Enhancing the fathead minnow fish embryo toxicity test: Optimizing embryo production and assessing the utility of additional test endpoints

Exploring Alternatives for Marine Toxicity Testing: Initial Evaluation of Fish Embryo and Mysid Tests

Current regulations require that toxicity assessments be performed using standardized toxicity testing methods, often using fish. Recent legislation in both the European Union and United States has mandated that toxicity testing alternatives implement the 3Rs of animal research (replacement, reduction, and refinement) whenever possible. There have been advances in the development of alternatives for freshwater assessments, but there is a lack of analogous developments for marine assessments. One potential alternative testing method is the fish embryo toxicity (FET) test, which uses fish embryos rather than older fish. In the present study, FET methods were applied to two marine model organisms, the sheepshead minnow and the inland silverside. Another potential alternative is the mysid shrimp survival and growth test, which uses an invertebrate model. The primary objective of the present study was to compare the sensitivity of these three potential alternative testing methods to two standardized fish-based tests using 3,4-dichloroaniline (DCA), a common reference toxicant. A secondary objective was to characterize the ontogeny of sheepshead minnows and inland silversides. This provided a temporal and visual guide that can be used to identify appropriately staged embryos for inclusion in FET tests and delineate key developmental events (e.g., somite development, eyespot formation, etc.). Comparison of the testing strategies for assessing DCA indicated that: (1) the standardized fish tests possessed comparable sensitivity to each other; (2) the mysid shrimp tests possessed comparable sensitivity to the standardized fish tests; (3) the sheepshead minnow and inland silverside FET tests were the least sensitive testing strategies employed; and (4) inclusion of sublethal endpoints (i.e., hatchability and pericardial edema) in the marine FETs increased their sensitivity. Environ Toxicol Chem 2024;00:1-15. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Fish Embryo Acute Toxicity Testing and the RTgill-W1 Cell Line as In Vitro Models for Whole-Effluent Toxicity (WET) Testing: An In Vitro/In Vivo Comparison of Chemicals Relevant for WET Testing

The fathead minnow (Pimephales promelas) fish embryo acute toxicity (FET) test was compared to the fish gill cells (RTgill-W1) in vitro assay and to the fish larvae acute toxicity test to evaluate their sensitivity for whole-effluent toxicity (WET) testing. The toxicity of 12 chemicals relevant for WET testing was compared as proof of principle. The concentrations lethal to 50% of a population (LC50) of embryos were compared to those in fish larvae and to the 50% effect concentration (EC50) in RTgill-W1 cells from previous literature. Along with traditional FET endpoints (coagulation, somite development, tail detachment, and heartbeat), cardiotoxicity was evaluated for WET applicability. Heart rate was measured at LC20 and LC50 values of six subselected chemicals (Cd, Cu, Ni, ammonia, 3,4-dichloraniline, and benzalkonium chloride). In addition, the toxicity of Cd and Ni was evaluated in RTgill-W1 cells exposed in a hypoosmotic medium to evaluate the effect that osmolarity may have on metal toxicity. A significant correlation was found between the FET and larvae LC50 values but not between the RTgill-W1 EC50 and FET LC50 values. Although sensitivity to Ni and Cd was found to increase with hypoosmotic conditions for FET and RTgill-W1 cells, a correlation was only found with removal of Ni from the analysis. Hypoosmotic conditions increased sensitivity with a significant correlation between RTgill-W1 cells and larvae. Cardiotoxicity was shown in three of the five subselected chemicals (Cd, Cu, and 3,4-dichloroaniline). Overall, both in vitro alternative models have shown good predictability of toxicity in fish in vivo for WET chemicals of interest. Environ Toxicol Chem 2022;41:2721-2731. © 2022 SETAC.

