An International Perspective on the Tools and Concepts for Effluent Toxicity Assessments in the Context of Animal Alternatives: Reduction in Vertebrate Use.

Since the 1940s, effluent toxicity testing has been used to assess potential ecological impacts of effluents and help determine necessary treatment options for environmental protection prior to release. Strategic combinations of toxicity tests, analytical tools, and biological monitoring have been developed. Because the number of vertebrates utilized in effluent testing is thought to be much greater than that used for individual chemical testing, there is a new need to develop strategies to reduce the numbers of vertebrates (i.e., fish) used. This need will become more critical as developing nations begin to use vertebrates in toxicity tests to assess effluent quality. A workshop was held to 1) assess the state of science in effluent toxicity testing globally; 2) determine current practices of regulators, industry, private laboratories, and academia; and 3) explore alternatives to vertebrate (fish) testing options and the inclusion of modified/new methods and approaches in the regulatory environment. No single approach was identified, because of a range of factors including regulatory concerns, validity criteria, and wider acceptability of alternatives. However, a suite of strategies in a weight-of-evidence approach would provide the flexibility to meet the needs of the environment, regulators, and the regulated community; and this "toolbox" approach would also support reduced reliance on in vivo fish tests. The present Focus article provides a brief overview of wastewater regulation and effluent testing approaches. Alternative methodologies under development and some of the limitations and barriers to regulatory approaches that can be selected to suit individual country and regional requirements are described and discussed. Environ Toxicol Chem 2018;37:2745-2757. © 2018 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Estimating incident ultraviolet radiation exposure in the northern Gulf of Mexico during the Deepwater Horizon oil spill.

Millions of barrels of oil were released into the Gulf of Mexico following the 2010 explosion of the Deepwater Horizon oil rig. Polycyclic aromatic hydrocarbons (PAHs) are toxic components of crude oil, which may become more toxic in the presence of ultraviolet (UV) radiation, a phenomenon known as photo-induced toxicity. The Deepwater Horizon spill impacted offshore and estuarine sites, where biota may be co-exposed to UV and PAHs. Penetration of UV into the water column is affected by site-specific factors. Therefore, measurements and/or estimations of UV are necessary when one is assessing the risk to biota posed by photo-induced toxicity. We describe how estimates of incident UV were determined for the area impacted by the Deepwater Horizon oil spill, using monitoring data from radiometers near the spill, in conjunction with reference spectra characterizing the composition of solar radiation. Furthermore, we provide UV attenuation coefficients for both near- and offshore sites in the Gulf of Mexico. These estimates are specific to the time and location of the spill, and fall within the range of intensities utilized during photo-induced toxicity tests performed in support of the Deepwater Horizon Natural Resource Damage Assessment (NRDA). These data further validate the methodologies and findings of phototoxicity tests included in the Deepwater Horizon NRDA, while underscoring the importance of considering UV exposure when assessing possible risks following oil spills. Environ Toxicol Chem 2018;37:1679-1687. © 2018 SETAC.

Review of the photo-induced toxicity of environmental contaminants.

Solar radiation is a vital component of ecosystem function. However, sunlight can also interact with certain xenobiotic compounds in a phenomenon known as photo-induced, photo-enhanced, photo-activated, or photo-toxicity. This phenomenon broadly refers to an interaction between a chemical and sunlight resulting in increased toxicity. Because most aquatic ecosystems receive some amount of sunlight, co-exposure to xenobiotic chemicals and solar radiation is likely to occur in the environment, and photo-induced toxicity may be an important factor impacting aquatic ecosystems. However, photo-induced toxicity is not likely to be relevant in all aquatic systems or exposure scenarios due to variation in important ecological factors as well as physiological adaptations of the species that reside there. Here, we provide an updated review of the state of the science of photo-induced toxicity in aquatic ecosystems.

Characterization of basic immune function parameters in the fathead minnow (Pimephales promelas), a common model in environmental toxicity testing.

