology and aquatic ecology, we often ignore responses of individuals and focus on average responses. However, both terrestrial and aquatic animals display consistent behavioral differences between individuals. The distribution of behavioral differences within a population contains vital information for predicting population responses to novel environmental challenges. Currently, individual data for behavioral and physiological traits of small marine invertebrates are few, partly because such variation is lost within published group means and assumed normality. We tested the combined effects of an inorganic contaminant (copper) and a biological stressor (i.e., chemical cues of a fish predator) on activity in a marine copepod. Although direct stress effects were weak, individuals behaved consistently differently, depending on the context. Individual differences in behavior were only expressed under the influence of kairomones, but not by copper exposure alone. This finding indicates that copepods express repeatable and context-dependent behavior. We also demonstrate how large variations in behavioral data can hide consistent differences between individuals. Environ Toxicol Chem 2020;39:1017-1026. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Contrasting Effects of Predation Risk and Copper on Copepod Respiration Rates.

Natural biotic and anthropogenic stressors can interact to alter contaminant toxicity. Energetic restrictions are potential mechanisms causing this pattern. To identify processes underlying observed effects of predation risk and copper (Cu) on delayed copepod age at maturity, we examined how these 2 stressors affect respiration rates. We tested 2 very different copepod species: the large, pelagic calanoid Calanus finmarchicus and the small, semibenthic harpacticoid Tigriopus brevicornis. Adult individuals were exposed for 12 h to the treatments: predation risk, Cu (23 µg L-1 ), combined predation risk and Cu (23 µg L-1 ), or control. Oxygen concentrations were monitored continuously. The 2 species differed in their responses. We found no clear effects of either stressor in C. finmarchicus. In T. brevicornis, predation risk increased respiration rates, whereas Cu alone had little impact. In contrast, combined exposure to predation risk and Cu interacted to reduce respiration rates to less than expected. We further observed an effect of sex because female-biased T. brevicornis replicates were more sensitive to both predation risk (increased respiration rates) and Cu exposure (reduced respiration rates). The present study provides further evidence that predation risk can interact with copepod responses toward Cu exposure. Interactive effects of biotic stressors ought to be considered to improve future marine environmental monitoring. Environ Toxicol Chem 2020;39:1765-1773. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

An affordable and automated imaging approach to acquire highly resolved individual data-an example of copepod growth in response to multiple stressors.

Individual trait variation is essential for populations to cope with multiple stressors and continuously changing environments. The immense number of possible stressor combinations and the influence of phenotypic variation makes experimental testing for effects on organisms challenging. The acquisition of such data requires many replicates and is notoriously laborious. It is further complicated when responses occur over short time periods. To overcome such challenges, we developed an automated imaging platform to acquire temporally highly resolved individual data. We tested this platform by exposing copepods to a combination of a biotic stressor (predator cues) and a toxicant (copper) and measured the growth response of individual copepods. We tested the automatically acquired data against published manually acquired data with much lower temporal resolution. We find the same general potentiating effects of predator cues on the adverse effects of copper, and the influence of an individual's clutch identity on its ability to resist stress, between the data obtained from low and high temporal resolution. However, when using the high temporal resolution, we also uncovered effects of clutch ID on the timing and duration of stage transitions, which highlights the importance of considering phenotypic variation in ecotoxicological testing. Phenotypic variation is usually not acknowledged in ecotoxicological testing. Our approach is scalable, affordable, and adjustable to accommodate both aquatic and terrestrial organisms, and a wide range of visually detectable endpoints. We discuss future extensions that would further widen its applicability.

Genotoxic Response and Mortality in 3 Marine Copepods Exposed to Waterborne Copper.

Copper (Cu) is an essential trace metal, but may also be toxic to aquatic organisms. Although many studies have investigated the cytotoxicity of Cu, little is known about the in vivo genotoxic potential of Cu in marine invertebrates. We investigated the genotoxicity of Cu in 2 pelagic calanoid copepods, Acartia tonsa and Temora longicornis, and the intertidal harpacticoid copepod Tigriopus brevicornis by exposing them for 6 and 72 h to waterborne Cu (0, 6, and 60 µg Cu/L). A subsequent 24-h period in filtered seawater was used to investigate delayed effects or recovery. Genotoxicity was evaluated as DNA strand breaks in individual copepods using the comet assay. Copper did not increase DNA strand breaks in any of the species at any concentration or time point. The treatment did, however, cause 100% mortality in A. tonsa following exposure to 60 µg Cu/L. Acartia tonsa and T. longicornis were more susceptible to Cu-induced mortality than the benthic harpacticoid T. brevicornis, which appeared to be unaffected by the treatments. The results show major differences in Cu susceptibility among the 3 copepods and also that acute toxicity of Cu to A. tonsa is not directly associated with genotoxicity. We also show that the comet assay can be used to quantify genotoxicity in individual copepods. Environ Toxicol Chem 2019;38:2224-2232. © 2019 SETAC.