Implications of dietary carbon incorporation in fish carbonates for the global carbon cycle

Marine bony fish are important participants in Earth's carbon cycle through their contributions to the biological pump and the marine inorganic carbon cycle. However, uncertainties in the composition and magnitude of fish contributions preclude their integration into fully coupled carbon-climate models. Here, we consider recent upwards revisions to global fish biomass estimates (2.7-9.5×) and provide new stable carbon isotope measurements that show marine fish are prodigious producers of carbonate with unique composition. Assuming the median increase (4.17×) in fish biomass estimates is linearly reflected in fish carbonate (ichthyocarbonate) production rate, marine fish are estimated to produce between 1.43 and 3.99 Pg CaCO3 yr-1, but potentially as much as 9.03 Pg CaCO3 yr-1. Thus, marine fish carbonate production is equivalent to or potentially higher than contributions by coccolithophores or pelagic foraminifera. New stable carbon isotope analyses indicate that a significant proportion of ichthyocarbonate is derived from dietary carbon, rather than seawater dissolved inorganic carbon. Using a statistical mixing model to derive source contributions, we estimate ichthyocarbonate contains up to 81 % dietary carbon, with average compositions of 28-56 %, standing in contrast to contents <10 % in other biogenic carbonate minerals. Results also indicate ichthyocarbonate contains 5.5-40.4 % total organic carbon. When scaled to the median revised global production of ichthyocarbonate, an additional 0.08 to 1.61 Pg C yr-1 can potentially be added to estimates of fish contributions to the biological pump, significantly increasing marine fish contributions to total surface carbon export. Our integration of geochemical and physiological analyses identifies an overlooked link between carbonate production and the biological pump. Since ichthyocarbonate production is anticipated to increase with climate change scenarios, due to ocean warming and acidification, these results emphasize the importance of quantitative understanding of the multifaceted role of marine fish in the global carbon cycle.

The role of marine fish-produced carbonates in the oceanic carbon cycle is determined by size, specific gravity, and dissolution rate

Rising CO2 emissions have heightened the necessity for increased understanding of Earth's carbon cycle to predict future climates. The involvement of marine planktonic species in the global carbon cycle has been extensively studied, but contributions by marine fish remain poorly characterized. Marine teleost fishes produce carbonate minerals ('ichthyocarbonates') within the lumen of their intestines which are excreted at significant rates on a global scale. However, we have limited understanding of the fate of excreted ichthyocarbonate. We analyzed ichthyocarbonate produced by three different marine teleosts for mol%MgCO3 content, size, specific gravity, and dissolution rate to gain a better understanding of ichthyocarbonate fate. Based on the species examined here, we report that 75 % of ichthyocarbonates are ≤0.91 mm in diameter. Analyses indicate high Mg2+ content across species (22.3 to 32.3 % mol%MgCO3), consistent with previous findings. Furthermore, ichthyocarbonate specific gravity ranged from 1.23 to 1.33 g/cm3, and ichthyocarbonate dissolution rates varied among species as a function of aragonite saturation state. Ichthyocarbonate sinking rates and dissolution depth were estimated for the Atlantic, Pacific, and Indian ocean basins for the three species examined. In the North Atlantic, for example, ~33 % of examined ichthyocarbonates are expected to reach depths exceeding 200 m prior to complete dissolution. The remaining ~66 % of ichthyocarbonate is estimated to dissolve and contribute to shallow water alkalinity budgets. Considering fish biomass and ichthyocarbonate production rates, our results support that marine fishes are critical to the global carbon cycle, contributing to oceanic alkalinity budgets and thereby influencing the ability of the oceans to neutralize atmospheric CO2.

A large aerobic scope and complex regulatory abilities confer hypoxia tolerance in larval toadfish, Opsanus beta, across a wide thermal range

Few studies have been performed on early-life stage toadfish, and none have addressed their tolerance to temperature and hypoxia despite large seasonal temperature fluctuations and daily hypoxia in their natural environment. The first directed captive breeding of Opsanus beta allowed the examination of larval oxygen demands and hypoxia tolerance across the range of their environmental temperatures (23-33 °C). Larval toadfish exhibited a surprisingly large aerobic scope across the tested temperature range. In response to progressive hypoxia, larval toadfish showed early metabolic depression and a low regulation index (RI), while juveniles had higher regulatory abilities but, unexpectedly, a lower aerobic scope. Larval and juvenile toadfish survived hours of severe hypoxia, but larval fish had a higher excessive post-hypoxia oxygen consumption, yet their metabolic rate returned to RMR in the same timeframe as the juveniles, likely due to their higher aerobic scope. We defined hypoxia tolerance using a physiological trait, p50, the oxygen tension in which oxygen uptake is reduced to 50 % of the metabolic rate at rest and determined it at all tested temperatures. Comparing these p50 values to environmental conditions in Florida Bay using hourly temperature and oxygen measurements from January 2014-October 2021 revealed that larval toadfish rarely experience < p50 conditions (11 % of events). Further, the median duration of these events was 3 h. The metabolic performance of larval toadfish combined with temperature and oxygen observations from their natural environment reveals the fascinating strategy in which larval toadfish survive diel hypoxia across seasons.