The fathead minnow (Pimephales promelas) is an environmental sentinel species, commonly used in toxicity testing. However, there is a lack of data regarding basic immune function in this species. To improve the usefulness of the fathead minnow as a model for basic immune function and immunotoxicity, this study sought to 1) compare the differential expression of immune function genes in naïve fathead minnows and 2) determine the effects of pathogen exposure on immune gene expression and spleen index. To accomplish this, kidney, spleen and liver tissue were collected three days post injection (dpi) from adult male fathead minnows from each of the following groups: 1) uninjected control 2) sham-injected (Hank's balanced salt solution) and 3) pathogen-injected (Yersinia ruckeri). Spleen tissue was also collected at seven and 14 dpi. Differential tissue expression of immune function genes was evaluated in naïve minnows and expression patterns were similar to those found in other fish species, with liver tissue generally having the highest amount of expression. Following pathogen injection, the expression of complement component 3 (c3) (4.4-fold, kidney; 2.5-fold, liver), interleukin 11 (il11) (4.8-fold, kidney; 15.2-fold, liver) and interleukin 1ß (il1ß) (8.2-fold, kidney; 17.2-fold, spleen; 2.6-fold, liver) were significantly upregulated. Elastase 2 (elas2) was significantly downregulated (5.8-fold) in liver tissue. A significant increase in spleen index at seven dpi was also observed in pathogen-injected minnows. This study has identified endpoints that are part of the normal response to pathogen in fathead minnows, an essential step toward the development of the fathead minnow as a model for immunotoxicity evaluations.

Photo-induced toxicity of Deepwater Horizon slick oil to blue crab (Callinectes sapidus) larvae.

The 2010 Deepwater Horizon oil spill resulted in the accidental release of approximately 700 million L of crude oil into the Gulf of Mexico. Photo-induced toxicity after co-exposure to ultraviolet (UV) radiation is 1 mechanism by which polycyclic aromatic hydrocarbons (PAHs) from oil spills may exert toxicity. Blue crab are an important commercial and ecological resource in the Gulf of Mexico, and their largely transparent larvae may make them sensitive to PAH photo-induced toxicity. The goal of the present study was to examine the sensitivity of early lifestage blue crab (Callinectes sapidus) zoea to slick oil collected during the Deepwater Horizon spill. Blue crab zoea were exposed to 1 of several dilutions of water accommodated fractions from 1 of 2 sources of oil and gradations of natural sunlight in a factorial design. Two 7-h solar exposures were carried out with a recovery period (dark) in between. Survival was found to be UV- and PAH-dependent. Toxicity was observed within the range of surface PAH concentrations reported in the Gulf of Mexico during the Deepwater Horizon spill. These findings indicate that early lifestage blue crab are sensitive to photo-induced toxicity from Deepwater Horizon slick oil.

Methylmercury Bioaccumulation in Stream Food Webs Declines with Increasing Primary Production.

Opposing hypotheses posit that increasing primary productivity should result in either greater or lesser contaminant accumulation in stream food webs. We conducted an experiment to evaluate primary productivity effects on MeHg accumulation in stream consumers. We varied light for 16 artificial streams creating a productivity gradient (oxygen production =0.048-0.71 mg O2 L(-1) d(-1)) among streams. Two-level food webs were established consisting of phytoplankton/filter feeding clam, periphyton/grazing snail, and leaves/shredding amphipod (Hyalella azteca). Phytoplankton and periphyton biomass, along with MeHg removal from the water column, increased significantly with productivity, but MeHg concentrations in these primary producers declined. Methylmercury concentrations in clams and snails also declined with productivity, and consumer concentrations were strongly correlated with MeHg concentrations in primary producers. Heterotroph biomass on leaves, MeHg in leaves, and MeHg in Hyalella were unrelated to stream productivity. Our results support the hypothesis that contaminant bioaccumulation declines with stream primary production via the mechanism of bloom dilution (MeHg burden per cell decreases in algal blooms), extending patterns of contaminant accumulation documented in lakes to lotic systems.

The fish embryo toxicity test as a replacement for the larval growth and survival test: A comparison of test sensitivity and identification of alternative endpoints in zebrafish and fathead minnows.

The fish embryo toxicity (FET) test has been proposed as an alternative to the larval growth and survival (LGS) test. The objectives of the present study were to evaluate the sensitivity of the FET and LGS tests in fathead minnows (Pimephales promelas) and zebrafish (Danio rerio) and to determine if the inclusion of sublethal metrics as test endpoints could enhance test utility. In both species, LGS and FET tests were conducted using 2 simulated effluents. A comparison of median lethal concentrations determined via each test revealed significant differences between test types; however, it could not be determined which test was the least and/or most sensitive. At the conclusion of each test, developmental abnormalities and the expression of genes related to growth and toxicity were evaluated. Fathead minnows and zebrafish exposed to mock municipal wastewater-treatment plant effluent in a FET test experienced an increased incidence of pericardial edema and significant alterations in the expression of genes including insulin-like growth factors 1 and 2, heat shock protein 70, and cytochrome P4501A, suggesting that the inclusion of these endpoints could enhance test utility. The results not only show the utility of the fathead minnow FET test as a replacement for the LGS test but also provide evidence that inclusion of additional endpoints could improve the predictive power of the FET test.

