2 Respiratory Gas Exchange, Aerobic Metabolism, and Effects of Hypoxia During Early Life

Developing rainbow trout (Oncorhynchus mykiss) lose branchial plasma accessible carbonic anhydrase expression with hatch and the transition to pH-sensitive, adult hemoglobin polymorphs

Salmonids possess a unique respiratory system comprised of three major components: highly pH-sensitive hemoglobins, red blood cell (RBC) intracellular pH (pHi) protection, and a heterogeneous distribution of plasma accessible carbonic anhydrase (paCA), specifically with absence of paCA at the gills. These characteristics are thought to have evolved to enhance oxygen unloading to the tissues while protecting uptake at the gills. Our knowledge of this system is detailed in adults, but little is known about it through development. Developing rainbow trout (Oncorhynchus mykiss) express embryonic RBCs containing hemoglobins that are relatively insensitive to pH; however, availability of gill paCA and RBC pHi protection is unknown. We show that pre-hatch rainbow trout express gill paCA, which is lost in correlation with the emergence of highly pH-sensitive adult hemoglobins and RBC pHi protection. Rainbow trout therefore exhibit a switch in respiratory strategy with hatch. We conclude that gill paCA likely represents an embryonic trait in rainbow trout and is constrained in adults due to their highly pH-sensitive hemoglobins.

Rapid embryonic development supports the early onset of gill functions in two coral reef damselfishes

The gill is one of the most important organs for growth and survival of fishes. Early life stages in coral reef fishes often exhibit extreme physiological and demographic characteristics that are linked to well-established respiratory and ionoregulatory processes. However, gill development and function in coral reef fishes is not well understood. Therefore, we investigated gill morphology, oxygen uptake and ionoregulatory systems throughout embryogenesis in two coral reef damselfishes, Acanthochromis polyacanthus and Amphiprion melanopus (Pomacentridae). In both species, we found key gill structures to develop rapidly early in the embryonic phase. Ionoregulatory cells appear on gill filaments 3-4 days post-fertilization and increase in density, whilst disappearing or shrinking in cutaneous locations. Primary respiratory tissue (lamellae) appears 5-7 days post-fertilization, coinciding with a peak in oxygen uptake rates of the developing embryos. Oxygen uptake was unaffected by phenylhydrazine across all ages (pre-hatching), indicating that haemoglobin is not yet required for oxygen uptake. This suggests that gills have limited contribution to respiratory functions during embryonic development, at least until hatching. Rapid gill development in damselfishes, when compared with that in most previously investigated fishes, may reflect preparations for a high-performance, challenging lifestyle on tropical reefs, but may also make reef fishes more vulnerable to anthropogenic stressors.

Harmful dinoflagellate Cochlodinium polykrikoides impairs the feeding behavior of larval sheepshead minnows (Cyprinodon variegatus)

Research evaluating the toxicity of the harmful dinoflagellate Cochlodinium (a.k.a. Margalefidinium) polykrikoides has been dominated by acute bioassays while the sublethal effects remain less well understood. This study examined the sublethal effects of C. polykrikoides exposure on the feeding behavior of larval estuarine fish. Sheepshead minnow (Cyprinodon variegatus) larvae were used in feeding experiments which assessed the total consumption of zooplankton prey (i.e., Artemia nauplii) over defined time periods. Larvae exposed to intermediate concentrations (i.e., 10²  cells ml^-1 ) of clonal cultures of C. polykrikoides saw statistically significant reductions (range = 10%-81%) in the Artemia consumed compared to controls (i.e., filtered seawater, culture media or nontoxin producing dinoflagellate). These reductions were found independent of whether the larvae were fed or starved prior to experimentation. As these concentrations are similar to those typically found during mild blooms or at the periphery of dense blooms, these findings have significant implications for the feeding behavior of ichthyoplankton.

Intraspecific variation in tolerance of warming in fishes

Intraspecific variation in key traits such as tolerance of warming can have profound effects on ecological and evolutionary processes, notably responses to climate change. The empirical evidence for three primary elements of intraspecific variation in tolerance of warming in fishes is reviewed. The first is purely mechanistic that tolerance varies across life stages and as fishes become mature. The limited evidence indicates strongly that this is the case, possibly because of universal physiological principles. The second is intraspecific variation that is because of phenotypic plasticity, also a mechanistic phenomenon that buffers individuals' sensitivity to negative impacts of global warming in their lifetime, or to some extent through epigenetic effects over successive generations. Although the evidence for plasticity in tolerance to warming is extensive, more work is required to understand underlying mechanisms and to reveal whether there are general patterns. The third element is intraspecific variation based on heritable genetic differences in tolerance, which underlies local adaptation and may define long-term adaptability of a species in the face of ongoing global change. There is clear evidence of local adaptation and some evidence of heritability of tolerance to warming, but the knowledge base is limited with detailed information for only a few model or emblematic species. There is also strong evidence of structured variation in tolerance of warming within species, which may have ecological and evolutionary significance irrespective of whether it reflects plasticity or adaptation. Although the overwhelming consensus is that having broader intraspecific variation in tolerance should reduce species vulnerability to impacts of global warming, there are no sufficient data on fishes to provide insights into particular mechanisms by which this may occur.

Acidification and hypoxia interactively affect metabolism in embryos, but not larvae, of the coastal forage fish Menidia menidia

Ocean acidification is occurring in conjunction with warming and deoxygenation as a result of anthropogenic greenhouse gas emissions. Multistressor experiments are critically needed to better understand the sensitivity of marine organisms to these concurrent changes. Growth and survival responses to acidification have been documented for many marine species, but studies that explore underlying physiological mechanisms of carbon dioxide (CO2) sensitivity are less common. We investigated oxygen consumption rates as proxies for metabolic responses in embryos and newly hatched larvae of an estuarine forage fish (Atlantic silverside, Menidia menidia) to factorial combinations of CO2×temperature or CO2×oxygen. Metabolic rates of embryos and larvae significantly increased with temperature, but partial pressure of CO2 (PCO2 ) alone did not affect metabolic rates in any experiment. However, there was a significant interaction between PCO2  and partial pressure of oxygen (PO2 ) in embryos, because metabolic rates were unaffected by PO2  level at ambient PCO2 , but decreased with declining PO2  under elevated PCO2 For larvae, however, PCO2  and PO2  had no significant effect on metabolic rates. Our findings suggest high individual variability in metabolic responses to high PCO2 , perhaps owing to parental effects and time of spawning. We conclude that early life metabolism is largely resilient to elevated PCO2  in this species, but that acidification likely influences energetic responses and thus vulnerability to hypoxia.

