Effects of elevated CO2 on metabolic rate and nitrogenous waste handling in the early life stages of yellowfin tuna (Thunnus albacares).

Ocean acidification is predicted to have a wide range of impacts on fish, but there has been little focus on broad-ranging pelagic fish species. Early life stages of fish are thought to be particularly susceptible to CO2 exposure, since acid-base regulatory faculties may not be fully developed. We obtained yellowfin tuna (Thunnus albacares) from a captive spawning broodstock population and exposed them to control or 1900 µatm CO2 through the first three days of development as embryos transitioned into yolk sac larvae. Metabolic rate, yolk sac depletion, and oil globule depletion were measured to assess overall energy usage. To determine if CO2 altered protein catabolism, tissue nitrogen content and nitrogenous waste excretion were quantified. CO2 exposure did not significantly impact embryonic metabolic rate, yolk sac depletion, or oil globule depletion, however, there was a significant decrease in metabolic rate at the latest measured yolk sac larval stage (36 h post fertilization). CO2-exposure led to a significant increase in nitrogenous waste excretion in larvae, but there were no differences in nitrogen tissue accumulation. Nitrogenous waste accumulated in embryos as they developed but decreased after hatch, coinciding with a large increase in nitrogenous waste excretion and increased metabolic rate in newly hatched larvae. Our results provide insight into how yellowfin tuna are impacted by increases in CO2 in early development, but more research with higher levels of replication is needed to better understand long-term impacts and acid-base regulatory mechanisms in this important pelagic fish.

Brief Oil Exposure Reduces Fitness in Wild Gulf of Mexico Mahi-Mahi (Coryphaena hippurus).

The Deepwater Horizon (DWH) disaster released 3.19 million barrels of crude oil into the Gulf of Mexico (GOM) in 2010, overlapping the habitat of pelagic fish populations. Using mahi-mahi (Coryphaena hippurus)─a highly migratory marine teleost present in the GOM during the spill─as a model species, laboratory experiments demonstrate injuries to physiology and behavior following oil exposure. However, more than a decade postspill, impacts on wild populations remain unknown. To address this gap, we exposed wild mahi-mahi to crude oil or control conditions onboard a research vessel, collected fin clip samples, and tagged them with electronic tags prior to release into the GOM. We demonstrate profound effects on survival and reproduction in the wild. In addition to significant changes in gene expression profiles and predation mortality, we documented altered acceleration and habitat use in the first 8 days oil-exposed individuals were at liberty as well as a cessation of apparent spawning activity for at least 37 days. These data reveal that even a brief and low-dose exposure to crude oil impairs fitness in wild mahi-mahi. These findings offer new perspectives on the lasting impacts of the DWH blowout and provide insight about the impacts of future deep-sea oil spills.

Characterizing the stress response in juvenile Delta smelt exposed to multiple stressors.

The Delta Smelt (Hypomesus transpacificus), once an abundant fish endemic to the Sacramento-San Joaquin Estuary, is now on the brink of extinction. Due to the high sensitivity of this species, knowledge of their stress response will be vital to their future survival and sustainability. Understanding the magnitude and kinetics of cortisol induction in Delta Smelt will provide valuable information when interpreting the degree of environmentally relevant stressors, such as warming and predator exposure. As little is known about the primary stress response and cortisol dynamics in Delta Smelt, the first aim of this study was to measure basal and maximal whole-body cortisol prior to and following exposure to a sublethal and significant netting stress at 17 and 21 °C. Our findings reveal that juvenile Delta Smelt held at 21 °C display an exacerbated stress response and a reduction in available energy compared to fish held at 17 °C. There was no evidence of the secondary stress response to the netting stress as whole-body glucose and lactate levels in treatment groups remained similar to basal values. The second aim of this study was to investigate the effect of a largemouth bass (Micropterus salmoides) predator cue, which was found to induce a significant increase in cortisol relative to control levels in juvenile Delta Smelt. Indices such as cortisol can be used as bioindicators of stress in the field and results from this study suggest that moderate temperatures and reduced predation are optimal release conditions during hatchery-based supplementation to minimize stress to this highly sensitive species.