A comparison of commercially-available automated and manual extraction kits for the isolation of total RNA from small tissue samples.

BACKGROUND: This study compared the performance of five commercially available kits in extracting total RNA from small eukaryotic tissue samples (<15 mg). Total RNA was isolated from fathead minnow (Pimephales promelas) tissues (spleen, blood, kidney, embryo, and larvae) using the Qiagen RNeasy® Plus Mini, Qiagen RNeasy® Plus Universal, Promega Maxwell® 16 LEV simplyRNA, Ambion MagMAX™-96 and Promega SimplyRNA HT kits. Kit performance was evaluated via measures of RNA quantity (e.g., total RNA amount) and quality (e.g., ratio of absorbance at 260 and 280 nm, RNA integrity number (RIN), presence of gDNA). RESULTS: With the exception of embryos, each kit generally extracted ≥5 µg of total RNA from each sample. With regard to RNA quality, the RINs of RNA samples isolated via the Plus Mini and Maxwell® 16 kits were consistently higher than those of samples extracted via the remaining three kits and for all tissues, these kits produced intact RNA with average RIN values ≥7. The Plus Universal and SimplyRNA HT kits produced moderately degraded (RIN values <7, but ≥5), while the RNA recovered via the MagMAX™ kit tended to exhibit a high degree of degradation (RIN values <5). CONCLUSIONS: Each kit was generally capable of extracting the amount of RNA required for most downstream gene expression applications suggesting that RNA yield is unlikely to be a limiting factor for any of the kits evaluated. However, differences in the quality of RNA extracted via each of the kits indicate that these kits may differ in their ability to yield RNA acceptable for some applications. Overall, the findings of this study demonstrate that there are practical differences between commercially available RNA extraction kits that should be taken into account when selecting extraction methods to be used for isolating RNA designated for gene expression analysis.

Alternative methods for toxicity assessments in fish: comparison of the fish embryo toxicity and the larval growth and survival tests in zebrafish and fathead minnows.

An increased demand for chemical toxicity evaluations has resulted in the need for alternative testing strategies that address animal welfare concerns. The fish embryo toxicity (FET) test developed for zebrafish (Danio rerio) is one such alternative, and the application of the FET test to other species such as the fathead minnow (Pimephales promelas) has been proposed. In the present study, the performances of the FET test and the larval growth and survival (LGS; a standard toxicity testing method) test in zebrafish and fathead minnows were evaluated. This required that testing methods for the fathead minnow FET and zebrafish LGS tests be harmonized with existing test methods and that the performance of these testing strategies be evaluated by comparing the median lethal concentrations of 2 reference toxicants, 3,4-dicholoraniline and ammonia, obtained via each of the test types. The results showed that procedures for the zebrafish FET test can be adapted and applied to the fathead minnow. Differences in test sensitivity were observed for 3,4-dicholoraniline but not ammonia; therefore, conclusions regarding which test types offer the least or most sensitivity could not be made. Overall, these results show that the fathead minnow FET test has potential as an alternative toxicity testing strategy and that further analysis with other toxicants is warranted in an effort to better characterize the sensitivity and feasibility of this testing strategy.

Acute photo-induced toxicity and toxicokinetics of single compounds and mixtures of polycyclic aromatic hydrocarbons in zebrafish.