Metabolic cost of osmoregulation by the gastro-intestinal tract in marine teleost fish

Introduction: Although dozens of studies have attempted to determine the metabolic cost of osmoregulation, mainly by comparing standard metabolic rates (SMR) in fish acclimated to different salinities, consensus is still lacking. Methods: In the present study, using the Gulf toadfish, Opsanus beta, we aimed to determine the metabolic cost of esophageal and intestinal osmoregulatory processes by estimating ATP consumption from known ion transport rates and pathways and comparing these estimates with measurements on isolated tissues. Further, we performed whole animal respirometry on fish acclimated to 9, 34 and 60 ppt. Results and Discussion: Our theoretical estimates of esophageal and intestinal osmoregulatory costs were in close agreement with direct measurements on isolated tissues and suggest that osmoregulation by these tissues amounts to ∼2.5% of SMR. This value agrees well with an earlier attempt to estimate osmoregulation cost from ion transport rates and combined with published measurements of gill osmoregulatory costs suggests that whole animal costs of osmoregulation in marine teleosts is ∼7.5% of SMR. As in many previous studies, our whole animal measurements were variable between fish and did not seem suited to determine osmoregulatory costs. While the esophagus showed constant metabolic rate regardless of acclimation salinity, the intestine of fish acclimated to higher salinities showed elevated metabolic rates. The esophagus and the intestine had 2.1 and 3.2-fold higher metabolic rates than corresponding whole animal mass specific rates, respectively. The intestinal tissue displays at least four different Cl- uptake pathways of which the Na+:Cl-:2 K+ (NKCC) pathway accounts for 95% of the Cl- uptake and is the most energy efficient. The remaining pathways are via apical anion exchange and seem to primarily serve luminal alkalinization and the formation of intestinal CaCO3 which is essential for water absorption.

Staying hydrated in sea water

Reduction of intestinal lumen osmotic pressure by formation of Ca(Mg)CO₃, "ichthyocarbonate", is essential for osmoregulation by the only vertebrate group, ray-finned fishes, widely capable of hydrating by ingesting seawater. Ichthyocarbonate formation and excretion is under elaborate physiological control and plays an important, yet still poorly defined, role in the oceanic carbon cycle.

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.

Toxicity of perfluoroalkyl substances (PFAS) toward embryonic stages of mahi-mahi (Coryphaena hippurus)

Perfluoroalkyl substances (PFAS) are highly persistent organic pollutants that have been detected in a wide array of environmental matrices and, in turn, diverse biota including humans and wildlife wherein they have been associated with a multitude of toxic, and otherwise adverse effects, including ecosystem impacts. In the present study, we developed a toxicity assay for embryonic stages of mahi-mahi (Coryphaena hippurus), as an environmentally relevant pelagic fish species, and applied this assay to the evaluation of the toxicity of "legacy" and "next-generation" PFAS including, respectively, perfluorooctanoic acid (PFOA) and several perfluoroethercarboxylic acids (PFECA). Acute embryotoxicity, in the form of lethality, was measured for all five PFAS toward mahi-mahi embryos with median lethal concentrations (LC₅₀) in the micromolar range. Consistent with studies in other similar model systems, and specifically the zebrafish, embryotoxicity in mahi-mahi generally (1) correlated with fluoroalkyl/fluoroether chain length and hydrophobicity, i.e., log P, of PFAS, and thus, aligned with a role of uptake in the relative toxicity; and (2) increased with continuous exposure, suggesting a possible role of development stage specifically including a contribution of hatching (and loss of protective chorion) and/or differentiation of target systems (e.g., liver). Compared to prior studies in the zebrafish embryo model, mahi-mahi was significantly more sensitive to PFAS which may be related to differences in either exposure conditions (e.g., salinity) and uptake, or possibly differential susceptibility of relevant targets, for the two species. Moreover, when considered in the context of the previously reported concentration of PFAS within upper sea surface layers, and co-localization of buoyant eggs (i.e., embryos) and other early development stages (i.e., larvae, juveniles) of pelagic fish species to the sea surface, the observed toxicity potentially aligns with environmentally relevant concentrations in these marine systems. Thus, impacts on ecosystems including, in particular, population recruitment are a possibility. The present study is the first to demonstrate embryotoxicity of PFAS in a pelagic marine fish species, and suggests that mahi-mahi represents a potentially informative, and moreover, environmentally relevant, ecotoxicological model for PFAS in marine systems.

The developing zebrafish kidney is impaired by Deepwater Horizon crude oil early-life stage exposure: A molecular to whole-organism perspective

Crude oil is known to induce developmental defects in teleost fish exposed during early life stages (ELSs). While most studies in recent years have focused on cardiac endpoints, evidence from whole-animal transcriptomic analyses and studies with individual polycyclic aromatic hydrocarbons (PAHs) indicate that the developing kidney (i.e., pronephros) is also at risk. Considering the role of the pronephros in osmoregulation, and the common observance of edema in oil-exposed ELS fish, surprisingly little is known regarding the effects of oil exposure on pronephros development and function. Using zebrafish (Danio rerio) ELSs, we assessed the transcriptional and morphological responses to two dilutions of high-energy water accommodated fractions (HEWAF) of oil from the Deepwater Horizon oil spill using a combination of qPCR and whole-mount in situ hybridization (WM-ISH) of candidate genes involved in pronephros development and function, and immunohistochemistry (WM-IHC). To assess potential functional impacts on the pronephros, three 24 h osmotic challenges (2 hypo-osmotic, 1 near iso-osmotic) were implemented at two developmental time points (48 and 96 h post fertilization; hpf) following exposure to HEWAF. Changes in transcript expression level and location specific to different regions of the pronephros were observed by qPCR and WM-ISH. Further, pronephros morphology was altered in crude oil exposed larvae, characterized by failed glomerulus and neck segment formation, and straightening of the pronephric tubules. The osmotic challenges at 96 hpf greatly exacerbated edema in both HEWAF-exposed groups regardless of osmolarity. By contrast, larvae at 48 hpf exhibited no edema prior to the osmotic challenge, but previous HEWAF exposure elicited a concentration-response increase in edema at hypo-osmotic conditions that appeared to have been largely alleviated under near iso-osmotic conditions. In summary, ELS HEWAF exposure impaired proper pronephros development in zebrafish, which coupled with cardiotoxic effects, most likely reduced or inhibited pronephros fluid clearance capacity and increased edema formation.