Respirometry and cutaneous oxygen flux measurements reveal a negligible aerobic cost of ion regulation in larval zebrafish (Danio rerio)

Fishes living in fresh water counter the passive loss of salts by actively absorbing ions through specialized cells termed ionocytes. Ionocytes contain ATP-dependent transporters and are enriched with mitochondria; therefore ionic regulation is an energy-consuming process. The purpose of this study was to assess the aerobic costs of ion transport in larval zebrafish (Danio rerio). We hypothesized that changes in rates of Na+ uptake evoked by acidic or low Na+ rearing conditions would result in corresponding changes in whole-body oxygen consumption (ṀO2 ) and/or cutaneous oxygen flux (JO2 ), measured at the ionocyte-expressing yolk sac epithelium using the scanning micro-optrode technique (SMOT). Larvae at 4 days post-fertilization (dpf) that were reared under low pH (pH 4) conditions exhibited a higher rate of Na+ uptake compared with fish reared under control conditions (pH 7.6), yet they displayed a lower ṀO2  and no difference in cutaneous JO2 Despite a higher Na+ uptake capacity in larvae reared under low Na+ conditions, there were no differences in ṀO2  and JO2  at 4 dpf. Furthermore, although Na+ uptake was nearly abolished in 2 dpf larvae lacking ionocytes after morpholino knockdown of the ionocyte proliferation regulating transcription factor foxi3a, ṀO2 and JO2  were unaffected. Finally, laser ablation of ionocytes did not affect cutaneous JO2 Thus, we conclude that the aerobic costs of ion uptake by ionocytes in larval zebrafish, at least in the case of Na+, are below detection using whole-body respirometry or cutaneous SMOT scans, providing evidence that ion regulation in zebrafish larvae incurs a low aerobic cost.

Low Oxygen Stress During Early Development Influences Regulation of Hypoxia-Response Genes in Farmed Atlantic Salmon (Salmo salar)

Survival and growth of developing salmonids are negatively affected by low oxygen levels within gravel nests in natural streams, and hypoxic stress is often experienced by farmed Atlantic salmon (Salmo salar) within hatcheries. Exposure to hypoxia during early development may have long-lasting effects by altering epigenetic marks and gene expression in oxygen regulatory pathways. Here, we examine the transcriptomic response to low dissolved oxygen (DO) in post-hatch salmon reared continuously in 30%, 60% or 100% DO from fertilization until start of feeding. RNA sequencing revealed multiple differentially expressed genes, including oxygen transporting hemoglobin embryonic α subunit (hbae) and EGLN3 family hypoxia-inducible factor 3 (egln3) which regulates the stability of hypoxia inducible factor 1α (HIF-1α). Both hbae and egln3 displayed expression levels inversely correlated to oxygen concentration, and DNA methylation patterns within the egln3 promoter were negatively associated with the transcript levels. These results suggest that epigenetic processes are influenced by low oxygen levels during early development in Atlantic salmon to upregulate hypoxia-response genes.

Thermal bottlenecks in the life cycle define climate vulnerability of fish

Species' vulnerability to climate change depends on the most temperature-sensitive life stages, but for major animal groups such as fish, life cycle bottlenecks are often not clearly defined. We used observational, experimental, and phylogenetic data to assess stage-specific thermal tolerance metrics for 694 marine and freshwater fish species from all climate zones. Our analysis shows that spawning adults and embryos consistently have narrower tolerance ranges than larvae and nonreproductive adults and are most vulnerable to climate warming. The sequence of stage-specific thermal tolerance corresponds with the oxygen-limitation hypothesis, suggesting a mechanistic link between ontogenetic changes in cardiorespiratory (aerobic) capacity and tolerance to temperature extremes. A logarithmic inverse correlation between the temperature dependence of physiological rates (development and oxygen consumption) and thermal tolerance range is proposed to reflect a fundamental, energetic trade-off in thermal adaptation. Scenario-based climate projections considering the most critical life stages (spawners and embryos) clearly identify the temperature requirements for reproduction as a critical bottleneck in the life cycle of fish. By 2100, depending on the Shared Socioeconomic Pathway (SSP) scenario followed, the percentages of species potentially affected by water temperatures exceeding their tolerance limit for reproduction range from ~10% (SSP 1-1.9) to ~60% (SSP 5-8.5). Efforts to meet ambitious climate targets (SSP 1-1.9) could therefore benefit many fish species and people who depend on healthy fish stocks.

Climate change impacts on fish reproduction are mediated at multiple levels of the brain-pituitary-gonad axis

Anthropogenic emissions of carbon dioxide in the atmosphere have generated rapid variations in atmospheric composition which drives major climate changes. Climate change related effects include changes in physico-chemical proprieties of sea and freshwater, such as variations in water temperature, salinity, pH/pCO₂ and oxygen content, which can impact fish critical physiological functions including reproduction. In this context, the main aim of the present review is to discuss how climate change related effects (variation in water temperature and salinity, increases in duration and frequency of hypoxia events, water acidification) would impact reproduction by affecting the neuroendocrine axis (brain-pituitary-gonad axis). Variations in temperature and photoperiod regimes are known to strongly affect sex differentiation and the timing and phenology of spawning period in several fish species. Temperature mainly acts at the level of gonad by interfering with steroidogenesis, (notably on gonadal aromatase activity) and gametogenesis. Temperature is also directly involved in the quality of released gametes and embryos development. Changes in salinity or water acidification are especially associated with reduction of sperm quality and reproductive output. Hypoxia events are able to interact with gonad steroidogenesis by acting on the steroids precursor cholesterol availability or directly on aromatase action, with an impact on the quality of gametes and reproductive success. Climate change related effects on water parameters likely influence also the reproductive behavior of fish. Although the precise mechanisms underlying the regulation of these effects are not always understood, in this review we discuss different hypothesis and propose future research perspectives.

