Exposure to ultraviolet radiation late in development increases the toxicity of oil to mahi-mahi (Coryphaena hippurus) embryos

Implications of biotic factors for toxicity testing in laboratory studies

There is an emerging call from scientists globally to advance the environmental relevance of laboratory studies, particularly within the field of ecotoxicology. To answer this call, we must carefully examine and elucidate the shortcomings of standardized toxicity testing methods that are used in the derivation of toxicity values and regulatory criteria. As a consequence of rapidly accelerating climate change, the inclusion of abiotic co-stressors are increasingly being incorporated into toxicity studies, with the goal of improving the representativeness of laboratory-derived toxicity values used in ecological risk assessments. However, much less attention has been paid to the influence of biotic factors that may just as meaningfully impact our capacity to evaluate and predict risks within impacted ecosystems. Therefore, the overarching goal is to highlight key biotic factors that should be taken into consideration during the experimental design and model selection phase. SYNOPSIS: Scientists are increasingly finding that lab reared results in toxicology might not be reflective of the external wild environment, we highlight in this review some key considerations when working between the lab and field.

Interactive effects of Louisiana Sweet Crude (LSC) thin oil sheens and ultraviolet light on mortality and swimming behavior of the larval Eastern oyster, Crassostrea virginica

The Eastern oyster (Crassostrea virginica) is an important commercial bivalve species which also has numerous ecological roles including biogeochemical cycling, providing habitat for larval fish and crustaceans, and reducing the impacts of coastal storms. Oil may pose a threat to oyster larvae swimming in the water column, leading to potential negative effects on survival, growth, and development. Oil toxicity may be further enhanced by chemical changes in the presence of sunlight. This study determined the toxicity of thin oil sheens with and without ultraviolet (UV) light, then examined the latent effects of the short term exposure on longer term survival and swimming ability. Larval C. virginica were exposed to four different oil sheen thicknesses for 24 h with either no UV light or 2-h UV light. Following the exposure, larvae were transferred to clean seawater and no UV light for 96 h. The presence of a 2-h UV light exposure significantly increased oyster mortality, indicating photo-enhanced toxicity. The LC50 for a 24-h oil sheen exposure without UV was 7.26 µm (23 µg/L PAH50) while a 2 h-UV exposure lowered the sheen toxicity threshold to 2.67 µm (10 µg/L PAH50). A previous 24-h oil sheen exposure (≥0.5 µm) led to latent effects on larval oyster survival, regardless of previous UV exposure. Sublethal impacts to larval oyster swimming behavior were also observed from the previous oil sheen exposure combined with UV exposure. This study provides new data for the toxicity of thin oil sheens to a sensitive early life stage of estuarine bivalve.

Role of Aryl Hydrocarbon Receptor and Oxidative Stress in the Regioselective Toxicities of Hydroxychrysenes in Embryonic Japanese Medaka (Oryzias latipes)

Oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) are environmental contaminants that can be created through oxidation of parent PAHs. Previous studies have found that 2-hydroxychrysene (2-OHCHR) caused anemia in embryonic Japanese medaka whereas 6-hydroxychrysene (6-OHCHR) did not, an example of regioselective toxicity. Anemia was prevented by cytochrome P450 (CYP) inhibition, which reduced the formation of the potential oxidatively active metabolite, 1,2-catechol, from 2-OHCHR. 2-OHCHR has also been found to be a four-fold more potent aryl hydrocarbon receptor (AhR) agonist compared with 6-OHCHR. These findings led us to hypothesize that AhR activation and/or oxidative stress play an important role in 2-OHCHR toxicity. Although treatments with the AhR agonists polychlorinated biphenyl (PCB)126 and 2-methoxychrysene (2-MeOCHR) did not cause significant toxicity, pretreatments with the AhR antagonist, CH-223191, reduced anemia by 97.2 ± 0.84% and mortality by 96.6 ± 0.69%. Aryl hydrocarbon receptor inhibition by the antagonist was confirmed by significant reductions (91.0 ± 9.94%) in induced ethoxyresorufin-O-deethylase activity. Thiobarbituric acid reactive substances concentrations were 32.9 ± 3.56% higher (p < 0.05) in 2-OHCHR treatments at 100 hours postfertilization compared with controls. Staining 2-OHCHR-treated embryos with the reactive oxygen species (ROS) scavenger 2',7'-dichlorofluorescin diacetate revealed 32.6 ± 2.69% of 2-OHCHR-treated embryos exhibiting high concentrations of ROS in caudal tissues, which is a site for embryonic hematopoiesis in medaka. Pretreatment with antioxidants, N-acetylcysteine (NAC) or vitamin E (Vit E) significantly reduced 2-OHCHR-induced anemia (NAC: 80.7 ± 1.12% and Vit E: 99.1 ± 0.43%) and mortality (NAC: 67.1 ± 1.69% and Vit E: 98.9 ± 0.66%). These results indicate that AhR may mediate 2-OHCHR toxicity through canonical signaling by up-regulating CYP1, enhancing the formation of reactive metabolites of 2-OHCHR that generate ROS within caudal hematopoietic tissues, potentially disrupting hematopoiesis, leading to anemia and subsequent mortality. Environ Toxicol Chem 2023;42:698-706. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Recommendations for advancing test protocols examining the photo-induced toxicity of petroleum and polycyclic aromatic compounds