The present study examined photo-induced toxicity and toxicokinetics for acute exposure to selected polycyclic aromatic hydrocarbons (PAHs) in zebrafish. Photo-enhanced toxicity from co-exposure to ultraviolet (UV) radiation and PAHs enhanced the toxicity and exhibited toxic effects at PAH concentrations orders of magnitude below effects observed in the absence of UV. Because environmental exposure to PAHs is usually in the form of complex mixtures, the present study examined the photo-induced toxicity of both single compounds and mixtures of PAHs. In a sensitive larval life stage of zebrafish, acute photo-induced median lethal concentrations (LC50s) were derived for 4 PAHs (anthracene, pyrene, carbazole, and phenanthrene) to examine the hypothesis that phototoxic (anthracene and pyrene) and nonphototoxic (carbazole and phenanthrene) pathways of mixtures could be predicted from single exposures. Anthracene and pyrene were phototoxic as predicted; however, carbazole exhibited moderate photo-induced toxicity and phenanthrene exhibited weak photo-induced toxicity. The toxicity of each chemical alone was used to compare the toxicity of mixtures in binary, tertiary, and quaternary combinations of these PAHs, and a predictive model for environmental mixtures was generated. The results indicated that the acute toxicity of PAH mixtures was additive in phototoxic scenarios, regardless of the magnitude of photo-enhancement. Based on PAH concentrations found in water and circumstances of high UV dose to aquatic systems, there exists potential risk of photo-induced toxicity to aquatic organisms.

Microscopic examination of skin in native and nonnative fish from Lake Tahoe exposed to ultraviolet radiation and fluoranthene.

The presence of nonnative species in Lake Tahoe (CA/NV), USA has been an ongoing concern for many decades, and the management of these species calls for an understanding of their ability to cope with the Lake's stressors and for an understanding of their potential to out-compete and reduce the populations of native species. Decreasing levels of ultraviolet radiation (UVR) due to eutrophication and increasing levels of phototoxic polycyclic aromatic hydrocarbons (PAHs) due to recreational activities may combine to affect the relative ability of native versus nonnative fish species to survive in the lake. Following a series of toxicity tests which exposed larvae of the native Lahontan redside minnow (Richardsonius egregius) and the nonnative warm-water bluegill sunfish (Lepomis macrochirus) to UVR and FLU, the occurrence of skin damage and/or physiologic defense mechanisms were studied using multiple microscopic techniques. The native minnow appeared to exhibit fewer instances of skin damage and increased instances of cellular coping mechanisms. This study supports the results of previous work conducted by the authors, who determined that the native redside minnow is the more tolerant of the two species, and that setting and adhering to a water quality standard for UVR transparency may aid in preventing the spread of the less tolerant nonnative bluegill and similar warm-water species.

Quantitative risk model for polycyclic aromatic hydrocarbon photoinduced toxicity in Pacific herring following the Exxon Valdez oil spill.

Phototoxicity occurs when exposure to ultraviolet radiation increases the toxicity of certain contaminants, including polycyclic aromatic hydrocarbons (PAHs). This study aimed to (1) develop a quantitative model to predict the risk of PAH phototoxicity to fish, (2) assess the predictive value of the model, and (3) estimate the risk of PAH phototoxicity to larval and young of year Pacific herring (Clupea pallasi) following the Exxon Valdez oil spill (EVOS) in Prince William Sound, Alaska. The model, in which median lethal times (LT50 values) are estimated from whole-body phototoxic PAH concentrations and ultraviolet A (UVA) exposure, was constructed from previously reported PAH phototoxicity data. The predictive value of this model was confirmed by the overlap of model-predicted and experimentally derived LT50 values. The model, along with UVA characterization data, was used to generate estimates for depths of de minimiz risk for PAH phototoxicity in young herring in 2003/2004 and immediately following the 1989 EVOS, assuming average and worst case conditions. Depths of de minimiz risk were estimated to be between 0 and 2 m deep when worst case UVA and PAH conditions were considered. A post hoc assessment determined that <1% of the young herring population would have been present at depths associated with significant risk of PAH phototoxicity in 2003/2004 and 1989.

Bayesian approach to potency estimation for aquatic toxicology experiments when a toxicant affects both fecundity and survival.

Chemicals in aquatic systems may impact a variety of endpoints including mortality, growth, or reproduction. Clearly, growth or reproduction will only be observed in organisms that survive. Because it is common to observe mortality in studies focusing on the reproduction of organisms, especially in higher concentration conditions, the resulting observed numbers of young become a mixture of zeroes and positive counts. Zeroes are recorded for organisms that die before having any young and living organisms with no offspring. Positive counts are recorded for living organisms with offspring. Thus, responses reflect both fecundity and mortality of the organisms used in such tests. In the present study, the authors propose the estimation of the concentration associated with a specified level of reproductive inhibition (RIp) using a Bayesian zero-inflated Poisson (ZIP) regression model. This approach allows any prior information and expert knowledge about the model parameters to be incorporated into the regression coefficients or RIp estimation. Simulation studies are conducted to compare the Bayesian ZIP regression model and classical methods. The Bayesian estimator outperforms the frequentist alternative by producing more precise point estimates with smaller mean square differences between RIp estimates and true values, narrower interval estimates with better coverage probabilities. The authors also applied their proposed model to a study of Ceriodaphnia dubia exposed to a test toxicant.