De Novo Assembly of the Nearly Complete Fathead Minnow Reference Genome Reveals a Repetitive but Compact Genome

The fathead minnow is a widely used model organism in environmental toxicology. The lack of a high-quality fathead minnow reference genome, however, has severely hampered its uses in toxicogenomics. We present the de novo assembly and annotation of the fathead minnow genome using long PacBio reads, Bionano and Hi-C scaffolding data, and large RNA-sequencing data sets from different tissues and life stages. The new annotated fathead minnow reference genome has a scaffold N50 of 12.0 Mbp and a complete benchmarking universal single-copy orthologs score of 95.1%. The completeness of annotation for the new reference genome is comparable to that of the zebrafish GRCz11 reference genome. The fathead minnow genome, revealed to be highly repetitive and sharing extensive syntenic regions with the zebrafish genome, has a much more compact gene structure than the zebrafish genome. Particularly, comparative genomic analysis with zebrafish, mouse, and human showed that fathead minnow homologous genes are relatively conserved in exon regions but had strikingly shorter intron regions. The new fathead minnow reference genome and annotation data, publicly available from the National Center for Biotechnology Information and the University of California Santa Cruz genome browser, provides an essential resource for aquatic toxicogenomic studies in ecotoxicology and public health. Environ Toxicol Chem 2022;41:448-461. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.

Life stage and endpoint sensitivity differences of fathead minnow (Pimephales promelas) to chemicals with various modes of action

Fish Embryo Acute Toxicity (FET) test was proposed as an alternative to the traditional test methods using larval or adult fish. However, whether fathead minnow (Pimephales promelas) embryo is appropriate for FET remains uncertain. In the present study, ecological threshold of toxicological concern (ecoTTC) values and uncertainty factors (UFs) for each Verhaar et al. category in P. promelas were identified by employing probabilistic ecological risk assessment (PERA) approach with chemical toxicity distributions (CTDs). The sensitivity among different life stages and toxicity among different mode of actions (MOAs) classes were comprehensively compared by CTD comparisons. The results showed that embryo exhibited the less or similar sensitivity compared to larva or adult for Verhaar et al. MOA classes (1-4) while adults were more sensitive, followed by embryo than larval for non-classified chemicals. Considering growth effect as endpoint to class 1, class 3, and non-classified chemicals on P. promelas embryo and larva was more sensitive than mortality. Non-classified chemicals especially inorganic compounds were most toxic to P. promelas embryo for the four concerned Verharr et al. MOA-specific chemical classes. This study also derived uncertainty factors (UFs) as 26.5 (9.8, 109) for embryo-to-larva, 6.26 (3.94, 11.0) for embryo-to-adult, 15.6 (10.1, 36.1) for mortality-to-growth, and 3.03 (1.86, 7.08) for mortality-to-reproduction, which can be applied for extrapolations of life stage-to-life stage and effect-to-effect to reduce the underestimating and overestimating risk by the use of default UF such as 10, 100 or 1000. Our findings are vital for feasibility of FET test of P. promelas for ecotoxicity testing and ecological risk assessment for chemicals with different MOAs.

Seawater intrusion: an appraisal of taxa at most risk and safe salinity levels

Seawater intrusion into low-lying coastal ecosystems carries environmental risks. Salinity levels at these coastal ecosystems may vary substantially, causing ecological effects from mortality to several sublethal endpoints, such as depression of rates of feeding, somatic growth, or reproduction. This review attempts to establish safe salinity levels for both terrestrial and freshwater temperate ecosystems by integrating data available in the literature. We have four specific objectives: (i) to identify the most sensitive ecological taxa to seawater intrusion; (ii) to establish maximum acceptable concentrations-environmental quality standards (MAC-EQSs) for sea water (SW) from species sensitivity distributions (SSDs); (iii) to compile from the literature examples of saline intrusion [to be used as predicted environmental concentrations (PECs)] and to compute risk quotients for the temperate zone; and (iv) to assess whether sodium chloride (NaCl) is an appropriate surrogate for SW in ecological risk assessments by comparing SSD-derived values for NaCl and SW and by comparing these with field data. Zooplankton, early life stages of amphibians and freshwater mussels were the most sensitive ecological receptors for the freshwater compartment, while soil invertebrates were the most sensitive ecological receptors for the terrestrial compartment. Hazard concentration 5% (HC₅ ) values, defined as the concentration (herein measured as conductivity) that affects (causes lethal or sublethal effects) 5% of the species in a distribution, computed for SW were over 22 and 40 times lower than the conductivity of natural SW (≈ 52 mS/cm) for the freshwater and soil compartment, respectively. This sensitivity of both compartments means that small increments in salinity levels or small SW intrusions might represent severe risks for low-lying coastal ecosystems. Furthermore, the proximity between HC₅ values for the soil and freshwater compartments suggests that salinized soils might represent an additional risk for nearby freshwater systems. This sensitivity was corroborated by the derivation of risk quotients using real saline intrusion examples (PECs) collected from the literature: risk was >1 in 34 out of 37 examples. By contrast, comparisons of HC₅ values obtained from SSDs in field surveys or mesocosm studies suggest that natural communities are more resilient to salinization than expected. Finally, NaCl was found to be slightly more toxic than SW, at both lethal and sublethal levels, and, thus, is suggested to be an acceptable surrogate for use in risk assessment.