Photo-induced toxicity of petroleum products and polycyclic aromatic compounds (PACs) is the enhanced toxicity caused by their interaction with ultraviolet radiation and occurs by two distinct mechanisms: photosensitization and photomodification. Laboratory approaches for designing, conducting, and reporting of photo-induced toxicity studies are reviewed and recommended to enhance the original Chemical Response to Oil Spills: Ecological Research Forum (CROSERF) protocols which did not address photo-induced toxicity. Guidance is provided on conducting photo-induced toxicity tests, including test species, endpoints, experimental design and dosing, light sources, irradiance measurement, chemical characterization, and data reporting. Because of distinct mechanisms, aspects of photosensitization (change in compound energy state) and photomodification (change in compound structure) are addressed separately, and practical applications in laboratory and field studies and advances in predictive modeling are discussed. One goal for developing standardized testing protocols is to support lab-to-field extrapolations, which in the case of petroleum substances often requires a modeling framework to account for differential physicochemical properties of the constituents. Recommendations are provided to promote greater standardization of laboratory studies on photo-induced toxicity, thus facilitating comparisons across studies and generating data needed to improve models used in oil spill science.

Co-exposure to UV radiation and crude oil increases acute embryotoxicity and sublethal malformations in the early life stages of Atlantic haddock (Melanogrammus aeglefinus)

Crude oil causes severe abnormalities in developing fish. Photomodification of constituents in crude oil increases its toxicity several fold. We report on the effect of crude oil, in combination with ultraviolet (UV) radiation, on Atlantic haddock (Melanogrammus aeglefinus) embryos. Accumulation of crude oil on the eggshell makes haddock embryos particularly susceptible to exposure. At high latitudes, they can be exposed to UV radiation many hours a day. Haddock embryos were exposed to crude oil (5-300 μg oil/L nominal loading concentrations) for three days in the presence and absence of UV radiation (290-400 nm). UV radiation partly degraded the eggs' outer membrane resulting in less accumulation of oil droplets in the treatment with highest oil concentration (300 μg oil/L). The co-exposure treatments resulted in acute toxicity, manifested by massive tissue necrosis and subsequent mortality, reducing LC50 at hatching stage by 60 % to 0.24 μg totPAH/L compared to 0.62 μg totPAH/L in crude oil only. In the treatment with nominal low oil concentrations (5-30 μg oil/L), only co-exposure to UV led to sublethal morphological heart defects. Including phototoxicity as a parameter in risk assessments of accidental oil spills is recommended.

Relationships between Isomeric Metabolism and Regioselective Toxicity of Hydroxychrysenes in Embryos of Japanese Medaka (Oryzias latipes)

Oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) are ubiquitous contaminants that can be formed through oxidation of parent PAHs. Our previous studies found 2-hydroxychrysene (2-OHCHR) to be significantly more toxic to Japanese medaka embryos than 6-hydroxychrysene (6-OHCHR), an example of regioselective toxicity. We have also previously identified a sensitive developmental window to 2-OHCHR toxicity that closely coincided with liver development, leading us to hypothesize that differences in metabolism may play a role in the regioselective toxicity. To test this hypothesis, Japanese medaka embryos were treated with each isomer for 24 h during liver development (52-76 hpf). Although 6-OHCHR was absorbed 97.2 ± 0.18% faster than 2-OHCHR, it was eliminated 57.7 ± 0.36% faster as a glucuronide conjugate. Pretreatment with cytochrome P450 inhibitor, ketoconazole, reduced anemia by 96.8 ± 3.19% and mortality by 95.2 ± 4.76% in 2-OHCHR treatments. Formation of chrysene-1,2-diol (1,2-CAT) was also reduced by 64.4 ± 2.14% by ketoconazole pretreatment. While pretreatment with UDP-glucuronosyltransferase inhibitor, nilotinib, reduced glucuronidation of 2-OHCHR by 52.4 ± 2.55% and of 6-OHCHR by 63.7 ± 3.19%, it did not alter toxicity for either compound. These results indicate that CYP-mediated activation, potentially to 1,2-CAT, may explain the isomeric differences in developmental toxicity of 2-OHCHR.

Transcriptomic responses and apoptosis in larval red drum (Sciaenops ocellatus) co-exposed to crude oil and ultraviolet (UV) radiation

Ultraviolet (UV) radiation can significantly increase the toxicity of polycyclic aromatic hydrocarbons (PAHs) in crude oil to early life stage (ELS) fishes through photo-induced /photo-enhanced toxicity. However, little is known about the sub-lethal effects and mechanisms of photo-induced PAH toxicity in ELS fishes. The present study investigated apoptosis and global transcriptomic effects in larval red drum (Sciaenops ocellatus) (24-72 h post-fertilization) following co-exposure to oil (0.29-0.30 μg/L ∑PAH₅₀) and UV. Apoptosis was quantified using the TUNEL assay, and transcriptomic effects were assessed using RNA sequencing analysis. Apoptotic fluorescence was greatest in the eyes and skin following 24 and 48 h co-exposure to oil and UV, indicating photo-induced toxicity. Consistent with these phenotypic responses, pathways associated with phototransduction, eye development, and dermatological disease were among the top predicted pathways impacted. The present study is the first to provide global transcriptomic analysis of UV and oil co-exposure in an ELS fish.