Differential tolerance of native and nonnative fish exposed to ultraviolet radiation and fluoranthene in Lake Tahoe (California/Nevada), USA.

Within Lake Tahoe (CA/NV), USA, multiple environmental stressors are present that can affect both native and nonnative fish species. Stressors include natural ultraviolet radiation (UVR) and polycyclic aromatic hydrocarbons (PAHs). Many PAHs, such as fluoranthene (FLU) are phototoxic to aquatic organisms in the presence of UVR. Decreasing levels of UVR due to eutrophication and increasing levels of PAHs due to recreational activities may combine to affect the relative ability of native versus nonnative fish species to survive in the lake. The objective of the present study was to examine the differential effects of exposure to different levels of UVR and phototoxic FLU in native and nonnative fish species. Responses to these changes in the native Lahontan redside minnow (Richardsonius egregius) and the nonnative warm-water bluegill sunfish (Lepomis macrochirus) were compared during toxicity tests, which were conducted in controlled outdoor exposures. Physiological defenses were also investigated in an attempt to elucidate ways each species may tolerate UVR and UVR + FLU exposures. It was determined that the native redside minnow is more tolerant to UVR and UVR + FLU exposure when compared to the nonnative bluegill. In addition, a natural UVR coping mechanism, increased pigmentation, is exhibited to a greater extent in the native redside. The present study will help determine the potential for a future successful invasion of the bluegill and similar species in Lake Tahoe and other oligotrophic, montane lakes that are susceptible to habitat alteration, nutrient inputs, and recreational activity.

Bayesian approach to estimating reproductive inhibition potency in aquatic toxicity testing.

Effectively and accurately assessing the toxicity of chemicals and their impact on the environment continues to be an important concern in ecotoxicology. Single experiments conducted by a particular laboratory commonly serve as the basis of toxicity risk assessment. These laboratories often have a long history of conducting experiments using particular protocols. In the present study, a Bayesian analysis for estimating potency based on a single experiment was formulated, which then served as the basis for incorporating the experimental information from historical controls. A Bayesian hierarchical model was developed to estimate the relative inhibition concentrations (RIp) of a toxicant and flexible ways of using historical control information were suggested. The methods were illustrated using a data set produced by the test for reproduction in Ceriodaphnia dubia in which the number of young produced over three broods was recorded. In addition, simulation studies were included to compare the Bayesian methods with previously proposed estimators of potency. The Bayesian methods gave more precise RIp estimates with smaller variation and nominal coverage probability offsetting a small negative bias in the point estimate. Incorporating historical control information in the Bayesian hierarchical model effectively uses the useful information from past similar experiments when estimating the RIp, and results in potency estimates that are more precise compared to frequentist methods.

Baseline characteristics and statistical implications for the OECD 210 fish early-life stage chronic toxicity test.

The fish toxicity assay most commonly used to establish chronic effects is the Organisation for Economic Co-operation and Development (OECD) 210, fish early-life stage test. However, the authors are not aware of any systematic analysis of the experimental design or statistical characteristics of the test since the test guideline was adopted nearly 20 years ago. Here, the authors report the results of an analysis of data compiled from OECD 210 tests conducted by industry labs. The distribution of responses observed in control treatments was analyzed, with the goal of understanding the implication of this variability on the sensitivity of the OECD 210 test guideline and providing recommendations on revised experimental design requirements of the test. Studies were confined to fathead minnows, rainbow trout, and zebrafish. Dichotomous endpoints (hatching success and posthatch survival) were examined for indications of overdispersion to evaluate whether significant chamber-to-chamber variability was present. Dichotomous and continuous (length, wet wt, dry wt) measurement endpoints were analyzed to determine minimum sample size requirements to detect differences from control responses with specified power. Results of the analysis indicated that sensitivity of the test could be improved by maximizing the number of replicate chambers per treatment concentration, increasing the acceptable level of control hatching success and larval survival compared to current levels, using wet weight measurements rather than dry weight, and focusing test efforts on species that demonstrate less variability in outcome measures. From these analyses, the authors provide evidence of the impact of expected levels of variability on the sensitivity of traditional OECD 210 studies and the implications for defining a target for future animal alternative methods for chronic toxicity testing in fish.