Elevated pCO2 and Hypoxia Alter the Acid-Base Homeostasis of Developing Sheepshead Minnows, Cyprinodon variegatus

Low dissolved oxygen and increased acidification are two environmental variables that concomitantly change in an estuarine environment, both of which are exacerbated by nutrient pollution and subsequent eutrophication. To better understand how estuarine residents compensate for daily fluctuations in these environmental variables, the interactive effects of acidification and hypoxia were assessed in developing sheepshead minnows (Cyprinodon variegatus) using a 2 by 2 factorial design over a 42-day exposure. Embryos were exposed to either acidic (partial pressure of CO₂, pCO₂, ~2000 μatm), hypoxic (reduced dissolved oxygen, ~2 mg l^-1), or combined acidic and hypoxic conditions and monitored for development, hatch rate, and survival. Changes in oxygen consumption, anaerobic metabolism, oxidative stress, and acid-base balance were evaluated at three life stages (embryo, larval, and juvenile fish) to discern if and how fish compensate for these stressors during development. The combination of acidification and hypoxia delayed hatching in embryos and significantly decreased oxygen consumption (p<0.001) in all three life-stages. Neither acidification, hypoxia, nor the combination of the stressors impacted the anaerobic metabolism or oxidative stress of juvenile fish, but acid-base equilibrium was disrupted by all three treatments in larval fish. Elevated carbonic anhydrase activity was observed in the multi-stress treatment in embryos and larval fish, but not in juvenile fish. These results show that developing sheepshead minnows can re-establish cellular homeostasis in compensating to acidified and hypoxic waters.

Oxythermal window drastically constraints the survival and development of European sturgeon early life phases

European sturgeon Acipenser sturio is an anadromous fish species being classified "critically endangered" with only one remaining population in the Gironde-Garonne-Dordogne basin (France). In the global warming context, this paper aims to determine the sensitivity of A. sturio early life phases to temperature and oxygen saturation. Embryos were experimentally exposed to a combination of temperature (12 to 30 °C) and oxygen (30 to 90% O₂ saturation) conditions. Lethal and sublethal effects were evaluated using embryonic mortality, hatching success, malformation rate, yolk sac resorption, tissue development and swimming speed. Embryonic survival peaked at 20 °C and no survival was recorded at 30 °C regardless of the associated oxygen saturation. No hatching occurred at 50% O₂ sat or below regardless of temperature. Malformation frequency appeared to be minimum at 20 °C and 90% O₂ sat. Swimming speed peaked at 16 °C. The temperature optimum of early life phases of A. sturio was determined to be close to 20 °C. Its upper tolerance limit is between 26 and 30 °C and its lower tolerance limit is below 12 °C. Oxygen depletion induces sublethal effects at 70% O₂ sat and lethal effects at 50% O₂ sat. Within the spawning period in the Gironde-Garonne-Dordogne basin, we identified yearly favourable oxythermal windows. Consequences of climate change would depend of the phenological adaptation of the species for its spawning period.

Matrix Metalloproteinase 13 Activity is Required for Normal and Hypoxia-Induced Precocious Hatching in Zebrafish Embryos

Hypoxia induces precocious hatching in zebrafish, but we do not have a clear understanding of the molecular mechanisms regulating the activation of the hatching enzyme or how these mechanisms trigger precocious hatching under unfavorable environmental conditions. Using immunohistochemistry, pharmacological inhibition of matrix metalloproteinase 13 (Mmp13), and in vivo zymography, we show that Mmp13a is present in the hatching gland just as embryos become hatching competent and that Mmp13a activity is required for both normal hatching and hypoxia-induced precocious hatching. We conclude that Mmp13a likely functions in activating the hatching enzyme zymogen and that Mmp13a activity is necessary but not sufficient for hatching in zebrafish. This study highlights the broad nature of MMP function in development and provides a non-mammalian example of extra-embryonic processes mediated by MMP activity.

Pipefish embryo oxygenation, survival, and development: egg size, male size, and temperature effects

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

Patterns of Fish Reproduction at the Interface between Air and Water

Although fishes by nature are aquatic, many species reproduce in such a way that their embryos are exposed to air either occasionally or constantly during incubation. We examine the ecological context and review specific examples of reproduction by fishes at the air-water interface, including fishes that do and do not breathe air. Four modes of reproduction at the air-water interface are described across 18 teleost orders, from fresh water, estuaries, and sea water. Mode 1, the most common type of reproduction by fishes at the air-water interface, includes 21 families of mostly marine teleosts that spawn in water onto a substrate surface, on vegetation, or into hollow objects such as shells that will later be continuously or occasionally exposed to air. Although the eggs are emerged into air, many of these species do not emerge into air as adults, and only about half of them breathe air. Mode 2 involves six families of freshwater fishes setting up and guarding a nest and guarding on the water surface, either with bubbles or in vegetation. Most of these species breathe air. In Mode 3, annual killifishes in at least two families in seasonally dry habitats bury eggs in mud in temporary pools, then die before the next generation emerges. These species neither guard nests nor breathe air. Mudskippers (Gobiidae) breathe air and use Mode 4, excavating burrows in a soft substrate and then storing air in a subterranean chamber. In a variation of Mode 4, eggs are placed on bubbles within a nesting burrow by swamp eels (Synbranchidae). No fishes from basal taxa are known to place their embryos where they will be exposed to air, although most of these species breathe air as adults. The widespread but still rare, diverse forms of fish reproduction at the air-water interface across a broad taxonomic spectrum suggest repeated independent evolutionary events and strong selection pressure for adult fishes to protect their embryos from hypoxic waters, aquatic predators, pathogens, and UV radiation. Air-breathing by adult fishes appears to be de-coupled from air exposure of developing embryos or aerial emersion of adults during spawning.