Ultraviolet avoidance by embryonic buoyancy control in three species of marine fish

Pelagic fish embryos are thought to float in or near surface waters for the majority of their development and are presumed to have little to no control over their mobility, rendering these embryos at high risk for damages associated with surface stressors such as ultraviolet radiation (UVR). We recently challenged these long-standing paradigms by characterizing a potential mechanism of stressor avoidance in early-life stage mahi-mahi (Coryphaena hippurus) in which embryos sense external cues, such as UVR, and modify their buoyancy to reduce further exposure. It is unknown whether embryos of other marine fish with pelagic spawning strategies have similar capabilities. To fill this knowledge gap, we investigated buoyancy change in response to UVR in three additional species of marine fish that utilize a pelagic spawning strategy: yellowfin tuna (Thunnus albacares), red snapper (Lutjanus campechanus), and cobia (Rachycentron canadum). Embryos of all three species displayed increased specific gravity and loss of buoyancy after exposures to environmentally relevant doses of UVR, a response that may be ubiquitous to fish with pelagic embryos. To gain further insight into this response, we investigated recovery of buoyancy, oxygen consumption, energy depletion, and photolyase induction in response to UVR exposures in at least one of the three species listed above.

Exposure of zebrafish larvae to water accommodated fractions of weathered crude oil alters steroid hormone concentrations with minimal effect on cholesterol

Crude oil has multiple toxic effects in fish, particularly during their early life stages. Recent transcriptomics studies have highlighted a potential effect on cholesterol homeostasis and biosynthesis, but have not investigated effects on steroid hormones, which are biosynthetically downstream metabolites of cholesterol. We exposed zebrafish (Danio rerio) embryos and larvae to 3 concentrations of a high energy water accommodated fraction (HEWAF) of crude oil and measured effects on cholesterol and steroid hormones at 48 and 96 h post fertilization (hpf). HEWAF exposure caused a small decrease in cholesterol at 96 hpf but not 48 hpf. HEWAF-exposed larvae had higher levels of androstenedione, testosterone, estradiol, cortisol, corticosterone, and progesterone at 96 hpf compared to controls, while effects at 48 hpf were more modest or not present. 2-Methoxyestradiol was lower following HEWAF exposure at both time points. Dihydrotestosterone was elevated in one HEWAF concentration at 48 hpf only. Our results suggest that hormone imbalance may be an important toxic effect of oil HEWAF exposure despite no major effect on their biosynthetic precursor cholesterol.

Stage-dependent and regioselective toxicity of 2- and 6-hydroxychrysene during Japanese medaka embryogenesis

Exposure to oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) at critical developmental time-points in fish models impairs red blood cell concentrations in a regioselective manner, with 2-hydroxychrysene being more potent than 6-hydroxychrysene. To better characterize this phenomenon, embryos of Japanese medaka (Oryzias latipes) were exposed to 2- or 6-hydroxychrysene (0.5, 2, or 5 μM) from 4 h-post-fertilization (hpf) to 7 d-post-fertilization. Following exposure, hemoglobin concentrations were quantified by staining fixed embryos with o-dianisidine (a hemoglobin-specific dye) and stained embryos were imaged using brightfield microscopy. Exposure to 2-hydroxychrysene resulted in a concentration-dependent decrease in hemoglobin relative to vehicle-exposed embryos, while only the highest concentration of 6-hydroxychrysene resulted in a significant decrease in hemoglobin. All tested concentrations of 2-hydroxychrysene also caused significant mortality (12.2 % ± 2.94, 38.9 % ± 14.4, 85.6 % ± 11.3), whereas mortality was not observed following exposure to 6-hydroxychrysene. Therefore, treatment of embryos with 2-hydroxychrysene at various developmental stages and durations was subsequently conducted to identify key developmental landmarks that may be targeted by 2-hydroxychrysene. A sensitive window of developmental toxicity to 2-hydroxychrysene was found between 52-100 hpf, with a 24 h exposure to 10 μM 2-hydroxychrysene resulting in significant anemia and mortality. Since exposure to 2-hydroxychrysene from 52 to 100 hpf, a window that includes liver morphogenesis in medaka, resulted in the highest magnitude of toxicity, liver development and function may have a role in 2-hydroxychrysene developmental toxicity.

Effects of ultraviolet radiation to Solea senegalensis during early development

Ultraviolet radiation (UVR) reaching the Earth surface is increasing and scarce information is available regarding effects of this stressor to early life stages of marine vertebrates. Therefore, this work aims to study the effects of UVR exposure during early development stages of the flatfish Solea senegalensis. Firstly, fish were exposed to UVR (six daily doses between 3.4 ± 0.08 and 8.6 ± 0.14 kJ m^-2) at the following moments: gastrula stage (24 h post fertilization, hpf), 1 and 2 days after hatching (dah, 48 and 72 hpf, respectively). In a second bioassay, fish at the beginning of metamorphosis were exposed to UVR (one or two daily doses of 7.2 ± 0.39 or 11.1 ± 0.49 kJ m^-2) and then maintained until the end of metamorphosis. Mortality and effects on development, growth and behaviour were evaluated at the end of both bioassays (3 dah and 18 dah, respectively). Biomarkers of neurotransmission (acetylcholinesterase, AChE), oxidative stress (catalase, CAT) and biotransformation (glutathione S-transferase, GST) were also determined at the end of the early larvae bioassay, and metamorphosis progression was evaluated during the second bioassay. UVR exposure caused distinct effects depending on life stage. Altered pigmentation, decreased growth, impaired fish behaviour and AChE and GST inhibition were observed at the earlier larval phase. Whereas, decrease in growth was the main effect observed at the metamorphosis stage. In summary, the exposure of S. senegalensis early stages to environmentally relevant UVR doses led to adverse responses at different levels of biological organization, which might lead to implications in later life stages.

