Early life-stage Deepwater Horizon crude oil exposure induces latent osmoregulatory defects in larval red drum (Sciaenops ocellatus).

Crude oil is known to induce developmental defects in teleost fish exposed during early-life stages (ELSs). A recent study has demonstrated that zebrafish (Danio rerio) larvae acutely exposed to Deepwater Horizon (DHW) crude oil showed transcriptional changes in key genes involved in early kidney (pronephros) development and function, which were coupled with pronephric morphological defects. Given the osmoregulatory importance of the kidney, it is unknown whether ELS effects arising from short-term crude exposures result in long-term osmoregulatory defects, particularly within estuarine fishes likely exposed to DWH oil following the spill. To address this knowledge gap, an acute 72 h exposure to red drum (Sciaenops ocellatus) larvae was performed using high-energy water-accommodated fractions (HEWAFs) of DWH weathered oil to analyze transcriptional changes in genes involved in pronephros development and function by quantitative PCR. To test the latent effects of oil exposure on osmoregulation ability, red drum larvae were first exposed to HEWAF for 24 h. Larvae were then reared in clean seawater for two weeks and a 96 h acute osmotic challenge test was performed by exposing the fish to waters with varying salinities. Latent effects of ELS crude oil exposure on osmoregulation were assessed by quantifying survival during the acute osmotic challenge test and analyzing transcriptional changes at 14 dpf. Results demonstrated that ELS crude oil exposure reduced survival of red drum larvae when challenged in hypoosmotic waters and that latent transcriptional changes in some target pronephric genes were evident, indicating that an affected kidney likely contributed to the increased mortality.

Exposure to Deepwater Horizon crude oil increases free cholesterol in larval red drum (Sciaenops ocellatus).

The 2010 Deepwater Horizon oil spill impacted over 2100 km of shoreline along the northern Gulf of Mexico, which coincided with the spawning season of many coastal species, including red drum (Sciaenops ocellatus). Red drum develop rapidly and are sensitive to crude oil exposure during the embryonic and larval periods. This study investigates the predictions from recent transcriptomic studies that cholesterol biosynthetic processes are impacted by oil exposure in fish early life stages. We found that red drum larvae exposed for 72 h to ΣPAH50 3.55-15.45 µg L-1 exhibited significantly increased pericardial area, a cardiotoxicity metric, but the expression of several genes targeted in the cholesterol synthesis pathway was not affected. However, whole-mount staining revealed significant increases in free cholesterol throughout the larval body (ΣPAH50 4.71-16.15 µg L-1), and total cholesterol followed an increasing trend (ΣPAH50 3.55-15.45 µg L-1). Cholesterol plays a critical role in fish embryo development and ion channel function. Therefore, the disruption of cholesterol homeostasis, as observed here, could play a role in the oil toxicity phenotype observed across many fish species.

Behavioral Changes in a Coastal Marine Fish Lead to Increased Predation Risk Following Oil Exposure.

Fishes exposed to crude oil have shown reduced sociability and poor habitat selection, which corresponded with increased predation risk. However, the contribution of oil-induced cardiorespiratory impairments to these findings is uncertain. This study explores the effect of oil exposure on predation risk in a model fish species, Sciaenops ocellatus, across a suite of physiological and behavioral end points to elucidate the mechanisms through which any observed effects are manifested. Using mesocosms to assess group predator avoidance, oil exposure to 36.3 µg l-1 ΣPAH reduced the time to 50% mortality from a mean time of 80.0 (74.1-86.0 95% confidence interval [CI]) min to 39.2 (35.6-42.8 95% CI) min. The influence of oil impaired cardiorespiratory and behavioral pathways on predation risk was assessed based on respiratory performance, swim performance, sociability, and routine activity. Swim trials demonstrated that cardiorespiratory and swim performance were unaffected by exposures to 26.6 or 100.8 µg l-1 ΣPAH. Interestingly, behavioral tests revealed that exposure to 26.6 µg l-1 ΣPAH increased distance moved, speed, acceleration, and burst activity. These data indicate that behavioral impairment is more sensitive than cardiorespiratory injury and may be a more important driver of downstream ecological risk following oil exposure in marine species.

The effects of acute crude oil exposure on growth and competition in red drum, Sciaenops ocellatus.