Body and organ metabolic rates of a cave fish, Triplophysa rosa: influence of light and ontogenetic variation

Triplophysa rosa is a typical species of cave-dwelling fish distributed throughout Wulong County, Chongqing, China. This study aimed to test whether T. rosa has a low metabolic level as a cave species and how the metabolic rate of this fish responds to light stimulation. The whole body and organ (including brain, heart, and liver) oxygen consumption rates ([Formula: see text]) and several blood parameters related to oxygen carrying were compared between T. rosa acclimated in constant dark and those in regular photoperiod conditions. No significant changes in any variables were observed between the regular photoperiod fish and the dark fish, suggesting that the metabolic consumption of T. rosa is not light sensitive, which may be attributed to the highly degraded eyes of this cave species. The average mass-specific resting [Formula: see text] of T. rosa was 38.3 mgO₂ kg^- 1 h^- 1 and was lower than many other fish species. One possible explanation for the low metabolic level of T. rosa can be due to its highly degraded eyes and small brain size. Whole-organ [Formula: see text] of the brain, heart, and liver were on average responsible for 8.18%, 3.55%, and 8.61% of the body resting [Formula: see text], respectively. Both heart mass and liver mass increased with increasing body mass; however, brain mass did not correlate with body mass. Maintaining a small brain size throughout ontogeny suggests energy-saving advantages for this cave species.

Good parenting may not increase reproductive success under environmental extremes

For species exhibiting parental care, the way in which parents adjust care behaviour to compensate for environmental change potentially influences offspring survival and, ultimately, population viability. Using the three-spined stickleback (Gasterosteus aculeatus) - a species in which males provide parental care by building and tending a nest and fanning the eggs - we examined how low dissolved oxygen (DO) levels affect paternal care, embryo development and survival. Although levels of nest tending were unaffected by DO level, we found that larger males fanned their embryos more under low oxygen conditions. This resulted in faster rates of embryo development within the clutches of these larger males, but reduced embryo survival at 7 days post-fertilization compared to clutches of smaller males. Our results suggest that although parents may attempt to compensate for environmental change via alterations to care behaviour, their ability to do so can be dependent on parental phenotype. This sets up the potential for oxygen levels to act on the strength and direction of selection within populations. We discuss possible explanations for the surprising result that supposedly adaptive changes in care behaviour by large males (i.e. increased fanning) led to reduced embryo survival at 7 days post-fertilization, and whether, as a consequence, acute environmental conditions may have the potential to overwhelm selection on sexual traits.

Cardio-respirometry disruption in zebrafish (Danio rerio) embryos exposed to hydraulic fracturing flowback and produced water

Hydraulic fracturing to extract oil and natural gas reserves is an increasing practice in many international energy sectors. Hydraulic fracturing flowback and produced water (FPW) is a hyper saline wastewater returned to the surface from a fractured well containing chemical species present in the initial fracturing fluid, geogenic contaminants, and potentially newly synthesized chemicals formed in the fracturing well environment. However, information on FPW toxicological mechanisms of action remain largely unknown. Both cardiotoxic and respirometric responses were explored in zebrafish (Danio rerio) embryos after either an acute sediment-free (FPW-SF) or raw/sediment containing (FPW-S) fraction exposure of 24 and 48 h at 2.5% and 5% dilutions. A 48 h exposure to either FPW fraction in 24-72 h post fertilization zebrafish embryos significantly increased occurrences of pericardial edema, yolk-sac edema, and tail/spine curvature. In contrast, larval heart rates significantly decreased after FPW fraction exposures. FPW-S, but not FPW-SF, at 2.5% doses significantly reduced embryonic respiration/metabolic rates (MO₂), while for 5% FPW, both fractions reduced MO₂. Expression of select cardiac genes were also significantly altered in each FPW exposure group, implicating a cardiovascular system compromise as the potential cause for reduced embryonic MO₂. Collectively, these results support our hypothesis that organics are major contributors to cardiac and respiratory responses to FPW exposure in zebrafish embryos. Our study is the first to investigate cardiac and respiratory sub-lethal effects of FPW exposure, demonstrating that FPW effects extend beyond initial osmotic stressors and verifies the use of respirometry as a potential marker for FPW exposure.

Metabolic plasticity in development: Synergistic responses to high temperature and hypoxia in zebrafish, Danio rerio

This study investigated interactions of temperature and hypoxia on metabolic plasticity and regulation in zebrafish, Danio rerio, in the first week of development. Larval morphometry, oxygen consumption, and metabolic responses to acute changes in temperature and oxygen were measured in larvae reared under four conditions, including control (28°C and partial pressures of oxygen [PO₂] of 21 kPa), high temperature (31°C), hypoxia (11 kPa), and the two stressors combined. Rearing conditions did not result in consistent morphometric changes; substantial metabolic adjustments, however, were evident. While acute temperature increase resulted in elevated oxygen consumption, with a Q₁₀ of 2.2 ± 0.08, early-staged larvae were able to compensate to chronic temperature rise as routine metabolic rates did not differ between 28°C and 31°C chronic treatments. In contrast, larval responses to chronic and acute hypoxia were similar, with ∼30% decrease in metabolic rates from normoxic values at both temperatures. Further, prior exposure to chronic hypoxia in conjunction with acute high temperature increased Q₁₀ by a factor of 2.5 from 2.2 ± 0.08 to 5.6 ± 0.19. Metabolic suppression by acute hypoxia was independent of any prior exposure conditions. In short, results from this study showed that zebrafish larvae exhibited surprising temperature resilience and metabolic plasticity to a 3°C temperature rise even in their first week of life. Yet exposure to a second stressor (hypoxia) resulted in elevated sensitivity to temperature change that may lead to bioenergetic imbalance due to synergetic effects of temperature and hypoxia on metabolic rates.