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.

An integrated systems-level model of the toxicity of brevetoxin based on high-resolution magic-angle spinning nuclear magnetic resonance (HRMAS NMR) metabolic profiling of zebrafish embryos

Brevetoxins (PbTx) are a well-recognized group of neurotoxins associated with harmful algal blooms, and specifically recurrent "Florida Red Tides," in marine waters that are linked to impacts on both human and ecosystem health including well-documented "fish kills" and marine mammal mortalities in affected coastal waters. Understanding mechanisms and pathways of PbTx toxicity enables identification of relevant biomarkers to better understand these environmental impacts, and improve monitoring efforts, in relation to this toxin. Toward a systems-level understanding of toxicity, and identification of potential biomarkers, high-resolution magic angle spinning nuclear magnetic resonance (HRMAS NMR) was utilized for metabolic profiling of zebrafish (Danio rerio) embryos, as an established toxicological model, exposed to PbTx-2 (the most common congener in marine waters). Metabolomics studies were, furthermore, complemented by an assessment of the toxicity of PbTx-2 in embryonic stages of zebrafish and mahi-mahi (Coryphaena hippurus), the latter representing an ecologically and geographically relevant marine species of fish, which identified acute embryotoxicity at environmentally relevant (i.e., parts-per-billion) concentrations in both species. HRMAS NMR analysis of intact zebrafish embryos exposed to sub-lethal concentrations of PbTx-2 afforded well-resolved spectra, and in turn, identification of 38 metabolites of which 28 were found to be significantly altered, relative to controls. Metabolites altered by PbTx-2 exposure specifically included those associated with (1) neuronal excitotoxicity, as well as associated neural homeostasis, and (2) interrelated pathways of carbohydrate and energy metabolism. Metabolomics studies, thereby, enabled a systems-level model of PbTx toxicity which integrated multiple metabolic, molecular and cellular pathways, in relation to environmentally relevant concentrations of the toxin, providing insight to not only targets and mechanisms, but potential biomarkers pertinent to environmental risk assessment and monitoring strategies.

A review of the toxicology of oil in vertebrates: what we have learned following the Deepwater Horizon oil spill

In the wake of the Deepwater Horizon (DWH) oil spill, a number of government agencies, academic institutions, consultants, and nonprofit organizations conducted lab- and field-based research to understand the toxic effects of the oil. Lab testing was performed with a variety of fish, birds, turtles, and vertebrate cell lines (as well as invertebrates); field biologists conducted observations on fish, birds, turtles, and marine mammals; and epidemiologists carried out observational studies in humans. Eight years after the spill, scientists and resource managers held a workshop to summarize the similarities and differences in the effects of DWH oil on vertebrate taxa and to identify remaining gaps in our understanding of oil toxicity in wildlife and humans, building upon the cross-taxonomic synthesis initiated during the Natural Resource Damage Assessment. Across the studies, consistency was found in the types of toxic response observed in the different organisms. Impairment of stress responses and adrenal gland function, cardiotoxicity, immune system dysfunction, disruption of blood cells and their function, effects on locomotion, and oxidative damage were observed across taxa. This consistency suggests conservation in the mechanisms of action and disease pathogenesis. From a toxicological perspective, a logical progression of impacts was noted: from molecular and cellular effects that manifest as organ dysfunction, to systemic effects that compromise fitness, growth, reproductive potential, and survival. From a clinical perspective, adverse health effects from DWH oil spill exposure formed a suite of signs/symptomatic responses that at the highest doses/concentrations resulted in multi-organ system failure.

Role of the cardiovascular system in ammonia excretion in early life stages of zebrafish (Danio rerio)

The purpose of this study was to investigate if the cardiovascular system is important for ammonia excretion in the early life stages of zebrafish. Morpholino knockdowns of cardiac troponin T (TNNT2) or vascular endothelial growth factor A (VEGFA) provided morphants with nonfunctional circulation. At the embryonic stage [30-36 h postfertilization (hpf)], ammonia excretion was not constrained by a lack of cardiovascular function. At 2 days postfertilization (dpf) and 4 dpf, morpholino knockdowns of TNNT2 or VEGFA significantly reduced ammonia excretion in all morphants. Expression of rhag, rhbg, and rhcgb showed no significant changes but the mRNA levels of the urea transporter (ut) were upregulated in the 4 dpf morphants. Taken together, rhag, rhbg, rhcgb, and ut gene expression and an unchanged tissue ammonia concentration but an increased tissue urea concentration, suggest that impaired ammonia excretion led to increased urea synthesis. However, in larvae anesthetized with tricaine or clove oil, ammonia excretion was not reduced in the 4 dpf morphants compared with controls. Furthermore, oxygen consumption was reduced in morphants regardless of anesthesia. These results suggest that cardiovascular function is not directly involved in ammonia excretion, but rather reduced activity and external convection may explain reduced ammonia excretion and compensatory urea accumulation in morphants with reduced cardiovascular function.