Multi-stressor Effects of Ultraviolet Light, Temperature, and Salinity on Louisiana Sweet Crude Oil Toxicity in Larval Estuarine Organisms

When oil is spilled into the environment its toxicity is affected by abiotic conditions. The cumulative and interactive stressors of chemical contaminants and environmental factors are especially relevant in estuaries where tidal fluctuations cause wide variability in salinity, temperature, and ultraviolet (UV) light penetration, which is an important modifying factor for polycyclic aromatic hydrocarbon (PAH) toxicity. Characterizing the interactions of multiple stressors on oil toxicity will improve prediction of environmental impacts under various spill scenarios. This study examined changes in crude oil toxicity with temperature, salinity, and UV light. Oil exposures included high-energy, water-accommodated fractions (HEWAFs) and thin oil sheens. Larval (24-48 h post hatch) estuarine species representing different trophic levels and habitats were evaluated. Mean 96 h LC₅₀ values for oil prepared as a HEWAF and tested under standard conditions (20 ppt, 25 °C, No-UV) were 62.5 µg/L tPAH₅₀ (mud snails), 198.5 µg/L (grass shrimp), and 774.5 µg/L (sheepshead minnows). Thin oil sheen 96 h LC₅₀ values were 5.3 µg/L tPAH₅₀ (mud snails), 14.7 µg/L (grass shrimp), and 22.0 µg/L (sheepshead minnows) under standard conditions. UV light significantly increased the toxicity of oil in all species tested. Oil toxicity also was greater under elevated temperature and lower salinity. Multi-stressor (oil combined with either increased temperature, decreased salinity, or both) LC₅₀ values were reduced to 3 µg/L tPAH₅₀ for HEWAFs and < 1.0 µg/L tPAH₅₀ for thin oil sheens. Environmental conditions at the time of an oil spill will significantly influence oil toxicity and organismal response and should be taken into consideration in toxicity testing and oil spill damage assessments.

Physiological Responses of Fish to Oil Spills

Millions of tons of oil are spilled in aquatic environments every decade, and this oil has the potential to greatly impact fish populations. Here, we review available information on the physiological effects of oil and polycyclic aromatic hydrocarbons on fish. Oil toxicity affects multiple biological systems, including cardiac function, cholesterol biosynthesis, peripheral and central nervous system function, the stress response, and osmoregulatory and acid-base balance processes. We propose that cholesterol depletion may be a significant contributor to impacts on cardiac, neuronal, and synaptic function as well as reduced cortisol production and release. Furthermore, it is possible that intracellular calcium homeostasis-a part of cardiotoxic and neuronal function that is affected by oil exposure-may be related to cholesterol depletion. A detailed understanding of oil impacts and affected physiological processes is emerging, but knowledge of their combined effects on fish in natural habitats is largely lacking. We identify key areas deserving attention in future research.

Planktonic Fiddler Crab (Uca longisignalis) Are Susceptible to Photoinduced Toxicity Following in ovo Exposure in Oiled Mesocosms

Benthic organisms may be exposed to polycyclic aromatic hydrocarbons (PAHs) in marine sediments as the result of oil spills. PAH photoinduced toxicity, which has been documented in a wide range of early life stage (ELS) aquatic biota, is a phenomenon by which ultraviolet (UV) radiation potentiates the toxicity of photodynamic PAHs (often leading to mortality). Fiddler crabs (Uca longisignalis) are important ecosystem engineers that influence biogeochemical cycles via burrowing. As gravid females burrow, their eggs may bioaccumulate PAHs from contaminated sediments, leading to in ovo exposure. Consequently, free-swimming larvae exposed to intense UV may be at risk for photoinduced toxicity. In the present study, mature fiddler crabs were bred on oiled sediments contaminated via simulated tidal flux. Gravid females were transferred to clean water after 10 days, and larvae were collected at hatch. While in ovo exposures to oil alone did not affect survival, offspring that were subsequently exposed to full spectrum sunlight in clean water experienced significant mortality that corresponded with in ovo exposures to sediments containing ≥1455 μg/kg tPAH₅₀. Results presented here provide evidence for the potential of photoinduced toxicity to occur in benthic organisms with free-swimming early life stages.