Investigating the Predictive Power of Three Potential Sublethal Endpoints for the Fathead Minnow Fish Embryo Toxicity Test: Snout-Vent Length, Eye Size, and Pericardial Edema

The fish embryo acute toxicity (FET) test is known to be less sensitive than the fish acute test for some chemicals, including neurotoxicants. Thus, there is an interest in identifying additional endpoints that can improve FET test performance. The goal of this project was to advance alternative toxicity testing methods by determining whether select developmental abnormalities-snout-vent length, eye size, and pericardial area-are linked to adverse alterations in ecologically-relevant behaviors and delayed mortality. Fathead minnow (Pimephales promelas) FET tests were conducted with 3,4-dicholoroaniline, cadmium, and perfluorooctanesulfonic acid (PFOS) and developmental abnormalities were quantified. Surviving eleutheroembryos were reared in clean water to 14 days post fertilization (dpf), during which time behaviors and mortality were evaluated. None of the abnormalities evaluated were predictive of behavioral alterations; however, embryos with ≥14% reductions in length or ≥3.54-fold increases in pericardial area had an 80% chance of mortality by 14 dpf. When these abnormalities were used as markers of mortality, the LC₅₀s for cadmium and PFOS were less than those calculated using only standardized FET test endpoints and similar to those obtained via larval fish tests, indicating that the snout-vent length and pericardial area warrant consideration as standard FET test endpoints.

Comparison of behavioral assays for assessing toxicant-induced alterations in neurological function in larval fathead minnows

Neuroactive compounds are routinely detected in surface waters at concentrations that pose potential threats to wildlife. Exposure to neurotoxicants can adversely affect exposed organism by altering ecologically-important behaviors (e.g., feeding and predator response) that are likely to have important repercussions for populations. These compounds can elicit behavioral effects at concentrations lower than those that induce overt toxicity as indicated by mortality or decreased growth. Though a wide variety of methods have been employed to assess the behavior of early life stage fish, it is unclear which assays are best suited for identifying ecologically-relevant behavioral changes following exposures to neurotoxicants. The goal of the present study was to promote the use of behavioral assays for assessing the behavioral impacts of exposure to neurotoxic compounds by comparing the performance of different behavioral assays in larval fish. To achieve this goal, the sensitivity and practicality of three behavioral assays (i.e., feeding, optomotor response, and C-start assays) were compared in larval fathead minnows exposed to a known neurotoxicant, chlorpyrifos. There were significant alterations in the performance of fathead minnow larvae in all three behavioral assays in response to a 12-d embryo-larval exposure to chlorpyrifos. However, feeding and C-start were the most practical of the selected assays, as they took less time and allowed for larger samples sizes. Further work to standardize behavioral testing methods, and to link alterations to ecologically-relevant behaviors, will help promote the use of these assays when investigating the potential environmental impacts of neurotoxic compounds.