Necrophagy by a benthic omnivore influences biomagnification of methylmercury in fish.

Omnivory has an important role in the movement of energy, nutrients, and contaminants between benthic and pelagic food webs. While top-predator fish are known to supplement a mostly piscivorous diet with benthic organisms, a more obscure benthic-pelagic coupling occurs when benthic invertebrates forage on fish carcasses, referred to as necrophagy. The combination of these two benthic-pelagic links, top-predator fish feeding on benthic organisms that have fed on dead fish, can generate a trophic feedback cycle that conserves energy and nutrients and may have implications for biomagnification of methylmercury (MeHg) in fish. We investigated the role of necrophagy by crayfish (Procambarus clarkii), via a trophic feedback cycle, on the biomagnification of MeHg in largemouth bass (Micropterus salmoides), a cosmopolitan top predator fish known to feed on crayfish. Controlled laboratory tests quantified the uptake of MeHg by both organisms from artificial and natural food (whole crayfish or bass tissue). Assimilation efficiency (AE) of MeHg was greater for bass fed crayfish (79±0.5%) than those fed artificial food (60±3%). Furthermore, AE of MeHg was greatest for largemouth bass fed crayfish that fed on MeHg-dosed dead fish (i.e., trophic feedback cycle; 94±17%). A model, parameterized with results of the laboratory experiments, was used to make steady-state projections of MeHg biomagnification factors. Model projections also indicate that MeHg biomagnification would be greatest for largemouth bass from a trophic feedback cycle. These results suggest that food web ecology has an important role in determining MeHg levels in predatory fish and underscore the need for further investigation into the magnitude that necrophagy may affect MeHg biomagnification in aquatic systems.

Regulatory aspects on the use of fish embryos in environmental toxicology.

Animal alternative tests are gaining serious consideration in an array of environmental sciences, particularly as they relate to sound management of chemicals and wastewater discharges. The ILSI Health and Environmental Sciences Institute and the European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) held an International Workshop on the Application of the Fish Embryo Test in March, 2008. This relatively young discipline is following advances in animal alternatives for human safety sciences, and it is advisable to develop a broad comparison of how animal alternative tests involving fish are viewed in a regulatory context over a wide array of authorities or advising bodies. These include OECD, Western Europe, North America, and Japan. This paper summarizes representative practices from these regions. Presently, the global regulatory environment has varying stances regarding the protection of fish for use as an experimental animal. Such differences have a long-term potential to lead to a lack of harmony in approaches to fish toxicity testing, especially for chemicals in commerce across multiple geographic regions. Implementation of alternative methods and approaches will be most successful if accepted globally, including methods of fish toxicity testing. An important area for harmonization would be in the interpretation of protected and nonprotected life stages of fish. Use of fish embryos represent a promising alternative and allow bridging to more technically challenging alternatives with longer prospective timelines, including cell-based assays, ecotoxicogenomics, and QSARs.

Ultraviolet radiation affects invasibility of lake ecosystems by warm-water fish.

Predicting where species invasions will occur remains a substantial challenge in ecology, but identifying factors that ultimately constrain the distribution of potential invaders could facilitate successful prediction. Whereas ultraviolet radiation (UVR) is recognized as an important factor controlling species distribution and community composition, the role of UVR in a habitat invasibility context has not been explored. Here we examine how underwater UVR can regulate warm-water fish invasion. In Lake Tahoe, California and Nevada, USA, established populations of exotic bluegill sunfish (Lepomis macrochirus) are currently limited to turbid, low-UVR embayments. An in situ incubation experiment that manipulated incident UVR exposure of larval bluegill, combined with an assessment of UVR exposure levels in nearshore habitats around Lake Tahoe, demonstrates that UVR can mediate habitat invasibility. Our findings suggest that the susceptibility to invasion by UVR sensitive species may increase in transparent aquatic systems threatened by declining water quality, and they highlight the importance of abiotic factors as regulators of invasion risk in ecosystems.