Effect of Louisiana sweet crude oil on a Pacific coral, Pocillopora damicornis

Recent oil spill responses such as the Deepwater Horizon event have underscored the need for crude oil ecotoxicological threshold data for shallow water corals to assist in natural resource damage assessments. We determined the toxicity of a mechanically agitated oil-seawater mixture (high-energy water-accommodated fraction, HEWAF) of a sweet crude oil on a branched stony coral, Pocillopora damicornis. We report the results of two experiments: a 96 h static renewal exposure experiment and a "pulse-chase" experiment of three short-term exposure durations followed by a recovery period in artificial seawater. Five endpoints were used to determine ecotoxicological values: 1) algal symbiont chlorophyll fluorescence, 2) a tissue regeneration assay and a visual health metric with three endpoints: 3) tissue integrity, 4) tissue color, and 5) polyp behavior. The sum of 50 entrained polycyclic aromatic hydrocarbons (tPAH50) was used as a proxy for oil exposure. For the 96 h exposure dose response experiment, dark-adapted maximum quantum yield (Fv/Fm) of the dinoflagellate symbionts was least affected by crude oil (EC₅₀ = 913 μg/L tPAH50); light-adapted effective quantum yield (EQY) was more sensitive (EC₅₀ =  428 μg/L tPAH50). In the health assessment, polyp behavior (EC₅₀ = 27 μg/L tPAH50) was more sensitive than tissue integrity (EC₅₀ = 806 μg/L tPAH50) or tissue color (EC₅₀ = 926 μg/L tPAH50). Tissue regeneration proved to be a particularly sensitive measurement for toxicity effects (EC₅₀ = 10 μg/L tPAH50). Short duration (6-24 h) exposures using 503 μg/L tPAH50 (average concentration) resulted in negative impacts to P. damicornis and its symbionts. Recovery of chlorophyll a fluorescence levels for 6-24 h oil exposures was observed in a few hours (Fv/Fm) to several days (EQY) following recovery in fresh seawater. The coral health assessments for tissue integrity and tissue color were not affected following short-term oil exposure durations, but the 96 h treatment duration resulted in significant decreases for both. A reduction in polyp behavior (extension) was observed for all treatment durations, with recovery observed for the short-term (6-24 h) exposures within 1-2 days following placement in fresh seawater. Wounded and intact fragments exposed to oil treatments were particularly sensitive, with significant delays observed in tissue regeneration. Estimating ecotoxicological values for P. damicornis exposed to crude oil HEWAFs provides a basis for natural resource damage assessments for oil spills in reef ecosystems. These data, when combined with ecotoxicological values for other coral reef species, will contribute to the development of species sensitivity models.

Exposure to Crude Oil Induces Retinal Apoptosis and Impairs Visual Function in Fish

Polycyclic aromatic hydrocarbons (PAHs) present in crude oil are known to impair visual development in fish. However, the underlying mechanism of PAH-induced toxicity to the visual system of fish is not understood. Embryonic zebrafish (Danio rerio) at 4 h post fertilization were exposed to weathered crude oil and assessed for visual function using an optokinetic response, with subsequent samples taken for immunohistochemistry and gene expression analysis. Cardiotoxicity was also assessed by measuring the heart rate, stroke volume, and cardiac output, as cardiac performance has been proposed to be a contributing factor to eye-associated malformations following oil exposure. Larvae exposed to the highest concentrations of crude oil (89.8 μg/L) exhibited an increased occurrence of bradycardia, though no changes in stroke volume or cardiac output were observed. However, genes important in eye development and phototransduction were downregulated in oil-exposed larvae, with an increased occurrence of cellular apoptosis, reduced neuronal connection, and reduced optokinetic behavioral response in zebrafish larvae.

Oil toxicity and implications for environmental tolerance in fish

Crude oil and its constituent chemicals are common environmental toxicants in aquatic environments worldwide, and have been the subject of intense research for decades. Importantly, aquatic environments are also the sites of numerous other environmental disturbances that can impact the endemic fauna. While there have been a number of attempts to explore the potential additive and synergistic effects of oil exposure and environmental stressors, many of these efforts have focused on the cumulative effects on typical toxicological endpoints (e.g. survival, growth, reproduction and cellular damage). Fewer studies have investigated the impact that oil exposure may have on the ability of exposed animals to tolerate typically encountered environmental stressors, despite the fact that this is an important consideration when placing oil spills in an ecological context. Here we review the available data and highlight potentially understudied areas relating to how oil exposure may impair organismal responses to common environmental stressors in fishes. We focused on four common environmental stressors in aquatic environments - hypoxia, temperature, salinity and acid-base disturbances - while also considering social stress and impacts on the hypothalamus-pituitary-interrenal axis. Overall, we believe the evidence supports treating the impacts of oil exposure on environmental tolerance as an independent endpoint of toxicity in fishes.

Mahi-mahi (Coryphaena hippurus) life development: morphological, physiological, behavioral and molecular phenotypes

Background

Mahi‐mahi (Coryphaena hippurus) is a commercially and ecologically important fish species that is widely distributed in tropical and subtropical waters. Biological attributes and reproductive capacities of mahi‐mahi make it a tractable model for experimental studies. In this study, life development of cultured mahi‐mahi from the zygote stage to adult has been described.

Results

A comprehensive developmental table has been created reporting development as primarily detailed observations of morphology. Additionally, physiological, behavioral, and molecular landmarks have been described to significantly contribute in the understanding of mahi life development.