Environmental change mediates mate choice for an extended phenotype, but not for mate quality

Sexual cues, including extended phenotypes, are expected to be reliable indicators of male genetic quality and/or provide information on parental quality. However, the reliability of these cues may be dependent on stability of the environment, with heterogeneity affecting how selection acts on such traits. Here, we test how environmental change mediates mate choice for multiple sexual traits, including an extended phenotype--the structure of male-built nests - in stickleback fish. First, we manipulated the dissolved oxygen (DO) content of water to create high or low DO environments in which male fish built nests. Then we recorded the mate choice of females encountering these males (and their nests), under either the same or reversed DO conditions. Males in high DO environments built more compact nests than those in low DO conditions and males adjusted their nest structure in response to changing conditions. Female mate choice for extended phenotype (male nests) was environmentally dependent (females chose more compact nests in high DO conditions), while female choice for male phenotype was not (females chose large, vigorous males regardless of DO level). Examining mate choice in this dynamic context suggests that females evaluate the reliability of multiple sexual cues, taking into account environmental heterogeneity.

Embryo oxygenation in pipefish brood pouches: novel insights

The pipefish brood pouch presents a unique mode of parental care that enables males to protect, osmoregulate, nourish and oxygenate the developing young. Using a very fine O2 probe, we assessed the extent to which males of the broad-nosed pipefish (Syngnathus typhle) oxygenate the developing embryos and are able to maintain pouch fluid O2 levels when brooding in normoxia (100% O2 saturation) and hypoxia (40% O2 saturation) for 24 days. In both treatments, pouch fluid O2 saturation levels were lower compared with the surrounding water and decreased throughout the brooding period, reflecting greater offspring demand for O2 during development and/or decreasing paternal ability to provide O2 to the embryos. Male condition (hepatosomatic index) was negatively affected by hypoxia. Larger males had higher pouch fluid O2 saturation levels compared with smaller males, and levels were higher in the bottom section of the pouch compared with other sections. Embryo size was positively correlated with O2 availability, irrespective of their position in the pouch. Two important conclusions can be drawn from our findings. First, our results highlight a potential limitation to brooding within the pouch and dismiss the notion of closed brood pouches as well-oxygenated structures promoting the evolution of larger eggs in syngnathids. Second, we provide direct evidence that paternal care improves with male size in this species. This finding offers an explanation for the documented strong female preference for larger partners because, in terms of oxygenation, the brood pouch can restrict embryo growth.

The evolutionary puzzle of egg size, oxygenation and parental care in aquatic environments

Offspring fitness generally improves with increasing egg size. Yet, eggs of most aquatic organisms are small. A common but largely untested assumption is that larger embryos require more oxygen than they can acquire through diffusion via the egg surface, constraining egg size evolution. However, we found no detrimental effects of large egg size on embryo growth and survival under hypoxic conditions. We tested this in the broad-nosed pipefish, Syngnathus typhle, whose males provide extensive care (nourishment, osmoregulation and oxygenation) to their young in a brood pouch on their bodies. We took advantage of this species' pronounced variation in egg size, correlating positively with female size, and tested the effect of hypoxia (40% dissolved oxygen) versus fully oxygenated (100%) water on embryo size and survival of large versus small eggs after 18 days of paternal brooding. Egg size did not affect embryo survival, regardless of O2 treatment. While hypoxia affected embryo size negatively, both large and small eggs showed similar reductions in growth. Males in hypoxia ventilated more and males with large eggs swam more, but neither treatment affected their position in the water column. Overall, our results call into question the most common explanation for constrained egg size evolution in aquatic environments.

Evolutionary ecology of pipefish brooding structures: embryo survival and growth do not improve with a pouch

For animals that reproduce in water, many adaptations in life‐history traits such as egg size, parental care, and behaviors that relate to embryo oxygenation are still poorly understood. In pipefishes, seahorses and seadragons, males care for the embryos either in some sort of brood pouch, or attached ventrally to the skin on their belly or tail. Typically, egg size is larger in the brood pouch group and it has been suggested that oxygen supplied via the pouch buffers the developing embryos against hypoxia and as such is an adaptation that has facilitated the evolution of larger eggs. Here, using four pipefish species, we tested whether the presence or absence of brood pouch relates to how male behavior, embryo size, and survival are affected by hypoxia, with normoxia as control. Two of our studied species Entelurus aequoreus and Nerophis ophidion (both having small eggs) have simple ventral attachment of eggs onto the male trunk, and the other two, Syngnathus typhle (large eggs) and S. rostellatus (small eggs), have fully enclosed brood pouches on the tail. Under hypoxia, all species showed lower embryo survival, while species with brood pouches suffered greater embryo mortality compared to pouchless species, irrespective of oxygen treatment. Behaviorally, species without pouches spent more time closer to the surface, possibly to improve oxygenation. Overall, we found no significant benefits of brood pouches in terms of embryo survival and size under hypoxia. Instead, our results suggest negative effects of large egg size, despite the protection of brood pouches.

Effects of increased temperature on metabolic activity and oxidative stress in the first life stages of marble trout (Salmo marmoratus)

Climate change may result in future alterations in thermal regime which could markedly affect the early developmental stages of cold water fish due to their expected high sensitivity to increasing temperature. In the present study, the effect of temperature increase of 2, 4 and 6°C on the oxygen consumption rate (R), the activity of respiratory electron transport system (ETS) and oxidative stress have been studied in four developmental stages of the marble trout (Salmo marmoratus)-eyed eggs, yolk-sac larvae and juveniles of 1 and 3 months. Oxygen consumption rate and ETS activity increased with level of development and with temperature in all four stages. ETS/R ratios decreased during development and correlated with temperature in eyed eggs, larvae and juveniles of 1 month, but not in juveniles of 3 months. Low ETS/R ratios at higher temperatures indicate stress response in eyed eggs, the most temperature sensitive developmental stage. Catalase (CAT) and glutathione reductase (GR) activities increased during development, but responded differently to elevated temperature in the different developmental stages. Stress in eyed eggs, caused by higher temperatures, resulted in increased oxygen consumption rate and increased activities of CAT and GR. Larvae were sensitive to increased temperature only at the highest experimental temperature of 16°C. Increased temperature did not stress the metabolism of the juveniles, since they were able to compensate their metabolic activity. The earlier developmental stages of marble trout are thus more sensitive to temperature increase than juveniles and therefore more endangered by higher water temperatures. This is the first report connecting oxygen consumption, ETS activity and ETS/R ratio with the activities of antioxidant enzymes in relation to increased temperature in salmonids.