Magnesium transport in the aglomerular kidney of the Gulf toadfish (Opsanus beta)

Despite having an aglomerular kidney, Gulf toadfish can survive in water ranging from nearly fresh up to 70 parts per thousand salinity. In hyperosmotic environments, the major renal function is to balance the passive Mg²⁺ load from the environment with an equal excretion. However, the molecular transporters involved in Mg²⁺ secretion are poorly understood. We investigated whether environmental MgCl₂ alone or in combination with elevated salinity affected transcriptional regulation of genes classically involved in renal Mg²⁺ secretion (slc41a1, slc41a3, cnnm3) together with three novel genes (trpm6, trpm7, claudin-19) and two isoforms of the Na⁺/K⁺-ATPase α-subunit (nka-α1a, nka-α1b). First, toadfish were acclimated to 5, 9, 35, or 60 ppt water (corresponding to ~ 7, 13, 50 and 108 mmol L^-1 ambient [Mg²⁺], respectively) and sampled at 24 h or 9 days. Next, the impact of elevated ambient [Mg²⁺] was explored by exposing toadfish to control (50 mmol L^-1 Mg²⁺), or elevated [Mg²⁺] (100 mmol L^-1) at a constant salinity for 7 days. Mg²⁺ levels in this experiment corresponded with levels in control and hypersaline conditions in the first experiment. A salinity increase from 5 to 60 ppt stimulated the level of all investigated transcripts in the kidney. In Mg²⁺-exposed fish, we observed a 14-fold increase in the volume of intestinal fluids and elevated plasma osmolality and [Mg²⁺], suggesting osmoregulatory challenges. However, none of the renal gene targets changed expression compared with the control group. We conclude that transcriptional regulation of renal Mg²⁺ transporters is induced by elevated [Mg²⁺] in combination with salinity rather than elevated ambient [Mg²⁺] alone.

The effects of acute temperature change and digestive status on in situ cardiac function in mahi-mahi (Coryphaena hippurus)

In this study, we investigated the effect of acute increases in temperature on cardiovascular function of mahi-mahi (Coryphaena hippurus). We also describe, for the first time, an artery that supplies the gastrointestinal tract that originates from the fourth branchial artery. We used vascular casting to verify the anatomical location of this unique celiaco-mesenteric artery. We predicted that blood flow in this vessel would be correlated with the digestive state of the animal. Increasing water temperature from 25.0 to 30.5 °C resulted in a linear increase in heart rate (f_H) from 165 ± 4 beats∙min^-1to 232 ± 7 beats∙min^-1. Over this temperature range, f_H strongly correlated with water temperature (R² = 0.79). At 31 °C f_H no longer correlated with water temperature, and at 34 °C f_H had dropped to 114 ± 19 beats∙min^-1. Furthermore, we found that mahi are capable of maintaining constant cardiac output over a temperature range from 25 to 31 °C. Cardiac function appeared to be compromised at temperatures >31 °C. In fed anesthetized fish, blood flow was pulsatile in the celiaco-mesenteric artery and was not in fasted fish. In fed fish, blood flow in the left celiaco-mesenteric artery was 1.99 ± 0.78 ml·min^-1·kg^-1 compared to the total cardiac output of 168.6 ± 12.7 ml·min^-1·kg^-1. The data suggest that mahi can differentially regulate gastric blood flow based on feeding state, which may explain the high digestive efficiency and very high growth rates of these pelagic predators.

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.

Exposure to Hydraulic Fracturing Flowback Water Impairs Mahi-Mahi (Coryphaena hippurus) Cardiomyocyte Contractile Function and Swimming Performance

Publicly available toxicological studies on wastewaters associated with unconventional oil and gas (UOG) activities in offshore regions are nonexistent. The current study investigated the impact of hydraulic fracturing-generated flowback water (HF-FW) on whole organism swimming performance/respiration and cardiomyocyte contractility dynamics in mahi-mahi (Coryphaena hippurus-hereafter referred to as "mahi"), an organism which inhabits marine ecosystems where offshore hydraulic fracturing activity is intensifying. Following exposure to 2.75% HF-FW for 24 h, mahi displayed significantly reduced critical swimming speeds (U_crit) and aerobic scopes (reductions of ∼40 and 61%, respectively) compared to control fish. Additionally, cardiomyocyte exposures to the same HF-FW sample at 2% dilutions reduced a multitude of mahi sarcomere contraction properties at various stimulation frequencies compared to all other treatment groups, including an approximate 40% decrease in sarcomere contraction size and a nearly 50% reduction in sarcomere relaxation velocity compared to controls. An approximate 8-fold change in expression of the cardiac contractile regulatory gene cmlc2 was also seen in ventricles from 2.75% HF-FW-exposed mahi. These results collectively identify cardiac function as a target for HF-FW toxicity and provide some of the first published data on UOG toxicity in a marine species.

A Mystery Tale: Nickel Is Fickle When Snails Fail-Investigating the Variability in Ni Toxicity to the Great Pond Snail

Dissolved Ni concentrations inhibiting the growth of juvenile great pond snails (Lymnaea stagnalis) have been documented to vary from about 1 to 200 µg L^-1 Ni. This variability makes L. stagnalis either a moderately sensitive or the most sensitive freshwater species to chronic Ni exposure tested to date. Given the role of sensitive species in environmental risk assessment frameworks, it is particularly important to understand this variability, i.e., to characterize the factors that modulate Ni toxicity and that may confound toxicity test outcomes when uncontrolled. In the present study, we tested if this variability was due to analytical (growth calculation: biomass versus growth rate), environmental (water quality), lab-specific practices, and/or snail population differences among earlier studies. Specifically, we reanalyzed previously published Ni toxicity data and conducted additional measurements of Ni aqueous speciation, short-term Ni uptake, and chronic Ni toxicity with test waters and snail cultures used in previous studies. Corrections for Ni bioavailability and growth calculations explained a large degree of variability in the published literature. However, a residual 16-fold difference remained puzzling between 2 studies: Niyogi et al. (2014) (low ECxs) and Crémazy et al. (2018) (high ECxs). Indeed, differences in metal bioavailability due to water chemistry, lab-specific practices, and snail population sensitivity could not explain the large variation in Ni toxicity in these 2 very similar studies. Other potentially important toxicity-modifying factors were not directly evaluated in the present work: test duration, diet, snail holding conditions, and snail age at onset of testing. The present analysis highlights the need for further studies to elucidate 1) the mechanisms of growth inhibition in Ni-exposed L. stagnalis and 2) the important abiotic and biotic factors affecting this biological response. Until these processes are understood, substantial uncertainties will remain about inclusion of this species in Ni environmental risk assessment. Integr Environ Assess Manag 2020;16:983-997. © 2020 SETAC.