Conclusion

Remarkably, despite the vast difference in adult size, many developmental landmarks of mahi map quite closely onto the development and growth of Zebrafish and other warm‐water, active Teleost fishes.

Mahi‐mahi is a tractable model for experimental studies high‐performance pelagic predatory fish species.

Biological attributes of mahi are reported in a comprehensive developmental table.

Physiological, behavioral and molecular landmarks are described through the life development.

Mahi has a rapid growth rate, but the developmental marks are similar to other teleost fishes.

Acute crude oil exposure alters mitochondrial function and ADP affinity in cardiac muscle fibers of young adult Mahi-mahi (Coryphaena hippurus)

Mitochondrial function is critical to support aerobic metabolism through the production of ATP, and deficiencies in mitochondrial bioenergetics will directly impact the performance capacity of highly aerobic tissues such as the myocardium. Cardiac function in fish has been shown to be negatively affected by crude oil exposure, however, the mechanism for this adverse response is largely unexplored. We hypothesized that lipophilic polycyclic aromatic hydrocarbons (PAHs) found in crude oil disrupt the electron transport system (ETS) ultimately leading to mitochondrial dysfunction. In this study, mitochondrial respiration and ADP affinity we measured using high resolution respirometery in permeabilized cardiac muscle fibers of young adult Mahi-mahi (Coryphaena hippurus) after an acute (24 h) whole animal crude oil exposure. Oil exposure reduced both complex I-fueled ADP stimulated respiration (OXPHOS_CI) and complex I,II-fueled ADP stimulated respiration (OXPHOS_{CI, CII}) by 33%,while complex II-fueled ADP stimulated respiration (OXPHOS_CII) was reduced by 25%. These changes were found without changes in enzyme activity or mitochondrial density between control and oil exposed Mahi. Additionally, mitochondrial affinity for ADP was decreased three-fold after acute exposure to crude oil. We purpose that acute crude oil exposure selectively impairs mitochondrial complexes of the electron transport system and ATP supply to the cell. This limited ATP supply could present several challenges to a predatory animal like the mahi; including a reliance on anaerobic metabolism and ultimately cell or tissue death as metabolic substrates are rapidly depleted. However, the impact of this impairment may only be evident under periods of increased aerobic metabolic demand.

Embryonic buoyancy control as a mechanism of ultraviolet radiation avoidance

Pelagic fish have long been presumed to produce buoyant embryos which float and hatch at or near surface waters. Due to their transparency and rapid development, mahi embryos are thought to be especially vulnerable to stressors occurring in surface waters, such as ultraviolet radiation (UVR) and increased temperatures. In the present study, we suggest a possibly critical mechanism of UVR avoidance by pelagic fish embryos. Specifically, we provide evidence that mahi embryos are able to sense UVR and may alter buoyancy as a means of reducing exposure to the most harmful stressors occurring in the upper layers of the water column. Further, once the UVR exposure was terminated, embryos displayed fast recovery of positive buoyancy indicating this response is rapidly dynamic and not pathological. The mechanism behind buoyancy control is not known, but evidence from the current study suggests that ammonia sequestration, as seen in multiple other fish species, is not the primary control mechanism employed by embryonic mahi. Finally, expression of antioxidant and UV repair enzymes were investigated to elucidate possible involvement in observed buoyancy changes and to explore alternative methods of repairing UVR damage.

Combined effects of elevated temperature and Deepwater Horizon oil exposure on the cardiac performance of larval mahi-mahi, Coryphaena hippurus

The 2010 Deepwater Horizon oil spill coincided with the spawning season of many pelagic fish species in the Gulf of Mexico. Yet, few studies have investigated physiological responses of larval fish to interactions between anthropogenic crude oil exposure and natural factors (e.g. temperature, oxygen levels). Consequently, mahi mahi (Coryphaena hippurus) embryos were exposed for 24 hours to combinations of two temperatures (26 and 30°C) and six concentrations of oiled fractions of weathered oil (from 0 to 44.1 μg ∑50PAHs·L^-1). In 56 hours post-fertilization larvae, heart rate, stroke volume and cardiac output were measured as indicators of functional cardiac phenotypes. Fluid accumulation and incidence of edema and hematomas were quantified as indicators of morphological impairments. At both 26 and 30°C, oil-exposed larvae suffered dose-dependent morphological impairments and functional heart failure. Elevation of temperature to 30°C appeared to induce greater physiological responses (bradycardia) at PAH concentrations in the range of 3.0–14.9 μg·L^-1. Conversely, elevated temperature in oil-exposed larvae reduced edema severity and hematoma incidence. However, the apparent protective role of warmer temperature does not appear to protect against enhanced mortality. Collectively, our findings show that elevated temperature may slightly decrease larval resilience to concurrent oil exposure.

Influence of variable ultraviolet radiation and oil exposure duration on survival of red drum (Sciaenops ocellatus) larvae

The toxicity of some polycyclic aromatic hydrocarbons (PAHs) increases with ultraviolet (UV) radiation. The intensity of UV radiation varies within aquatic ecosystems, potentially providing reprieves during which tissue repair may occur. Transient/short-term PAH exposure prior to UV exposure may initiate metabolism/clearance, potentially affecting outcomes. Larval Sciaenops ocellatus were exposed to oil and UV radiation, using either variable photoperiods or pre-UV oil exposure durations. Shorter PAH exposures exhibited greater toxicity, as did exposure to shorter photoperiods. Environ Toxicol Chem 2018;37:2372-2379. © 2018 SETAC.