Low concentration of 2,4,6-tribromophenol (TBP) represents a risk to South American silver catfish Ramdia quelen (Quoy and Gaimard, 1824) population

The 2,4,6-tribromophenol (TBP) is an environmental persistent pollutant widely used as flame retardant, antimicrobial and insecticide agent in wood preservation and plastic production. Currently, TBP is found in environmental compartments such as soil, freshwater, groundwater, sewage sludge and domestic dust, but the effects to biota and the risk of exposure to aquatic vertebrates are still scarce. In the present study, Rhamdia quelen fish embryos (8 h post-fertilization - hpf) were exposed to 0.3 and 3.0 μg L^-1 of TBP until 96 hpf. Biochemical biomarkers, hatching, survival and larvae/embryo malformations were evaluated after exposure. Additionally, a mathematical model was proposed to evaluate the effects along further generations. The results showed that TBP decreased the survival level but did not cause significant difference in the hatching rates. After 72 and 96 hpf, individuals from the highest tested concentration group showed more severe malformations than individuals from control and the lower concentrations groups. The deformities were concentrated on the embryos facial region where the sensorial structures related to fish behavior are present. The biochemical biomarkers revealed both oxidative stress and neurotoxicity signs after exposure to the contaminant, while the application of the mathematical model showed a decrease of population in both tested TBP concentrations. In conclusion, the current results demonstrated that TBP is toxic to R. quelen embryos and represents a risk to population after early life stage exposure.

Characterizing Crude Oil Toxicity to Early-Life Stage Fish Based On a Complex Mixture: Are We Making Unsupported Assumptions?

Numerous studies of the water-soluble fraction (WSF) from crude oil have concluded that polycyclic aromatic hydrocarbons (PAHs) are the primary causative agents for early life stage (ELS) fish toxicity. Noteworthy is the lack of studies demonstrating that the sum of PAHs are capable of causing toxic effects in ELS fish at the low levels claimed (0.1-5 μg/L) without being part of a complex crude oil mixture. Crude oil and the WSF are composed of thousands of other compounds that co-occur and likely contribute to crude oil toxicity. Based on the available data, it appears that the syndrome of effects (lower heart rate, edemas, and morphological abnormalities) for ELS fish exposed to the aqueous fraction of a crude oil mixture is commonly observed in studies exposing fish embryos to high concentrations of a variety of compounds and may be a nonspecific response. We conclude that the available data support the hypothesis that this syndrome of effects is likely the result of baseline toxicity (not receptor based) due to membrane disruption and resulting alteration in ion (e.g., calcium and potassium) homeostasis. We acknowledge the possibility of some compounds in the WSF capable of causing a specific receptor based toxicity response to ELS fish; however, such compounds have not been identified nor their receptor characterized. Concluding that PAHs are the main toxic compounds for crude oil exposure is misleading and does not result in guideline values that can be useful for environmental protection. Water quality guidelines for any single chemical or suite of chemicals must be based on a complete understanding of exposure concentrations, mechanism of action, potency, and resulting response. This review focuses on the toxic effects reported for fish embryos and the purported toxic concentrations observed in the aqueous phase of an oil/water mixture, the known levels of toxicity for individual PAHs, a toxic unit approach for characterizing mixtures, and the potential molecular initiating event for ELS toxicity in fish. This review also has implications for a large number of studies exposing ELS fish to a variety of compounds at high concentrations that result in a common baseline toxic response.

Identifying sublethal endpoints for evaluating neurotoxic compounds utilizing the fish embryo toxicity test

