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.

Is hypoxia vulnerability in fishes a by-product of maximum metabolic rate?

The metabolic index concept combines metabolic data and known thermal sensitivities to estimate the factorial aerobic scope of animals in different habitats, which is valuable for understanding the metabolic demands that constrain species' geographical distributions. An important assumption of this concept is that the O2 supply capacity (which is equivalent to the rate of oxygen consumption divided by the environmental partial pressure of oxygen: ) is constant at O2 tensions above the critical O2 threshold (i.e. the where O2 uptake can no longer meet metabolic demand). This has led to the notion that hypoxia vulnerability is not a selected trait, but a by-product of selection on maximum metabolic rate. In this Commentary, we explore whether this fundamental assumption is supported among fishes. We provide evidence that O2 supply capacity is not constant in all fishes, with some species exhibiting an elevated O2 supply capacity in hypoxic environments. We further discuss the divergent selective pressures on hypoxia- and exercise-based cardiorespiratory adaptations in fishes, while also considering the implications of a hypoxia-optimized O2 supply capacity for the metabolic index concept.

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.

Hypoxia acclimation increases novelty response strength during fast-starts in the African mormyrid, Marcusenius victoriae.

Many fishes perform quick and sudden swimming maneuvers known as fast-starts to escape when threatened. In pulse-type weakly electric fishes these responses are accompanied by transient increases in the rate of electric signal production known as novelty responses. While novelty responses may increase an individual's information about their surroundings, they are aerobically powered and may come at a high energetic cost when compared to fast-starts, which rely primarily on anaerobic muscle. The juxtaposition between two key aspects of fast-starts in these fishes - the aerobic novelty response and the anaerobic swimming performance - makes them an interesting model for studying effects of hypoxia on escape performance and sensory information acquisition. We acclimated the hypoxia-tolerant African mormyrid Marcusenius victoriae to either high or low dissolved oxygen (DO) levels for 8weeks, after which fast-starts and novelty responses were quantified under both high (normoxic) and low-DO (hypoxic) test conditions. Hypoxia-acclimated fish exhibited higher maximum curvature than normoxia-acclimated fish. Displacement of normoxia-acclimated fish was not reduced under acute hypoxic test conditions. Novelty responses were given upon each startle, whether or not the fish performed a fast-start; however, novelty responses associated with fast-starts were significantly stronger than those without, suggesting a functional link between fast-start initiation and the motor control of the novelty response. Overall, hypoxia-acclimated individuals produced significantly stronger novelty responses during fast-starts. We suggest that increased novelty response strength in hypoxia-acclimated fish corresponds to an increased rate of sensory sampling, which may compensate for potential negative effects of hypoxia on higher-level processing.

How temperature-induced variation in musculoskeletal anatomy affects escape performance and survival of zebrafish (Danio rerio).

Fishes are particularly sensitive to the effects of environmental conditions during early development, which can significantly impact adult morphology, performance, and survival. Previous research has highlighted the sensitivity of fishes to the effects of temperature during early development on vertebral number and muscle composition, which are both important determinants of an individual's swimming performance. In this study, we investigated the effect of developmental temperature on vertebral and muscle variation, and the subsequent effect of any variation on burst swimming performance in zebrafish (Danio rerio). Following development at a range of temperatures, all individuals were shifted to and maintained at a common temperature before startle responses were recorded and individuals were analyzed for either vertebral number or muscle composition. Our results indicate that developmental temperature does not significantly affect muscle composition, but can affect an individual's vertebral number, and that individuals with more vertebrae achieved greater displacement and velocities during C-start performance. To determine the ecological importance of this vertebral variation and to identify the potential selective factors behind it, we exposed populations of zebrafish with various vertebral numbers to native predators, needlenose garfish (Xenentodon cancila). We found that only caudal vertebral number was related to survival, and that survivors had the same caudal vertebral number across developmental temperatures. Overall, this work highlights the importance of including variation in musculoskeletal anatomy when investigating what is driving selection in fishes. J. Exp. Zool. 325A:25-40, 2016. © 2015 Wiley Periodicals, Inc.

Linking vertebral number to performance of aquatic escape responses in the axolotl (Ambystoma mexicanum).

Environmental conditions during early development in ectothermic vertebrates can lead to variation in vertebral number among individuals of the same species. It is often seen that individuals of a species raised at cooler temperatures have more vertebrae than individuals raised at warmer temperatures, although the functional consequences of this variation in vertebral number on swimming performance are relatively unclear. To investigate this relationship, we tested how vertebral number in axolotls (Ambystoma mexicanum) affected performance of aquatic escape responses (C-starts). Axolotls were reared at four temperatures (12-24°C) encompassing their natural thermal range and then transitioned to a mean temperature (18°C) three months before C-starts were recorded. Our results showed variation in vertebral number, but that variation was not significantly affected by developmental temperature. C-start performance among axolotls was significantly correlated with caudal vertebral number, and individuals with more caudal vertebrae were able to achieve greater curvature more quickly during their responses than individuals with fewer vertebrae. However, our results show that these individuals did not achieve greater displacements or velocities, and that developmental temperature did not have any effect on C-start performance. We highlight that the most important aspects of escape swim performance (i.e., how far individuals get from a threat and how quickly they move the most important parts of the body away from that threat) are consistent across individuals regardless of developmental temperature and morphological variation.

Habitat-specific foraging and sex determine mercury concentrations in sympatric benthic and limnetic ecotypes of threespine stickleback.

Mercury (Hg) is a widespread environmental contaminant known for the neurotoxicity of its methylated forms, especially monomethylmercury, which bioaccumulates and biomagnifies in aquatic food webs. Mercury bioaccumulation and biomagnification rates are known to vary among species utilizing different food webs (benthic vs limnetic) within and between systems. The authors assessed whether carbon and nitrogen stable isotope values and total Hg (THg) concentrations differed between sympatric benthic and limnetic ecotypes and sexes of threespine stickleback fish (Gasterosteus aculeatus) from Benka Lake, Alaska, USA. The mean THg concentration in the limnetic ecotype was significantly higher (difference between benthic and limnetic means equals 26 mg/kg dry wt or 16.1%) than that of the benthic ecotype. Trophic position and benthic carbon percentage utilized were both important determinants of THg concentration; however, the 2 variables were of approximately equal importance in females, whereas trophic position clearly explained more of the variance than benthic carbon percentage in males. Additionally, strong sex effects (mean difference between females and males equals 45 mg/kg dry wt or 29.4%) were observed in both ecotypes, with female fish having lower THg concentrations than males. These results indicate that trophic ecology and sex are both important determinants of Hg contamination even within a single species and lake and likely play a role in governing Hg concentrations in higher trophic levels.