Developmental and latent effects of diluted bitumen exposure on early life stages of sockeye salmon (Oncorhynchus nerka)

The early life stages of Pacific salmon are at risk of environmental exposure to diluted bitumen (dilbit) as Canada's oil sands industry continues to expand. The toxicity and latent effects of dilbit exposure were assessed in sockeye salmon (Oncorhynchus nerka) exposed to water-soluble fractions of dilbit (WSFd) from fertilization to the swim-up stage, and then reared in clean water for 8 months. Mortality was significantly higher in WSFd-exposed embryos, with cumulative mortality up to 4.6-fold higher in exposed relative to unexposed embryos. The sublethal effects of WSFd exposure included transcriptional up-regulation of cyp1a, a concentration-dependent delay in the onset and progression of hatching, as well as increased prevalence of developmental deformities at total polycyclic aromatic hydrocarbon (TPAH) concentrations ≥35 μg L^-1. Growth and body composition were negatively affected by WSFd exposure, including a concentration-specific decrease in soluble protein concentration and increases in total body lipid and triglyceride concentrations. Mortality continued during the first 2 months after transferring fish to clean water, reaching 53% in fish exposed to 100 μg L^-1 TPAH; but there was no latent impact on swimming performance, heart mass, or heart morphology in surviving fish after 8 months. A latent effect of WSFd exposure on brain morphology was observed, with fish exposed to 4 μg L^-1 TPAH having significantly larger brains compared to other treatment groups after 8 months in clean water. This study provides comprehensive data on the acute, sub-chronic, and latent impacts of dilbit exposure in early life stage sockeye, information that is critical for a proper risk analysis of the impact of a dilbit spill on this socioeconomically important fish species.

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.

Photo-induced toxicity following exposure to crude oil and ultraviolet radiation in 2 Australian fishes

Some polycyclic aromatic hydrocarbons (PAHs), components of crude oil, are known to cause increased toxicity when organisms are co-exposed with ultraviolet radiation, resulting in photo-induced toxicity. The photodynamic characteristics of some PAHs are of particular concern to places like Australia with high ultraviolet radiation levels. The objective of the present study was to characterize the photo-induced toxicity of an Australian North West Shelf oil to early life stage yellowtail kingfish (Seriola lalandi) and black bream (Acanthopagrus butcheri). The fish were exposed to high-energy water accommodated fractions for 24 to 36 h. During the exposure, the fish were either co-exposed to full-intensity or filtered natural sunlight and then transferred to clean water. At 48 h, survival, cardiac effects, and spinal deformities were assessed. Yellowtail kingfish embryos co-exposed to oil and full-spectrum sunlight exhibited decreased hatching success and a higher incidence of cardiac arrhythmias, compared with filtered sunlight. A significant increase in the incidence of pericardial edema occurred in black bream embryos co-exposed to full-spectrum sunlight. These results highlight the need for more studies investigating the effects of PAHs and photo-induced toxicity under environmental conditions relevant to Australia. Environ Toxicol Chem 2018;37:1359-1366. © 2018 SETAC.

Photo-induced toxicity in early life stage fiddler crab (Uca longisignalis) following exposure to Deepwater Horizon oil

The 2010 explosion of the Deepwater Horizon (DWH) oil rig led to the release of millions of barrels of oil in the Gulf of Mexico. Oil in aquatic ecosystems exerts toxicity through multiple mechanisms, including photo-induced toxicity following co-exposure with UV radiation. The timing and location of the spill coincided with both fiddler crab reproduction and peak yearly UV intensities, putting early life stage fiddler crabs at risk of injury due to photo-induced toxicity. The present study assessed sensitivity of fiddler crab larvae to photo-induced toxicity during co-exposure to a range of environmentally relevant dilutions of high-energy water accommodated fractions of DWH oil, and either <10, 50, or 100% ambient sunlight, achieved with filters that allowed for variable UV penetration. Solar exposures (duration: 7-h per day) were conducted for two consecutive days, with a dark recovery period (duration: 17-h) in between. Survival was significantly decreased in treatments the presence of >10% UV and relatively low concentrations of oil. Results of the present study indicate fiddler crab larvae are sensitive to photo-induced toxicity in the presence of DWH oil. These results are of concern, as fiddler crabs play an important role as ecosystem engineers, modulating sediment biogeochemical processes via burrowing action. Furthermore, they occupy an important place in the food web in the Gulf of Mexico.

Photoenhanced Toxicity of Weathered Crude Oil in Sediment and Water to Larval Zebrafish

Solar radiation exposure can increase the toxicity of bioaccumulated oil compounds in a diversity of aquatic species. We investigated the photoenhanced toxicity of weathered South Louisiana crude oil in sediment and water accommodated fractions (WAF) to larval zebrafish. Larvae were first exposed for 24 h to one of six treatments: no oil (sediment or water), 7.5 g oil/kg sediment, oil-only WAF, oil WAF plus the dispersant Corexit 9500A, or dispersant alone. Larvae were then exposed to high or low levels of sunlight in control water for 3 or 3.5 h. Hydrocarbon concentrations were measured in exposure media, including alkanes, polycyclic aromatic compounds and total petroleum hydrocarbons. Significant phototoxicity was observed in larvae exposed to oiled sediment, oil-only WAF, and oil plus dispersant WAF. The results indicated that petroleum from the northern Gulf of Mexico can be phototoxic to larval fish exposed to oil in either the water column or sediment.