Fish embryos are increasingly being utilized in aquatic toxicity testing, as evidenced by the Organisation for Economic Co-operation and Development's approval of the fish embryo acute toxicity (FET) test. However, the FET test only allows for the estimation of acute toxicity, whereas other test methods such as the larval growth and survival (LGS) test allow for the estimation of both acute and chronic toxicity. Additionally, it has been demonstrated that the FET test is less sensitive than other test methods for some neurotoxic compounds. To address these limitations, efforts to identify sublethal endpoints that increase FET test sensitivity and allow for the prediction of sublethal adverse effects have begun. As such, the objectives of the current study were 1) to compare estimated LC50 values from the FET and LGS test for three known neurotoxicants: fluoride (F), nickel (Ni), and cadmium (Cd) and 2) to evaluate the responsiveness of potential sublethal endpoints for the FET test related to growth (i.e., wet weight and snout-vent length), neurological development (i.e., spontaneous contraction frequency and eye size), and cardiovascular function (i.e., heart rate and pericardial area). The calculated LC50 values from the F and Cd FET test were significantly higher than those from the LGS test, demonstrating that the FET test is less sensitive than the LGS test for neurotoxic compounds. Only Cd exposure resulted in alterations in any of the sublethal endpoints investigated. Embryos/eleutheroembryos exposed to Cd displayed alterations in length, eye size, and pericardial area at concentrations five-fold less than the estimated LC50 value, suggesting that for Cd the inclusion of these sublethal endpoints would improve the sensitivity of the FET test. Overall, these results provide evidence that for some neurotoxicants, the inclusion of sublehtal endpoints may improve the utility of the FET test; however, further research utilizing a broader range of neurotoxicants with differing mechanisms of action, is needed to fully establish such endpoints in the context of routine FET test.

Heat stress upregulates G-T mismatch binding activities in zebrafish (Danio rerio) embryos preexposed and nonexposed to a sublethal level of cadmium (Cd)

G-T mispair frequently appears in eukaryotic DNA due to the spontaneous deamination of 5-methylcytosine paired with guanine and is therefore an important target for DNA mismatch repair (MMR). Our earlier studies showed the downregulation of G-T binding activities in cadmium (Cd)-exposed (Danio rerio) embryos. Since elevation of water temperature was reported to increase Cd toxicity in zebrafish, this study explored whether heat stress affected zebrafish mismatch binding capacity in the absence or presence of Cd. Heat stress (37 °C for 30 min) induced heat shock protein 70 mRNA expression in embryos at 10 and 24 h post fertilization (hpf). Heat stress weakly upregulated normal G-T sensing machinery and inhibited G-T recognition activity in embryos preexposed to 3 μM Cd for 9 h. Either heat shock or a 23-h Cd treatment alone caused a 1.7-fold stimulation of G-T binding capacity in 24 hpf embryos and heat stress of Cd-preexposed embryos further enhanced G-T binding activity to 2.5 fold of control. Normal and Cd-downregulated loop binding activities in 10 and 24 hpf embryos were almost unreactive to heat shock. Heat stress-upregulated G-T sensing in nonexposed, but not in Cd-preexposed, 24 hpf embryos correlated with stronger gene activities encoding MMR-linked mismatch detecting factors MutS homolog 2 and 6 plus a higher DNA binding activity of the transcription factor Sp1 that regulates msh2/msh6 expression. Our results suggested the importance of heat shock response in facilitating the correction of G-T mismatch in developing zebrafish even under Cd exposure.

Development of cardiovascular and neurodevelopmental metrics as sublethal endpoints for the Fish embryo toxicity test

The fathead minnow fish embryo toxicity (FET) test has been proposed as a more humane alternative to current toxicity testing methods as younger organisms are thought to experience less distress during toxicant exposure. However, the FET test protocol does not include endpoints that allow for the prediction of sublethal adverse outcomes, limiting its utility relative to other test types. Researchers have proposed the development of sublethal endpoints for the FET test to increase its utility. The present study 1) developed methods for previously unmeasured sublethal metrics in fathead minnows (i.e., spontaneous contraction frequency and heart rate) and 2) investigated the responsiveness of several sublethal endpoints related to growth (wet wt, length, and growth-related gene expression), neurodevelopment (spontaneous contraction frequency, eye size, and neurodevelopmental gene expression), and cardiovascular function and development (pericardial area, heart rate, and cardiovascular system-related gene expression) as additional FET test metrics using the model toxicant 3,4-dichloroaniline. Of the growth, neurological, and cardiovascular endpoints measured, length, eye size, and pericardial area were found to be more responsive than the other endpoints evaluated. Future studies linking alterations in these endpoints to longer-term adverse impacts are needed to fully evaluate the predictive power of these metrics in chemical and whole-effluent toxicity testing. Environ Toxicol Chem 2018;37:2530-2541. © 2018 SETAC.