Thermal stress alters expression of genes involved in one carbon and DNA methylation pathways in Atlantic cod embryos

One-carbon (1-C) metabolism is essential for normal embryonic development through its regulation of DNA methylation and cell proliferation. With consideration to the potential future anthropogenic oceanic warming, we studied the effects of both acute thermal stress and continuous thermal stress (10°C) during Atlantic cod embryo development on the expression levels of genes associated with the 1-C metabolism, including DNA methyltransferases. We conducted a phylogenetic analysis of vertebrate DNA methyltransferases to determine the number and similarity of DNMT found in Atlantic cod. This analysis revealed that Atlantic cod have one maintenance dnmt (dnmt1) and five de novo DNMTs (dnmt4, dnmt3, dnmt3b, dnmt3aa, dnmt3ab). Stage specific changes in expression levels occurred for all genes analyzed. The effect of acute thermal stress was evaluated during early development. Compared to controls these experiments showed significant alterations in expression levels of several genes, that in some instances were reversed at later stages of development. A significant effect of continuous thermal stress was found in gastrula embryos where lower mRNA expression levels of 1-C metabolism, de novo DNMTs and cell proliferation genes were detected. One exception was the maintenance DNMT, which was only sensitive to acute and not continuous thermal stress. DNA methylation status indicated that blastula embryos were hypomethylated compared to spermatozoa and late gastrula stages. In post-gastrula stage, however, continuous thermal stress resulted in a higher degree of DNA methylation compared to controls. These data reveal that the regulation of epigenetically important transcripts in the 1-C metabolism of Atlantic cod embryos is sensitive to thermal stress.

Expression pattern and biochemical properties of zebrafish N-acetylglutamate synthase

The urea cycle converts ammonia, a waste product of protein catabolism, into urea. Because fish dispose ammonia directly into water, the role of the urea cycle in fish remains unknown. Six enzymes, N-acetylglutamate synthase (NAGS), carbamylphosphate synthetase III, ornithine transcarbamylase, argininosuccinate synthase, argininosuccinate lyase and arginase 1, and two membrane transporters, ornithine transporter and aralar, comprise the urea cycle. The genes for all six enzymes and both transporters are present in the zebrafish genome. NAGS (EC 2.3.1.1) catalyzes the formation of N-acetylglutamate from glutamate and acetyl coenzyme A and in zebrafish is partially inhibited by L-arginine. NAGS and other urea cycle genes are highly expressed during the first four days of zebrafish development. Sequence alignment of NAGS proteins from six fish species revealed three regions of sequence conservation: the mitochondrial targeting signal (MTS) at the N-terminus, followed by the variable and conserved segments. Removal of the MTS yields mature zebrafish NAGS (zfNAGS-M) while removal of the variable segment from zfNAGS-M results in conserved NAGS (zfNAGS-C). Both zfNAGS-M and zfNAGS-C are tetramers in the absence of L-arginine; addition of L-arginine decreased partition coefficients of both proteins. The zfNAGS-C unfolds over a broader temperature range and has higher specific activity than zfNAGS-M. In the presence of L-arginine the apparent Vmax of zfNAGS-M and zfNAGS-C decreased, their Km(app) for acetyl coenzyme A increased while the Km(app) for glutamate remained unchanged. The expression pattern of NAGS and other urea cycle genes in developing zebrafish suggests that they may have a role in citrulline and/or arginine biosynthesis during the first day of development and in ammonia detoxification thereafter. Biophysical and biochemical properties of zebrafish NAGS suggest that the variable segment may stabilize a tetrameric state of zfNAGS-M and that under physiological conditions zebrafish NAGS catalyzes formation of N-acetylglutamate at the maximal rate.

Ontogeny of ornithine-urea cycle gene expression in zebrafish (Danio rerio)

Although the majority of adult teleosts excrete most of their nitrogenous wastes as ammonia, several fish species are capable of producing urea early in development. In zebrafish, it is unclear whether this results from a functional ornithine-urea cycle (O-UC) and, if so, how it might be regulated. This study examined the spatiotemporal patterns of gene expression of four major O-UC enzymes: carbamoyl phosphate synthase III (CPSIII), ornithine transcarboxylase, arginosuccinate synthetase, and arginosuccinate lyase, using real-time PCR and whole mount in situ hybridization. In addition, we hypothesized that CPSIII gene expression was epigenetically regulated through methylation of its promoter, a widespread mode of differential gene regulation between tissues and life stages in vertebrates. Furthermore, to assess CPSIII functionality, we used morpholinos to silence CPSIII in zebrafish embryos and assessed their nitrogenous waste handling during development, and in response to ammonia injections. Our results suggest that mRNAs of O-UC enzymes are expressed early in zebrafish development and colocalize to the embryonic endoderm. In addition, the methylation status of CPSIII promoter is not consistent with the patterns of expression observed in developing larvae or adult tissues, suggesting other means of transcriptional regulation of this enzyme. Finally, CPSIII morphants exhibited a transient reduction in CPSIII enzyme activity 24 h postfertilization, which was paralleled by reduced urea production during development and in response to an ammonia challenge. Overall, we conclude that the O-UC is functional in zebrafish embryos, providing further evidence that the capacity to produce urea via the O-UC is widespread in developing teleosts.