A marine teleost, Opsanus beta, compensates acidosis in hypersaline water by H+ excretion or reduced HCO3- excretion rather than HCO3- uptake

Increases in ambient salinity demand parallel increases in intestinal base secretion for maintenance of osmoregulatory status, which is likely the cause of a transient acidosis following transfer of euryhaline fish from freshwater to seawater. It was predicted that transfer of the marine Gulf toadfish (Opsanus beta) from seawater (35 ppt) to hypersaline (60 ppt) seawater (HSW) would lead to a transient acidosis that would be compensated by increases in branchial acid excretion to offset the acid-base disturbance. Toadfish exposed to HSW showed a significant decrease in blood pH and [HCO₃^-] but no increase in pCO₂, followed by a full recovery after 48-96 h. A similar metabolic acidosis and recovery was found when fish were exposed to 60-ppt HCO₃^--free seawater (HEPES-buffered), which may suggest that compensation for intestinal base loss during hypersaline treatment is from gill H⁺ excretion rather than gill HCO₃^- uptake. However, we cannot rule out that reduced branchial HCO₃^- excretion contributed to an increase in net acid excretion. Since colchicine prevents full compensation, translocation of H⁺ and/or HCO₃^- transporters between cytosolic compartments and plasma membrane fractions might be involved in compensating for the hypersalinity-induced acidosis. Translocation of transporters rather than de novo synthesis may represent a faster and less energetically demanding response to rapidly fluctuating and high salinities encountered by toadfish in their natural environment.

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.

The potential for salt toxicity: Can the trans-epithelial potential (TEP) across the gills serve as a metric for major ion toxicity in fish?

An emerging Multi-Ion Toxicity (MIT) model for assessment of environmental salt pollution is based on the premise that major ion toxicity to aquatic organisms is related to a critical disturbance of the trans-epithelial potential across the gills (ΔTEP), which can be predicted by electrochemical theory. However, the model has never been evaluated physiologically. We directly tested key assumptions by examining the individual effects of eight different salts (NaCl, Na₂SO₄, MgCl₂, MgSO₄, KCl, K₂SO₄, CaCl₂, and CaSO₄) on measured TEP in three different fish species (fathead minnow, Pimephales promelas = FHM; channel catfish, Ictalurus punctatus = CC; bluegill, Lepomis macrochirus = BG). A geometric concentration series based on previously reported 96-h LC50 values for FHM was used. All salts caused concentration-dependent increases in TEP to less negative/more positive values in a pattern well-described by the Michaelis-Menten equation. The ΔTEP responses for different salts were similar to one another within each species when concentrations were expressed as a percentage of the FHM LC50. A plateau was reached at or before 100 % of the LC50 where the ΔTEP values were remarkably consistent, with only 1.4 to 2.2-fold variation. This relative uniformity in the ΔTEP responses contrasts with 28-fold variation in salt concentration (in mmol L^-1), 9.6-fold in total dissolved solids, and 7.9-fold in conductivity at the LC50. The Michaelis-Menten Km values (salt concentrations causing 50 % of the ΔTEP_max) were positively related to the 96-h LC50 values. ΔTEP responses were not a direct effect of osmolarity in all species and were related to specific cation rather than specific anion concentrations in FHM. These responses were stable for up to 24 h in CC. The results provide strong physiological support for the assumptions of the MIT model, are coherent with electrochemical theory, and point to areas for future research.

Is aquaporin-3 involved in water-permeability changes in the killifish during hypoxia and normoxic recovery, in freshwater or seawater?

Aquaporins are the predominant water-transporting proteins in vertebrates, but only a handful of studies have investigated aquaporin function in fish, particularly in mediating water permeability during salinity challenges. Even less is known about aquaporin function in hypoxia (low oxygen), which can profoundly affect gill function. Fish deprived of oxygen typically enlarge gill surface area and shrink the water-to-blood diffusion distance, to facilitate oxygen uptake into the bloodstream. However, these alterations to gill morphology can result in unfavorable water and ion fluxes. Thus, there exists an osmorespiratory compromise, whereby fish must try to balance high branchial gas exchange with low ion and water permeability. Furthermore, the gills of seawater and freshwater teleosts have substantially different functions with respect to osmotic and ion fluxes; consequently, hypoxia can have very different effects according to the salinity of the environment. The purpose of this study was to determine what role aquaporins play in water permeability in the hypoxia-tolerant euryhaline common killifish (Fundulus heteroclitus), in two important osmoregulatory organs-the gills and intestine. Using immunofluorescence, we localized aquaporin-3 (AQP3) protein to the basolateral and apical membranes of ionocytes and enterocytes, respectively. Although hypoxia increased branchial AQP3 messenger-RNA expression in seawater and freshwater, protein abundance did not correlate. Indeed, hypoxia did not alter AQP3 protein abundance in seawater and reduced it in the cell membranes of freshwater gills. Together, these observations suggest killifish AQP3 contributes to reduced diffusive water flux during hypoxia and normoxic recovery in freshwater and facilitates intestinal permeability in seawater and freshwater.