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.

Changes in microRNA-mRNA Signatures Agree with Morphological, Physiological, and Behavioral Changes in Larval Mahi-Mahi Treated with Deepwater Horizon Oil.

In this study, we performed a systematic evaluation of global microRNA-mRNA interactions associated with the developmental toxicity of Deepwater Horizon oil using a combination of integrated mRNA and microRNA deep sequencing, expression profiling, gene ontology enrichment, and functional predictions by a series of advanced bioinformatic tools. After exposure to water accommodated fraction (WAF) of both weathered slick oil (0.5%, 1%, and 2%) and source oil (0.125%, 0.25%, and 0.5%) from the Deep Water Horizon oil spill, four dose-dependent miRNAs were identified, including three up-regulated (miR-23b, miR-34b, and miR-181b) and one down-regulated miRNAs (miR-203a) in mahi-mahi hatchings exposed from 6 h postfertilization (hpf) to 48 hpf. Consistent with morphological, physiological, and behavioral changes, the target genes of these miRNAs were largely involved in the development of the cardiovascular, visual, nervous system and associated toxicity pathways, suggesting that miRNAs play an essential role in regulating the responses to oil exposure. The results obtained from this study improve our understanding of the role of miRNAs and their target genes in relation to dose-dependent oil toxicity and provide the potential of using miRNAs as novel biomarkers in future oil studies.

Time- and Oil-Dependent Transcriptomic and Physiological Responses to Deepwater Horizon Oil in Mahi-Mahi (Coryphaena hippurus) Embryos and Larvae.

The Deepwater Horizon (DWH) oil spill contaminated the spawning habitats for numerous commercially and ecologically important fishes. Exposure to the water accommodated fraction (WAF) of oil from the spill has been shown to cause cardiac toxicity during early developmental stages across fishes. To better understand the molecular events and explore new pathways responsible for toxicity, RNA sequencing was performed in conjunction with physiological and morphological assessments to analyze the time-course (24, 48, and 96 h post fertilization (hpf)) of transcriptional and developmental responses in embryos/larvae of mahi-mahi exposed to WAF of weathered (slick) and source DWH oils. Slick oil exposure induced more pronounced changes in gene expression over time than source oil exposure. Predominant transcriptomic responses included alteration of EIF2 signaling, steroid biosynthesis, ribosome biogenesis and activation of the cytochrome P450 pathway. At 96 hpf, slick oil exposure resulted in significant perturbations in eye development and peripheral nervous system, suggesting novel targets in addition to the heart may be involved in the developmental toxicity of DHW oil. Comparisons of changes of cardiac genes with phenotypic responses were consistent with reduced heart rate and increased pericardial edema in larvae exposed to slick oil but not source oil.

Effects of turbidity, temperature and predation cue on the stress response of juvenile delta smelt.

The San Francisco Estuary (SFE) is one of the most degraded ecosystems in the United States, and organisms that inhabit it are exposed to a suite of environmental stressors. The delta smelt (Hypomesus transpacificus), a small semi-anadromous fish endemic to the SFE and considered an indicator species, is close to extinction in the wild. The goal of this study was to investigate how environmental alterations to the SFE, such as reductions in turbidities, higher temperatures and increased prevalence of invasive predators affect the physiology and stress response of juvenile delta smelt. Juvenile delta smelt were exposed to two temperatures (17 and 21°C) and two turbidities (1-2 and 10-11 NTU) for 2 weeks. After the first week of exposure, delta smelt were exposed to a largemouth bass (Micropterus salmoides) predator cue at the same time every day for 7 days. Fish were measured and sampled on the first (acute) and final (chronic) day of exposures to predator cues and later analyzed for whole-body cortisol, glucose, lactate, and protein. Length and mass measurements were used to calculate condition factor of fish in each treatment. Turbidity had the greatest effect on juvenile delta smelt and resulted in reduced cortisol, increased glucose and lactate, and greater condition factor. Elevated temperatures reduced available energy in delta smelt, indicated by lower glucose and total protein, whereas predator cue exposure had negligible effects on their stress response. This is the first study to show reduced cortisol in juvenile delta smelt held in turbid conditions and adds to the growing data that suggest this species performs best in moderate temperatures and turbidities. Multistressor experiments are necessary to understand the capacity of delta smelt to respond to the multivariate and dynamic changes in their natural environment, and results from this study should be considered for management-based conservation efforts.