Redox regulation in Atlantic cod (Gadus morhua) embryos developing under normal and heat-stressed conditions

With regard to predicted oceanic warming, we studied the effects of heat stress on the redox system during embryonic development of Atlantic cod (Gadus morhua), with emphasis on the glutathione balance, activities of key antioxidant enzymes, and their mRNA levels. The embryos were incubated at optimal temperature for development (6 °C) or slightly above the threshold temperature (10 °C). The regulation of all the redox-related parameters measured at optimum development was highly dynamic and complex, indicating the importance of both maternal and zygotic contributions to maintaining redox equilibrium. Development at 10 °C caused a significantly higher mortality at the blastula and early gastrula stages, indicating severe stress. Measures of the glutathione redox couple showed a significantly more reduced state in embryos at 10 °C compared to 6 °C at the post-gastrula stages. Mean normalized expression of nrf2, trxred, g6pd, gclc, nox1, CuZnsod, and mt in embryos kept at 10 °C revealed stage-specific significantly reduced mRNA levels. Activities of antioxidant enzymes changed both during ontogenesis and in response to temperature, but did not correlate with mRNA levels. As the embryos need a tightly regulated redox environment to coordinate between growth and differentiation, these findings suggest that the altered redox balance might participate in inducing phenotypic changes caused by elevated temperature.

The ontogeny of regulatory control of the rainbow trout (Oncorhynchus mykiss) heart and how this is influenced by chronic hypoxia exposure

Salmonid embryos develop in cool waters over relatively long periods of time. Interestingly, hypoxic conditions have been found to be relatively common in some nesting sites (redds). The goals of this study were to determine the ontogeny of cardiac regulation in rainbow trout early life stages and how this is influenced by chronic hypoxia. The heart rate response to cholinergic and adrenergic receptor stimulation or inhibition was measured in individuals reared in normoxic (100% O2 saturation) or hypoxic (30% O2 saturation) conditions from fertilization to embryonic stages 22, 26 and 29, and larval stages 30 and 32. In normoxia, heart rate increased in response to β-adrenergic receptor stimulation (isoproterenol) as early as embryonic stage 22, and decreased with the antagonist propranolol after this stage. Cholinergic stimulation (acetylcholine) was ineffective at all stages, but atropine (acetylcholine antagonist) increased heart rate at larval stage 32. This demonstrates that cardiac β-adrenergic receptors are functional at early life stages, while cholinergic receptors are not responsive until after hatching. Collectively, embryos had cardio-acceleration control mechanisms in place just after the heartbeat stage, while cardio-inhibitory control was not functional until after hatching. Chronic hypoxia exposure triggered bradycardia, increased the response to adrenergic stimulation in embryos and larvae, and delayed the onset of cholinergic control in larvae. In non-motile stages, therefore, survival in chronic low oxygen may depend on the ability to alter the cardiac ontogenic program to meet the physiological requirements of the developing fish.

The energetics of embryonic growth

Embryos typically operate under much tighter energy constraints than older animals. This has had a profound impact on how energy is stored, mobilized and partitioned. The result is sometimes quite different ways of doing things. Growth, in particular, is a much more important activity during development. Compared with adults, specific growth rates (g) are extremely high (≥150%day(-1) for some fish). Production efficiencies are also much higher, particularly for early stages where values of 80-90% are not uncommon. Higher production efficiencies are possible, in part, because of lower unit costs at high g. Unlike in adults, the unit cost of growth does not appear to be fixed during early life. Energy also tends to be partitioned in a different manner, with compensatory partitioning being much more important during early life. Other differences include much higher routine metabolic intensities, smaller aerobic scopes and approximately isometric scaling of routine metabolism. The implications for ontogenetic growth models are discussed.

Oxygen requirements of zebrafish (Danio rerio) embryos in embryo toxicity tests with environmental samples

The zebrafish embryo test is a widely used bioassay for the testing of chemicals, effluents and other types of environmental samples. Oxygen depletion in the testing of sediments and effluents is especially important and may be a confounding factor in the interpretation of apparent toxicity. In order to identify oxygen levels critical to early developmental stages of zebrafish, oxygen consumption of zebrafish embryos between 0 and 96h post-fertilization, minimum oxygen levels required by the embryos for survival as well as the effects of oxygen depletion following exposure to model sediments were determined. No significant effects on zebrafish embryo development were observed for oxygen concentrations between 7.15 and 3.33mg/L, whereas at concentrations between 3.0and 2.0mg/L minor developmental retardations were observed, yet without any pathological consequences. Oxygen concentrations lower than 0.88mg/L were 100% lethal. In the sediment contact tests with zebrafish embryos, native sediments rich in organic materials rapidly developed strongly hypoxic conditions, particularly at the sediment-water interface (0 to 500μm distance to the sediment).

Rhesus glycoprotein and urea transporter genes in rainbow trout embryos are upregulated in response to alkaline water (pH 9.7) but not elevated water ammonia

Recent studies have shown that genes for the putative ammonia transporter, Rhesus glycoproteins (Rh) and the facilitated urea transporter (UT) are expressed before hatching in rainbow trout (Oncorhychus mykiss Walbaum) embryos. We tested the hypothesis that Rh and UT gene expressions are regulated in response to environmental conditions that inhibit ammonia excretion during early life stages. Eyed-up embryos (22 days post-fertilization (dpf)) were exposed to control (pH 8.3), high ammonia (1.70 mmol l(-1) NH4HCO3) and high pH (pH 9.7) conditions for 48h. With exposure to high water ammonia, ammonia excretion rates were reversed, tissue ammonia concentration was elevated by 9-fold, but there were no significant changes in mRNA expression relative to control embryos. In contrast, exposure to high water pH had a smaller impact on ammonia excretion rates and tissue ammonia concentrations, whereas mRNA levels for the Rhesus glycoprotein Rhcg2 and urea transporter (UT) were elevated by 3.5- and 5.6-fold, respectively. As well, mRNAs of the genes for H+ATPase and Na+/H+ exchanger (NHE2), associated with NH3 excretion, were also upregulated by 7.2- and 13-fold, respectively, in embryos exposed to alkaline water relative to controls. These results indicate that the Rhcg2, UT and associated transport genes are regulated in rainbow trout embryos, but in contrast to adults, there is no effect of high external ammonia at this stage of development.