Na+ K+ ATPase isoform switching in zebrafish during transition to dilute freshwater habitats

Na⁺ K⁺ ATPase (NKA) is crucial to branchial ion transport as it uses the energy from ATP to move Na⁺ against its electrochemical gradient. When fish encounter extremely dilute environments the energy available from ATP hydrolysis may not be sufficient to overcome thermodynamic constraints on ion transport. Yet many fish species-including zebrafish-are capable of surviving in dilute environments. Despite much study, the physiological mechanisms by which this occurs remain poorly understood. Here, we demonstrate that zebrafish acclimated to less than 10 µM Na⁺ water exhibit upregulation of a specific NKA α subunit ( zatp1a1a.5) that, unlike most NKA heterotrimers, would result in transfer of only a single Na⁺ and K⁺ per ATP hydrolysis reaction. Thermodynamic models demonstrate that this change is sufficient to reduce the activation energy of NKA, allowing it to overcome the adverse electrochemical gradient imposed by dilute freshwater. Importantly, upregulation of zatp1a1a.5 also coincides with the recovery of whole body Na⁺ post-transfer, which occurs within 24 h. While these structural modifications are crucial for allowing zebrafish to survive in ion-poor environments, phylogenetic and structural analysis of available α subunits from a range of teleosts suggests this adaptation is not widely distributed.

Salt-water acclimation of the estuarine crocodile Crocodylus porosus involves enhanced ion transport properties of the urodaeum and rectum

Estuarine crocodiles, Crocodylus porosus, inhabit freshwater, estuarine and marine environments. Despite being known to undertake extensive movements throughout and between hypo-osmotic and hyperosmotic environments, little is known about the role of the cloaca in coping with changes in salinity. We report here that, in addition to the well-documented functional plasticity of the lingual salt glands, the middle of the three cloacal segments (i.e. the urodaeum) responds to increased ambient salinity to enhance solute-coupled water absorption. This post-renal modification of urine serves to conserve water when exposed to hyperosmotic environments and, in conjunction with lingual salt gland secretions, enables C. porosus to maintain salt and water balance and thereby thrive in hyperosmotic environments. Isolated epithelia from the urodaeum of 70% seawater-acclimated C. porosus had a strongly enhanced short-circuit current (an indicator of active ion transport) compared with freshwater-acclimated crocodiles. This enhanced active ion absorption was driven by increased Na+/K+-ATPase activity, and possibly enhanced proton pump activity, and was facilitated by the apical epithelial Na+ channel (ENaC) and/or the apical Na+/H+ exchanger (NHE2), both of which are expressed in the urodaeum. NHE3 was expressed at very low levels in the urodaeum and probably does not contribute to solute-coupled water absorption in this cloacal segment. As C. porosus does not appear to drink water of salinities above 18 ppt, observations of elevated short-circuit current in the rectum as well as a trend for increased NHE2 expression in the oesophagus, the anterior intestine and the rectum suggest that dietary salt intake may stimulate salt and possibly water absorption by the gastrointestinal tract of C. porosus living in hyperosmotic environments.

Exposure to Crude Oil from the Deepwater Horizon Oil Spill Impairs Oil Avoidance Behavior without Affecting Olfactory Physiology in Juvenile Mahi-Mahi (Coryphaena hippurus)

The understanding of the detection threshold and behavioral response of fishes in response to crude oil is critical to predicting the effects of oil spills on wild fish populations. The Deepwater Horizon oil spill released approximately 4.9 million barrels of crude oil into the northern Gulf of Mexico in 2010, overlapping spatially and temporally with the habitat of many pelagic fish species. Yet, it is unknown whether highly migratory species, such as mahi-mahi (Coryphaena hippurus), might detect and avoid oil contaminated waters. We tested the ability of control and oil-exposed juvenile mahi-mahi (15-45 mm) to avoid two dilutions of crude oil in a two-channel flume. Control fish avoided the higher concentration (27.1 μg/L Σ₅₀PAH), while oil-exposed (24 h, 18.0 μg/L Σ₅₀PAH) conspecifics did not. Electro-olfactogram (EOG) data demonstrated that both control and oil-exposed (24 h, 14.5 μg/L Σ₅₀PAH) juvenile mahi-mahi (27-85 mm) could detect crude oil as an olfactory cue and that oil-exposure did not affect the EOG amplitude or duration in response to oil or other cues. These results show that a brief oil exposure impairs the ability of mahi-mahi to avoid oil and suggests that this alteration likely results from injury to higher order central nervous system processing rather than impaired olfactory physiology.

Intra-Specific Difference in the Effect of Salinity on Physiological Performance in European Perch (Perca fluviatilis) and Its Ecological Importance for Fish in Estuaries

Changes in environmental salinity challenge fish homeostasis and may affect physiological performance, such as swimming capacity and metabolism, which are important for foraging, migration, and escaping predators in the wild. The effects of salinity stress on physiological performance are largely species specific, but may also depend on intra-specific differences in physiological capabilities of sub-populations. We measured critical swimming speed (U_crit) and metabolic rates during swimming and at rest at salinities of 0 and 10 in European perch (Perca fluviatilis) from a low salinity tolerance population (LSTP) and a high salinity tolerance population (HSTP). U_crit of LSTP was significantly reduced at a salinity of 10 yet was unaffected by salinity change in HSTP. We did not detect a significant cost of osmoregulation, which should theoretically be apparent from the metabolic rates during swimming and at rest at a salinity of 0 compared to at a salinity of 10 (iso-osmotic). Maximum metabolic rates were also not affected by salinity, indicating a modest tradeoff between respiration and osmoregulation (osmo-respiratory compromise). Intra-specific differences in effects of salinity on physiological performance are important for fish species to maintain ecological compatibility in estuarine environments, yet render these sub-populations vulnerable to fisheries. The findings of the present study are therefore valuable knowledge in conservation and management of estuarine fish populations.