Crude oil is a well-known toxicant that reduces cardiorespiratory performance in acutely exposed fishes. While toxic effects can manifest in death in severe cases, the ecological consequences of sub-lethal exposure remain uncertain. This study investigated the impact of crude oil exposure on long-term social competition, growth, and metabolic performance in a coastal species, the red drum (Sciaenops ocellatus). Fish were acutely exposed to either control or one of two environmentally relevant oil concentrations and reared together in groups of 15 (5 from each exposure concentration) for eight weeks under resource-rich or resource-limited scenarios. Relative to controls, a 41.3% and 45.9% reduction in the specific growth rate was-observed following exposure to 25.3 and 53.4 µg l-1 ΣPAH respectively under resource-limited conditions. These fish were subsequently sampled for metabolic performance and common indicators of social subordination including reduced glucocorticoid receptors in the gill and caudal fin damage. The reduction in specific growth rate coincided with a 15.1% and 17.3% reduction in standard metabolic rate; however, maximum metabolic rate and aerobic scope were unaffected. Additionally, measures of social subordination showed no differences between oil-exposed and control fish. These results reinforce the hypothesis that acute oil exposure can have prolonged sub-lethal effects that compromise the ability of exposed individuals to perform effectively in their environment, including gathering and/or metabolizing food. Furthermore, this work highlights the premise that oil spills can be more detrimental in already at-risk ecosystems.

Oil exposure alters social group cohesion in fish.

Many animal taxa live in groups to increase foraging and reproductive success and aid in predator avoidance. For fish, a large proportion of species spend all or part of their lives in groups, with group coordination playing an important role in the emergent benefits of group-living. Group cohesion can be altered by an array of factors, including exposure to toxic environmental contaminants. Oil spills are one of the most serious forms of pollution in aquatic systems, and while a range of effects of acute oil exposure on animal physiology have been demonstrated, sub-lethal effects on animal behavior are relatively under-studied. Here we used an open-field behavioral assay to explore influence of acute oil exposure on social behavior in a gregarious fish native to the Gulf of Mexico, Atlantic croaker (Micropogonias undulatus). We used two oil concentrations (0.7% and 2% oil dilution, or 6.0 ± 0.9 and 32.9 ± 5.9 µg l-1 ΣPAH50 respectively) and assays were performed when all members of a group were exposed, when only one member was exposed, and when no individuals were exposed. Shoal cohesion, as assessed via mean neighbor distance, showed significant impairment following acute exposure to 2% oil. Fish in oil-exposed groups also showed reduced voluntary movement speed. Importantly, overall group cohesion was disrupted when even one fish within a shoal was exposed to 2% oil, and the behavior of unexposed in mixed groups, in terms of movement speed and proximity to the arena wall, was affected by the presence of these exposed fish. These results demonstrate that oil exposure can have adverse effects on fish behavior that may lead to reduced ecological success.

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.

mRNA-miRNA-Seq Reveals Neuro-Cardio Mechanisms of Crude Oil Toxicity in Red Drum ( Sciaenops ocellatus).

Polycyclic aromatic hydrocarbons (PAHs) present in crude oil can cause global gene dysregulation and developmental impairment in fish. However, the mechanisms that alter gene regulation are not well understood. In this study, larval red drum ( Sciaenops ocellatus) were exposed to water accommodated fractions of source oil (6.8, 13.7, and 35.9 µg/L total PAHs) and weathered slick oil (4.7, 8.1, and 18.0 µg/L total PAHs) from the Deepwater Horizon (DWH) oil spill. The global mRNA-microRNA functional networks associated with the toxicity of DWH oil were explored by next-generation sequencing and in-depth bioinformatics analyses. Both source and slick oil significantly altered the expression of miR-18a, miR-27b, and miR-203a across all exposure concentrations. Consistent with the observed concentration-dependent morphological changes, the target mRNAs of these microRNAs were predominantly involved in neuro-cardio system development processes and associated key signaling pathways such as axonal guidance signaling, cAMP-response-element-binding protein signaling in neurons, calcium signaling, and nuclear-factor-of-activated T cells signaling in cardiac hypertrophy. The results indicated that the developmental toxicity of crude oil may result from the abnormal expression of microRNAs and associated target genes, especially for the nervous system. Moreover, we provide a case study for systematic toxicity evaluation leveraging mRNA-microRNA-seq data using nonmodel species.

Effects of Deepwater Horizon crude oil on ocular development in two estuarine fish species, red drum (Sciaenops ocellatus) and sheepshead minnow (Cyprinodon variegatus).