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.

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.

Impacts of a local music festival on fish stress hormone levels and the adjacent underwater soundscape.

An understudied consequence of coastal urbanization on marine environments is sound pollution. While underwater anthropogenic sounds are recognized as a threat to aquatic organisms, little is known about the effects of above-surface coastal sound pollution on adjacent underwater soundscapes and the organisms inhabiting them. Here, the impact of noise from the 2019 Ultra Music Festival® in Miami, FL, USA was assessed at the University of Miami Experimental Hatchery (UMEH) located directly adjacent to the music festival and on underwater sound levels in Bear Cut, a nearby water channel. In addition, stress hormone levels in fish held at UMEH were measured before and during the festival. Air sound levels recorded at UMEH during the Ultra Music Festival did not exceed 72 dBA and 98 dBC. The subsurface sound intensity levels in the low frequency band increased by 2-3 dB re 1 µPa in the adjacent waterway, Bear Cut, and by 7-9 dB re 1 µPa in the fish tanks at UMEH. Gulf toadfish (Opsanus beta) housed in the UMEH tanks experienced a 4-5 fold increase in plasma cortisol, their main stress hormone, during the first night of the Ultra Music Festival compared to two baseline samples taken 3 weeks and 4 days before Ultra. While this study offers preliminary insights into this type of sound pollution, more research is needed to conclude if Ultra caused a stress response in wild organisms and to fully understand the implications of this type of sound pollution.

Impacts of deepwater horizon oil on fish.

An explosion on the Deepwater Horizon (DWH) oil rig in 2010 lead to the largest marine oil spill to occur in US history, resulting in significant impacts to the ecosystems and organisms in the Northern Gulf of Mexico (GoM). The present review sought to summarize and discuss findings from the 50+ peer-reviewed publications reporting effects of DWH oil exposure on teleost fish, and concludes that oil toxicity is a multi-target, multi-organ syndrome with substantial species-specific sensitivity differences. Of the 15 species tested with characterized exposures, 20% show effects at concentrations <1 µg l-1 while 50% display effects at <8.6 µg l-1 ΣPAH50, concentrations well within the range of reported environmental levels during the spill. Cardiotoxic effects are among the most frequently reported endpoints in DWH oil exposure studies and are thought to have significant downstream effects on fitness and survival. However, additional and possibly cardio-toxic independent impacts on sensory function and behavior are reported at very low exposure concentrations (< 1 µg l-1 ∑PAH50) and are clearly deserving of further study. Available information about modes of action leading to different categories of effects are summarized in the present review. An overview of the literature illustrates that early life stages (ELS) are approximately 1-order of magnitude more sensitive than corresponding later life stages, but also illustrates that adults can be impacted at concentrations as low as 4 µg l-1 ΣPAH50. The majority of studies exploring DWH oil toxicity in fish are performed using acute exposures (1-2 days), mid-range test temperatures (26-28 °C) and measure effects at the molecular to organismal levels, leaving a pressing need for more long-term exposures, exposures at the upper and lower levels of GoM relevant temperatures, and studies investigating population level impacts.

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.

Effects of corexit 9500A and Corexit-crude oil mixtures on transcriptomic pathways and developmental toxicity in early life stage mahi-mahi (Coryphaena hippurus).