The effects of Deepwater Horizon crude oil on ammonia and urea handling in mahi-mahi (Coryphaena hippurus) early life stages

Many ecologically important fishes, including mahi-mahi (Coryphaena hippurus), and their offspring were directly exposed to crude oil following the Deepwater Horizon (DWH) oil spill. Early life stage fish are especially vulnerable to the toxicity of crude oil-derived polycyclic aromatic hydrocarbons (PAHs). In teleosts, yolk sac proteins are the main energy source during development and are usually catabolized into ammonia or urea among other byproducts. Although excretion of these waste products is sensitive to oil exposure, we know little about the underlying mechanisms of this process. In this study, we examined the effects of crude oil on ammonia and urea handling in the early life stages of mahi. Mahi embryos exposed to 30-32 μg L^-1 ∑PAH exhibited increased urea excretion rates and greater accumulation of urea in the tissues before hatch suggesting that ammonia, which is highly toxic, was converted into less-toxic urea. Oil-exposed embryos (6.3-32 μg L^-1 ∑PAH) displayed significantly increased tissue ammonia levels at 42 hpf and upregulated mRNA levels of ammonia transporters (Rhag, Rhbg and Rhcg1) from 30 to 54 hpf. However, despite increased accumulation and higher expression of ammonia transporters, the larvae exposed to higher ∑PAH (30 μg L^-1 ∑PAH) showed reduced ammonia excretion rates after hatch. Together, the increased production of nitrogenous waste reinforces previous work that increased energy demand in oil-exposed embryos is fueled, at least in part, by protein metabolism and that urea synthesis plays a role in ammonia detoxification in oil-exposed mahi embryos. To our knowledge, this study is the first to combine physiological and molecular approaches to assess the impact of crude-oil on both nitrogenous waste excretion and accumulation in the early life stages of any teleosts.

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.

The osmorespiratory compromise in the euryhaline killifish: water regulation during hypoxia

Freshwater- and seawater-acclimated Fundulus heteroclitus were exposed to acute hypoxia (10% air saturation, 3 h), followed by normoxic recovery (3 h). In both salinities, ventilation increased and heart rate fell in the classic manner, while Ṁ _O2  initially declined by ∼50%, with partial restoration by 3 h of hypoxia, and no O₂ debt repayment during recovery. Gill paracellular permeability (measured with [¹⁴C] PEG-4000) was 1.4-fold higher in seawater, and declined by 50% during hypoxia with post-exposure overshoot to 188%. A similar pattern with smaller changes occurred in freshwater. Drinking rate (also measured with [¹⁴C] PEG-4000) was 8-fold higher in seawater fish, but declined by ∼90% during hypoxia in both groups, with post-exposure overshoots to ∼270%. Gill diffusive water flux (measured with ³H₂O) was 1.9-fold higher in freshwater fish, and exhibited a ∼35% decrease during hypoxia, which persisted throughout recovery, but was unchanged during hypoxia in seawater fish. Nevertheless, freshwater killifish gained mass while seawater fish lost mass during hypoxia, and these changes were not corrected during normoxic recovery. We conclude that this hypoxia-tolerant teleost beneficially reduces gill water permeability in a salinity-dependent fashion during hypoxia, despite attempting to simultaneously improve Ṁ _O2 , but nevertheless incurs a net water balance penalty in both freshwater and seawater.

Damsels in Distress: Oil Exposure Modifies Behavior and Olfaction in Bicolor Damselfish (Stegastes partitus)

In fishes, olfactory cues evoke behavioral responses that are crucial to survival; however, the receptors, olfactory sensory neurons, are directly exposed to the environment and are susceptible to damage from aquatic contaminants. In 2010, 4.9 million barrels of crude oil were released into the northern Gulf of Mexico from the Deepwater Horizon disaster, exposing marine organisms to this environmental contaminant. We examined the ability of bicolor damselfish (Stegastes partitus), exposed to the water accommodated fraction (WAF) of crude oil, to respond to chemical alarm cue (CAC) using a two-channel flume. Control bicolor damselfish avoided CAC in the flume choice test, whereas WAF-exposed conspecifics did not. This lack of avoidance persisted following 8 days of control water conditions. We then examined the physiological response to CAC, brine shrimp rinse, bile salt, and amino acid cues using the electro-olfactogram (EOG) technique and found that WAF-exposed bicolor damselfish were less likely to detect CAC as an olfactory cue but showed no difference in EOG amplitude or duration compared to controls. These data indicate that a sublethal WAF exposure directly modifies detection and avoidance of CAC beyond the exposure period and may suggest reduced predator avoidance behavior in oil-exposed fish in the wild.

Impacts of Deepwater Horizon Crude Oil on Mahi-Mahi (Coryphaena hippurus) Heart Cell Function

Deepwater Horizon crude oil is comprised of polycyclic aromatic hydrocarbons that cause a number of cardiotoxic effects in marine fishes across all levels of biological organization and at different life stages. Although cardiotoxic impacts have been widely reported, the mechanisms underlying these impairments in adult fish remain understudied. In this study, we examined the impacts of crude oil on cardiomyocyte contractility and electrophysiological parameters in freshly isolated ventricular cardiomyocytes from adult mahi-mahi (Coryphaena hippurus). Cardiomyocytes directly exposed to oil exhibited reduced contractility over a range of environmentally relevant concentrations (2.8-12.9 μg l^-1∑PAH). This reduction in contractility was most pronounced at higher stimulation frequencies, corresponding to the upper limits of previously measured in situ mahi heart rates. To better understand the mechanisms underlying impaired contractile function, electrophysiological studies were performed, which revealed oil exposure prolonged cardiomyocyte action potentials and disrupted potassium cycling (9.9-30.4 μg l^-1∑PAH). This study is the first to measure cellular contractility in oil-exposed cardiomyocytes from a pelagic fish. Results from this study contribute to previously observed impairments to heart function and whole-animal exercise performance in mahi, underscoring the advantages of using an integrative approach in examining mechanisms of oil-induced cardiotoxicity in marine fish.