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus(Rh) glycoproteins

Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells,whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H+ excretion and Na+uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na+ uptake and acid excretion. In this model, Rhag facilitates NH3 flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical “Na+/NH +4 exchange complex” consisting of several membrane transporters (Rhcg, V-type H+-ATPase, Na+/H+ exchanger NHE-2 and/or NHE-3, Na+ channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase(CA-2) and basolateral Na+/HCO –3cotransporter (NBC-1) and Na+/K+-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH3 by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H+ ions created by the catalysed or non-catalysed hydration of expired metabolic CO2. Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO2 transporters,their responses to feeding, and their roles in early life stages prior to the full development of gills.

Hypoxia, nitrogen, and fisheries: integrating effects across local and global landscapes

Anthropogenic nutrient enrichment and physical characteristics result in low dissolved oxygen concentrations (hypoxia) in estuaries and semienclosed seas throughout the world. Published research indicates that within and near oxygen-depleted waters, finfish and mobile macroinvertebrates experience negative effects that range from mortality to altered trophic interactions. Chronic exposure to hypoxia and fluctuating oxygen concentrations impair reproduction, immune responses, and growth. We present an analysis of hypoxia, nitrogen loadings, and fisheries landings in 30 estuaries and semien-closed seas worldwide. Our results suggest that hypoxia does not typically reduce systemwide fisheries landings below what would be predicted from nitrogen loadings, except where raw sewage is released or particularly sensitive species lose critical habitat. A number of compensatory mechanisms limit the translation of local-scale effects of hypoxia to the scale of the whole system. Hypoxia is, however, a serious environmental challenge that should be considered in fisheries management strategies and be a direct target of environmental restoration.

Tribute to P. L. Lutz: a message from the heart--why hypoxic bradycardia in fishes?

The sensing and processing of hypoxic signals, the responses to these signals and the modulation of these responses by other physical and physiological factors are an immense topic filled with numerous novel and exciting discoveries. Nestled among these discoveries, and in contrast to mammals, is the unusual cardiac response of many fish to environmental hypoxia - a reflex slowing of heart rate. The afferent and efferent arms of this reflex have been characterised, but the benefits of the hypoxic bradycardia remain enigmatic since equivocal results have emerged from experiments examining the benefit to oxygen transfer across the gills. The main thesis developed here is that hypoxic bradycardia could afford a number of direct benefits to the fish heart, largely because the oxygen supply to the spongy myocardium is precarious (i.e. it is determined primarily by the partial pressure of oxygen in venous blood, Pv(O(2))) and, secondarily, because the fish heart has an unusual ability to produce large increases in cardiac stroke volume (V(SH)) that allow cardiac output to be maintained during hypoxic bradycardia. Among the putative benefits of hypoxic bradycardia is an increase in the diastolic residence time of blood in the lumen of the heart, which offers an advantage of increased time for diffusion, and improved cardiac contractility through the negative force-frequency effect. The increase in V(SH) will stretch the cardiac chambers, potentially reducing the diffusion distance for oxygen. Hypoxic bradycardia could also reduce cardiac oxygen demand by reducing cardiac dP/dt and cardiac power output, something that could be masked at cold temperature because of a reduced myocardial work load. While the presence of a coronary circulation in certain fishes decreases the reliance of the heart on Pv(O(2)), hypoxic bradycardia could still benefit oxygen delivery via an extended diastolic period during which peak coronary blood flow occurs. The notable absence of hypoxic bradycardia among fishes that breathe air during aquatic hypoxia and thereby raise their Pv(O(2)), opens the possibility that that the evolutionary loss of hypoxic bradycardia may have coincided with some forms of air breathing in fishes. Experiments are needed to test some of these possibilities. Ultimately, any potential benefit of hypoxic bradycardia must be placed in the proper context of myocardial oxygen supply and demand, and must consider the ability of the fish heart to support its routine cardiac power output through glycolysis.

Complex phenotype-environment associations revealed in an East African cyprinid

Environmental factors influence phenotypes directly, as well as indirectly via trait correlations and interactions with other environmental variables. Using nine populations of the African cyprinid Barbus neumayeri, we employed path analysis to examine direct, indirect and total effects of two environmental variables, water flow (WF) and dissolved oxygen (DO), on several morphological traits. WF and DO directly influenced relative gill size, body shape and caudal fin shape in manners consistent with a priori predictions. Indirect effects also played an important role in the system: (1) strong, oppositely signed direct and indirect effects of WF on body shape resulted in a nonsignificant total effect; (2) DO had no direct effect on body shape, but a strong total effect via indirect effects on gill size; (3) WF indirectly influenced gill size via effects on DO. Only through examination of multiple environmental parameters and multiple traits can we hope to understand complex relationships between environment and phenotype.

Parameters influencing the dissolved oxygen in the boundary layer of rainbow trout (Oncorhynchus mykiss) embryos and larvae

We investigated the influence of oxygen demand (developmental stage) and supply (hypoxia, water flow rate, the chorion and body movements) on the oxygen concentration within the boundary layer next to the chorion of embryos or skin of larvae of rainbow trout (Oncorhynchus mykiss). Oxygen microelectrodes were used to measure dissolved oxygen (DO) within the boundary layer of trout embryos and larvae. As the embryos and larvae developed, the DO gradient and the thickness of the boundary layer increased. The DO concentration within the boundary layer next to the chorion or skin surface decreased as the DO concentration in the free-stream water decreased. A decrease in water flow rate increased the magnitude of the gradient and thickness of the boundary layer. In normoxia, the DO in the perivitelline fluid inside the chorion was 16±3.0% saturation at 31 days post fertilization, indicating that the chorion was a significant barrier to oxygen diffusion. The number of body movements did not change when embryos were exposed to hypoxia before hatching, but after hatching, hypoxia resulted in a decrease in body movements of the larvae. Taken together, our data indicate that the oxygen boundary layer around trout embryos and larvae depends on both the oxygen demand and supply. The factors that significantly impacted boundary layer oxygen were developmental stage, free-stream oxygen levels, water flow rate, and the presence of the chorion.