Polycyclic aromatic hydrocarbons (PAHs) present in crude oil have been shown to cause the dysregulation of genes important in eye development and function, as well as morphological abnormalities of the eye. However, it is not currently understood how these changes in gene expression are manifested as deficits in visual function. Embryonic red drum (Sciaenops ocellatus) and sheepshead minnow (Cyprinodon variegatus) were exposed to water accommodated fractions (WAFs) of weathered crude oil and assessed for visual function using an optomotor response assay in early life-stage larvae, with subsequent samples taken for histological analysis of the eyes. Larvae of both species exposed to increasing concentrations of oil exhibited a reduced optomotor response. The mean diameters of retinal layers, which play an important role in visual function and image processing, were significantly reduced in oil-exposed sheepshead larvae, though not in red drum larvae. The present study provides evidence that weathered crude oil has a significant effect on visual function in early life-stage fishes.

Social competition in red drum (Sciaenops ocellatus) is influenced by crude oil exposure.

The present study examined impacts of crude oil exposure on dyad competition in juvenile red drum. Following the 2010 Deepwater Horizon oil spill, it has become well established that oil exposure can constrain maximum metabolic rate, reduce aerobic scope and exercise performance in marine fish. Aerobic scope is one of the physiological characteristics that is a known determinant of dominance in fish social hierarchy formation. As such, oil exposure may predispose individuals to subordinate social status, complete with the concomitant ecological costs. We tested this hypothesis on the gregarious Gulf of Mexico species, the red drum (Sciaenops ocellatus). Using a standard dyad, one-on-one, test design, we first assessed the parameters - including size and aerobic scope- that predict social dominance. Of the tested parameters, only aerobic scope was predictive of social dominance, with dominant individuals consistently having higher aerobic scopes than subordinates. Hierarchy formation between individuals exposed to one of two oil concentrations (5.7 ± 0.5 and 9.0 ± 0.2 µg l-1 ΣPAH50) and unexposed conspecifics were then investigated. As hypothesized, fish exposed to both oil concentrations were more likely to be subordinate than what would occur by random chance. These results demonstrate that the physiological constraints imposed by oil exposure can affect social status and behavior in fishes, which can have downstream consequences for ecological fitness.

The effects of oil induced respiratory impairment on two indices of hypoxia tolerance in Atlantic croaker (Micropogonias undulatus).

The Gulf of Mexico was home to the Deepwater Horizon oil spill, and is also known to exhibit seasonal declines in oxygen availability. Oil exposure in fish is known to impact oxygen uptake through cardiac impairment, which raises questions about the additive effects of these two stressors. Here we explore this question on the Atlantic croaker using two measures of hypoxia tolerance: critical oxygen threshold (Pcrit), and time to loss of equilibrium (LOE). We first demonstrated that 24 h exposure to 10.1 and 23.2 µg l-1 ΣPAH50 significantly impaired oxygen uptake. There was no effect of exposure on Pcrit or LOE. Exposure did result in significantly different repeatability between pre- and post-exposure Pcrit, suggesting that hypoxia tolerant individual may see greater impacts following exposure. These results suggest oil exposure does not have wide scale detrimental outcomes for hypoxia tolerance in fish, yet there may be fine scale impairments of ecological significance.

Cardiac function and survival are affected by crude oil in larval red drum, Sciaenops ocellatus.

Following exposure to weathered and non-weathered oil, lethal and sub-lethal impacts on red drum larvae were assessed using survival, morphological, and cardiotoxicity assays. The LC50 for red drum ranged from 14.6 (10.3-20.9) to 21.3 (19.1-23.8) µgl-1 ΣPAH with no effect of exposure timing during the pre-hatch window or oil weathering. Similarly, morphological deformities showed dose responses in the low ppb range. Cardiac output showed similar sensitivity resulting in a major 70% reduction after exposure to 2.6µgl-1 ΣPAH. This cardiac failure was driven by reduced stroke volume rather than bradycardia, meaning that in some species, cardiac function is more sensitive than previously thought. After the Deepwater Horizon oil spill, much of this type of work has primarily focused on pelagic species with little known about fast developing estuarine species. These results demonstrate similarity sensitivity of the red drum as their pelagic counter parts, and more importantly, that cardiac function is dramatically reduced in concert with pericardial edema.