Crude oil and polycyclic aromatic hydrocarbon (PAH) exposure in early life stage fish has been well-characterized to induce phenotypic malformations such as altered heart development and other morphological impacts. The effects of chemical oil dispersants on toxicity are more controversial. To better understand how chemical dispersion of oil can impact toxicity in pelagic fish, embryos of mahi-mahi (Coryphaena hippurus) were exposed to three concentrations of the chemical dispersant Corexit 9500A, or Corexit 9500A-oil mixtures (chemically enhanced water accommodated fractions: CEWAF) of Deepwater Horizon crude oil for 48 h. RNA sequencing, gene ontology enrichment, and phenotypic measurements were conducted to assess toxicity. Exposure to Corexit 9500A altered expression of less than 50 genes at all concentrations (2.5, 5, and 10 mg/L nominal concentration) and did not induce acute mortality or phenotypic malformations, corroborating other studies showing minimal effects of Corexit 9500A on developing mahi-mahi embryos. CEWAF preparations contained environmentally relevant ∑PAH concentrations ranging from 1.4 to 3.1 µg/L and similarly did not alter larval morphology. Differentially expressed genes and significantly altered pathways related to cardiotoxicity, visual impairments, and Ca2+ homeostasis reinforced previous work that expression of genes associated with the heart and eye are highly sensitive molecular endpoints in oil-exposed early life stage fish. Differential expression and gene ontology pathways were similar across the three CEWAF treatments, indicating that increased chemical dispersion did not alter molecular outcomes within the range tested here. In addition, significant sublethal molecular responses occurred in the absence of observable phenotypic changes to the heart, indicating that effects of oil on early life stage fish may not be completely dependent on cardiac function.

Deepwater Horizon crude oil exposure alters cholesterol biosynthesis with implications for developmental cardiotoxicity in larval mahi-mahi (Coryphaena hippurus).

During the spring and summer of 2010, the Deepwater Horizon (DWH) oil well released over three million barrels of crude oil into the Gulf of Mexico. As the oil dispersed it contaminated ecosystems that support numerous Gulf species including mahi-mahi (Coryphaena hippurus). The timing of the spill, and location of the surface slick, coincided with the spawning of many species in the region, raising concerns over embryonic and larval exposure. Numerous abnormalities due to crude oil exposure have been documented in fish early life stages, including cardiotoxicity; however, knowledge of the molecular mechanisms that cause these phenotypes is still limited. Several transcriptomic studies have presented cholesterol biosynthesis as one of the top enriched pathways following PAH exposure. In this study we exposed mahi-mahi embryos to DWH oil collected from the surface slick. At exposures ranging from ∑PAH 1.69 µg/L to ∑PAH 5.99 µg/L, the resulting larvae demonstrated significant increases in farnesyl-diphosphate farnesyltransferase 1 (fdft1) and an upward trend in 3-Hydroxy-3-Methylglutaryl-CoA Reductase (hmgcr) expression, genes that encode key enzymes in the cholesterol biosynthetic pathway. In addition to the increased expression of genes in cholesterol biosynthetic pathway, a significant decrease in total cholesterol was observed in larval homogenates, at ∑PAH 8.3 µg/L. These data confirm earlier transcriptomic studies and show that oil may diminish cholesterol and adversely impact numerous cellular functions due to altered membrane stability.

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.

Combined effects of hypoxia or elevated temperature and Deepwater Horizon crude oil exposure on juvenile mahi-mahi swimming performance.

This study examined potential interactive effects of co-exposure to Deepwater Horizon (DWH) crude oil (∼30 µg L-1 ΣPAHs) for 24 h and either hypoxia (2.5 mg O2 L-1; 40% O2 saturation) or elevated temperature (30 °C) on the swimming performance of juvenile mahi-mahi (Coryphaena hippurus). Additionally, effects of shorter duration exposures to equal or higher doses of oil alone either prior to swimming or during the actual swim trial itself were examined. Only exposure to hypoxia alone or combined with crude oil elicited significant decreases in critical swimming speed (Ucrit) and to a similar extent (∼20%). In contrast, results indicate that elevated temperature might ameliorate some effects of oil exposure on swimming performance and that effects of shorter duration exposures are either reduced or delayed.