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.

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.

Special issue on aquaculture: New opportunities to address global food supply for comparative biochemistry and physiology

This article serves as an introduction to a Virtual Special Issue of Comparative Biochemistry and Physiology (CBP) focused on aquaculture. CBP has not traditionally had a focus on aquaculture, and the Editors sought to use this Special Issue to identify opportunities for synergy between traditional comparative physiology and applied physiology, such as aquaculture. Each of the four CBP journals has a dedicated special issue, with manuscripts that span the breadth of vertebrate and invertebrate species cultured around the globe. This overview is intended to identify the major themes of the submissions, as well as articulate a vision for the types of aquaculture-focused research that are well suited for CBP publications.

Ontogeny of urea and ammonia transporters in mahi-mahi (Coryphaena hippurus) early life stages

The mechanism(s) of ammonia and urea excretion in freshwater fish have received considerable attention; however, parallel investigations of seawater fish, specifically in the early life stages are scarce. The first objective of this study was to evaluate the patterns of ammonia and urea excretion in mahi-mahi (Coryphaena hippurus) up to 102  hours post fertilization (hpf). Similar to other teleosts, mahi embryos are ureotelic before hatch and gradually switch to being ammoniotelic around the time of hatch. The second objective was to characterize mRNA levels of ammonia transporters (Rhag, Rhbg, Rhcg1 and Rhcg2), as well as urea transporter (UT) and sodium hydrogen exchangers (NHE3 and NHE2) during mahi development. As predicted, the mRNA levels of the Rhesus glycoprotein (Rh) genes, especially Rhag, Rhbg and the UT gene were highly consistent with the ontogeny of ammonia and urea excretion rates. Further, the localization of each transporter was examined in larvae collected at 60 and 102 hpf using in situ hybridization. Rhag was expressed in the gills, yolk sac, and operculum. Rhbg was expressed in the gills and upper mouth. Rhcg1 and NHE3 were co-localized in the sub-operculum, and Rhcg2 was expressed in the skin. Together, these results indicate that urea excretion is critical for ammonia detoxification during embryonic development and that Rh proteins are involved in ammonia excretion via gills and yolk sac, possibly facilitated by NHE3.

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.

Maximum salinity tolerance and osmoregulatory capabilities of European perch Perca fluviatilis populations originating from different salinity habitats

Although considered a stenohaline freshwater species, European perch (Perca fluviatilis) inhabit brackish waters. The present study determined the maximum salinity tolerance and osmoregulatory capability on individuals originating from brackish water and from freshwater populations. The fish were acclimated for 3 weeks to salinities of 0, 10, 12.5, 15, 17.5 and 20 after an initial stepwise increase to the target salinity. The maximum salinity tolerance was determined as the test salinity below which the fish could not acclimate and lost equilibrium. Blood plasma osmolality was measured if the fish had not lost equilibrium after the acclimation period. The maximum salinity tolerance was 17.5 for brackish water European perch and 10 for fresh water European perch. The high salinity tolerance of the brackish water European perch was caused by their ability to both hyper- and hypo-osmoregulate, whereas the freshwater originating fish could only hyper-osmoregulate. The results showed that maximum salinity tolerances and osmoregulatory capabilities depends on the origin habitat salinity. Due to genetic differentiation between European perch populations in brackish and fresh water, the possibility of brackish water European perch being a subspecies of European perch is discussed, yet vital knowledge concerning heritability of salinity tolerance traits is still missing. Regardless of species status, within-species plasticity in the ability to cope with varying salinities have substantial ecological and conservation implications and underlines the need for managing brackish water and freshwater European perch stocks separately.

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.

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 (U_crit) 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.

Physiological responses of corals to ocean acidification and copper exposure

Acidification and land-based sources of pollution have been linked to widespread declines of coral cover in coastal reef ecosystems. In this study, two coral species, Acropora cervicornis and Pocillopora damicornis were exposed to increased copper at two CO₂ levels for 96 h. Copper accumulation and anti-oxidant enzyme activities were measured. Copper accumulation only increased in A. cervicornis zooxanthellae and corresponded with photosynthetic toxicity. Enzyme activities in both coral species were affected; however, A. cervicornis was more sensitive than P. damicornis, and zooxanthellae were more affected than animal fractions of holobionts. Generally, activities of all anti-oxidant enzymes increased, with copper exposure in corals; whereas, activities of glutathione reductase and to some degree glutathione peroxidase were observed due to increasing CO₂ exposure alone. Exposure to copper in combination with higher CO₂ resulted in a synergistic response in some cases. These results provide insight into mechanisms of copper and CO₂ impacts in corals.

Behavioural salinity preference of juvenile yellow perch Perca flavescens

The present study determined the behavioural salinity preference of a freshwater stock of juvenile yellow perch Perca flavescens acclimated to salinities of 0 and 10. The preferred salinities ranged between 7·3 and 13·0 (mean ± s.d. = 10·4 ± 1·7; n = 13) with no significant effect of acclimation salinity. The results showed that juvenile P. flavescens prefers near isoosmotic salinities, which could be due to a lowered energetic cost of osmoregulation.