Combined effects of oil exposure, temperature and ultraviolet radiation on buoyancy and oxygen consumption of embryonic mahi-mahi, Coryphaena hippurus

The Deepwater Horizon oil spill occurred in the summer of 2010 and coincided with the spawning window of the ecologically and economically important pelagic fish mahi-mahi (Coryphaena hippurus). During summer months, early life stage mahi-mahi were likely also exposed to other naturally occurring stressors such as increased temperature and ultraviolet radiation (UV). Previous research has shown that co-exposure to oil and additional natural stressors can affect the timing and duration of negative buoyancy in mahi-mahi embryos. The current study aimed to elucidate the factors affecting the onset of negative buoyancy and to also explore possible mechanisms behind buoyancy change. Embryos co-exposed to oil and/or increased temperature and UV radiation displayed early onset of negative buoyancy with concurrent increases in oxygen consumption and sinking rates, which are normally only seen during the period directly preceding hatch. Results also suggest a behavioral response in which embryos avoid UV radiation by sinking down the water column but reestablish positive buoyancy once the UV radiation is removed. These findings imply that embryos can dynamically change their position in the water column in response to external cues and thus may have much greater control over buoyancy than previously thought.

Developmental toxicity of hydroxylated chrysene metabolites in zebrafish embryos

One of the primary sources of polycyclic aromatic hydrocarbons (PAHs) in marine environments is oil. Photochemical oxidation and microbial transformation of PAH-containing oils can result in the formation of oxygenated products. Among the PAHs in crude oil, chrysene is one of the most persistent within the water column and may be transformed to 2- and 6-hydroxychrysene (OHCHR). Both of these compounds have been shown to activate (2-OHCHR) and antagonize (6-OHCHR) the estrogen receptor (ER). Previous studies in our lab have shown that estrogen can significantly alter zebrafish development. However, little is known about the developmental toxicity of hydroxylated PAHs. Zebrafish embryos were exposed to 0.5-10μM of 2- or 6-OHCHR from 2h post-fertilization (hpf) until 76hpf. A significant decrease in survival was observed following exposure to 6-OHCHR - but not 2-OHCHR. Both OHCHRs significantly increased the percentage of overall deformities after treatment. In addition to cardiac malformations, ocular and circulatory defects were also observed in embryos exposed to both compounds, while 2-OHCHR generally resulted in a higher prevalence of effect. Moreover, treatment with 2-OHCHR resulted in a significant decrease in hemoglobin levels. ER nor G-Protein coupled estrogen receptor (GPER) antagonists and agonists did not rescue the observed defects. We also analyzed the expression of cardiac-, eye- and circulation-related genes previously shown to be affected by oil. Rhodopsin mRNA expresssion was significantly decreased by both compounds equally. However, exposure to 2-OHCHR significantly increased the expression of the hematopoietic regulator, runx1 (runt related transcription factor 1). These results indicate the toxicity of oxygenated photoproducts of PAHs and suggest that other targets and signaling pathways may contribute to developmental toxicity of weathered oil. Our findings also demonstrate the regio-selective toxicity of hydroxy-PAHs in the effects on eye and circulatory development and raise the need to identify mechanisms and ecological risks of oxy-PAHs to fish populations.

Developmental transcriptomic analyses for mechanistic insights into critical pathways involved in embryogenesis of pelagic mahi-mahi (Coryphaena hippurus)

Mahi-mahi (Coryphaena hippurus) is a commercially and ecologically important species of fish occurring in tropical and temperate waters worldwide. Understanding early life events is crucial for predicting effects of environmental stress, which is largely restricted by a lack of genetic resources regarding expression of early developmental genes and regulation of pathways. The need for anchoring developmental stages to transcriptional activities is highlighted by increasing evidence on the impacts of recurrent worldwide oil spills in this sensitive species during early development. By means of high throughput sequencing, we characterized the developmental transcriptome of mahi-mahi at three critical developmental stages, from pharyngula embryonic stage (24 hpf) to 48 hpf yolk-sac larva (transition 1), and to 96 hpf free-swimming larva (transition 2). With comparative analysis by multiple bioinformatic tools, a larger number of significantly altered genes and more diverse gene ontology terms were observed during transition 2 than transition 1. Cellular and tissue development terms were more significantly enriched in transition 1, while metabolism related terms were more enriched in transition 2, indicating a switch progressing from general embryonic development to metabolism during the two transitions. Special focus was given on the most significant common canonical pathways (e.g. calcium signaling, glutamate receptor signaling, cAMP response element-binding protein signaling, cardiac β-adrenergic signaling, etc.) and expression of developmental genes (e.g. collagens, myosin, notch, glutamate metabotropic receptor etc.), which were associated with morphological changes of nervous, muscular, and cardiovascular system. These data will provide an important basis for understanding embryonic development and identifying molecular mechanisms of abnormal development in fish species.