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.

Morphology and cardiac physiology are differentially affected by temperature in developing larvae of the marine fish mahi-mahi (Coryphaena hippurus).

Cardiovascular performance is altered by temperature in larval fishes, but how acute versus chronic temperature exposures independently affect cardiac morphology and physiology in the growing larva is poorly understood. Consequently, we investigated the influence of water temperature on cardiac plasticity in developing mahi-mahi. Morphological (e.g. standard length, heart angle) and physiological cardiac variables (e.g. heart rate fH, stroke volume, cardiac output) were recorded under two conditions by imaging: (i) under acute temperature exposure where embryos were reared at 25°C up to 128 h post-fertilization (hpf) and then acutely exposed to 25 (rearing temperature), 27 and 30°C; and (ii) at two rearing (chronic) temperatures of 26 and 30°C and performed at 32 and 56 hpf. Chronic elevated temperature improved developmental time in mahi-mahi. Heart rates were 1.2-1.4-fold higher under exposure of elevated acute temperatures across development (Q10≥2.0). Q10 for heart rate in acute exposure was 1.8-fold higher compared to chronic exposure at 56 hpf. At same stage, stroke volume was temperature independent (Q10∼1.0). However, larvae displayed higher stroke volume later in stage. Cardiac output in developing mahi-mahi is mainly dictated by chronotropic rather than inotropic modulation, is differentially affected by temperature during development and is not linked to metabolic changes.

Assessment of early life stage mahi-mahi windows of sensitivity during acute exposures to Deepwater Horizon crude oil.

Windows of exposure to a weathered Deepwater Horizon oil sample (slick A) were examined for early life stage mahi-mahi (Coryphaena hippurus) to determine whether there are developmental periods of enhanced sensitivity during the course of a standard 96-h bioassay. Survival was assessed at 96 h following oil exposures ranging from 2 h to 96 h and targeting 3 general periods of development, namely the prehatch phase, the period surrounding hatch, and the posthatch phase. In addition, 3 different oil preparations were used: high- and low-energy water accommodated fractions of oil and very thin surface slicks of oil (∼1 µm). The latter 2 were used to distinguish between effects due to direct contact with the slick itself and the water underlying the slick. Considering the data from all 3 exposure regimes, it was determined that the period near or including hatch was likely the most sensitive. Furthermore, toxicity was not enhanced by direct contact with slick oil. These findings are environmentally relevant given that the concentrations of polycyclic aromatic hydrocarbons eliciting mortality from exposures during the sensitive periods of development were below or near concentrations measured during the active spill phase. Environ Toxicol Chem 2017;36:1887-1895. © 2016 SETAC.

Effects of Deepwater Horizon crude oil exposure, temperature and developmental stage on oxygen consumption of embryonic and larval mahi-mahi (Coryphaena hippurus).

The timing and location of the 2010 Deepwater Horizon (DWH) incident within the Gulf of Mexico resulted in crude oil exposure of many commercially and ecologically important fish species, such as mahi-mahi (Coryphaena hippurus), during the sensitive early life stages. Previous research has shown that oil exposure during the embryonic stage of predatory pelagic fish reduces cardiac function - a particularly important trait for fast-swimming predators with high aerobic demands. However, it is unclear whether reductions in cardiac function translate to impacts on oxygen consumption in these developing embryos and larvae. A 24-channel optical-fluorescence oxygen-sensing system for high-throughput respiration measurements was used to investigate the effects of oil exposure, temperature and developmental stage on oxygen consumption rates in embryonic and larval mahi-mahi. Oil-exposed developing mahi-mahi displayed increased oxygen consumption, despite clear cardiac deformities and bradycardia, confirming oxygen uptake and delivery from a source other than the circulatory system. In addition to metabolic rate measurements, nitrogenous waste excretion was measured to test the hypothesis that increased energy demand was fueled by protein catabolism. This is the first study to our knowledge that demonstrates increased energy demand and energy depletion in oil-exposed developing mahi-mahi.