Breathing patterns and associated cardiovascular changes in intermittently breathing animals: (Partially) correcting a semantic quagmire.

Many animal species do not breathe in a continuous, rhythmic fashion, but rather display a variety of breathing patterns characterized by prolonged periods between breaths (inter-breath intervals), during which the heart continues to beat. Examples of intermittent breathing abound across the animal kingdom, from crustaceans to cetaceans. With respect to human physiology, intermittent breathing-also termed 'periodic' or 'episodic' breathing-is associated with a variety of pathologies. Cardiovascular phenomena associated with intermittent breathing in diving species have been termed 'diving bradycardia', 'submersion bradycardia', 'immersion bradycardia', 'ventilation tachycardia', 'respiratory sinus arrhythmia' and so forth. An examination across the literature of terminology applied to these physiological phenomena indicates, unfortunately, no attempt at standardization. This might be viewed as an esoteric semantic problem except for the fact that many of the terms variously used by different authors carry with them implicit or explicit suggestions of underlying physiological mechanisms and even human-associated pathologies. In this article, we review several phenomena associated with diving and intermittent breathing, indicate the semantic issues arising from the use of each term, and make recommendations for best practice when applying specific terms to particular cardiorespiratory patterns. Ultimately, we emphasize that the biology-not the semantics-is what is important, but also stress that confusion surrounding underlying mechanisms can be avoided by more careful attention to terms describing physiological changes during intermittent breathing and diving.

Temperature and hypoxia trigger developmental phenotypic plasticity of cardiorespiratory physiology and growth in the parthenogenetic marbled crayfish, Procambarus virginalis Lyko, 2017.

Attempting to differentiate phenotypic variation caused by environmentally-induced alterations in gene expression from that caused by actual allelic differences can be experimentally difficult. Environmental variables must be carefully controlled and then interindividual genetic differences ruled out as sources of phenotypic variation. We investigated phenotypic variability of cardiorespiratory physiology as well as biometric traits in the parthenogenetically-reproducing marbled crayfish Procambarus virginalis Lyko, 2017, all offspring being genetically identical clones. Populations of P. virginalis were reared from eggs tank-bred at four different temperatures (16, 19, 22 and 25 °C) or two different oxygen levels (9.5 and 20 kPa). Then, at Stage 3 and 4 juvenile stages, physiological (heart rate, oxygen consumption) and morphological (carapace length, body mass) variables were measured. Heart rate and oxygen consumption measured at 23 °C showed only small effects of rearing temperature in Stage 3 juveniles, with larger effects evident in older, Stage 4 juveniles. Additionally, coefficients of variation were calculated to compare our data to previously published data on P. virginalis as well as sexually-reproducing crayfish. Comparison revealed that carapace length, body mass and heart rate (but not oxygen consumption) indeed showed lower, yet notable coefficients of variation in clonal crayfish. Yet, despite being genetically identical, significant variation in their morphology and physiology in response to different rearing conditions nonetheless occurred in marbled crayfish. This suggests that epigenetically induced phenotypic variation might play a significant role in asexual but also sexually reproducing species.

Short communication: Baroreflex function in embryonic emus (Dromiceius novaehollandiae).

The baroreflex involves cardiovascular homeostatic mechanisms that buffer the system against acute deviations in arterial blood pressure. It is comprised of the cardiac limb which involves adjustments in heart rate and the peripheral limb which involves adjustments in vascular resistance. This negative feedback loop mechanism has been investigated in numerous species of adult vertebrates, however our understanding of the maturation and functional importance of the reflex in developing animals remains poorly understood. In egglaying species, our knowledge of this mechanism is limited to the domestic chicken embryo and the embryonic alligator. While each of these species possess a cardiac baroreflex prior to hatching, they differ in the timing when it becomes functional, with the embryonic chicken possessing the reflex at 90% of incubation, while the alligator possesses the reflex at 70% of incubation. In an effort to determine if bird species might share similar patterns of active baroreflex function, we studied embryonic emus (Dromiceius novaehollandiae). However, we hypothesized that emus would possess a pattern of baroreflex function similar to that of the American alligator given the emu embryo possesses functional vagal tone at 70% of incubation, possibly indicating a more mature collection of cardiovascular control mechanism than those found in embryonic chickens. Our findings illustrate that emu embryos possess a hypotensive baroreflex at 90% of incubation. Therefore, our data fail to support our original hypothesis. While only two species of birds have been studied in this context, it could indicate that baroreflex function is not essential for cardiovascular homeostasis in birds for the majority of in ovo development.

Insufficient reporting of experimental variables as a cause for nonreproducibility in animal physiology? A case study.

Nonreproducibility in scientific investigations has been explained by inadequately reporting methodology, honest error, and even misconduct. We hypothesized that, within the field of animal physiology, the most parsimonious explanation for nonreproducibility is inadequate reporting of key methodological details. We further hypothesized that implementation of relatively recently released reporting guidelines has positively impacted journal article quality, as measured by completeness of the methodology descriptions. We analyzed 84 research articles published in five primarily organismal animal physiology journals in 2008-2010 (generally before current guidelines) and 2018-2020. Compliance for reporting 34 variables referring to biology, experiments, and data collection was assessed. Reporting compliance was just ∼61% in 2008-2010, rising only slightly to 67.5% for 2018-2020. Only 21% of the reported variables showed significant differences across the period from 2008-2020. We conclude that, despite attempts by societies and journals to promote greater reporting compliance, such efforts have so far been relatively unsuccessful in the field of animal physiology.

Metabolic cost of development, regeneration, and reproduction in the planarian Schmidtea mediterranea.

Planaria are known for their ability to completely regenerate upon fissioning or experimental amputation. Yet, metabolic costs of regeneration have not been directly measured in planaria. Our goal was to establish the relationships between oxygen consumption (V̇O2), regeneration, and reproductive mode for asexual and sexual strains of Schmidtea mediterranea. We hypothesized that V̇O2 would vary by regeneration day for both sexual and asexual S. mediterranea, reflecting different costs of tissue reconstruction, but with an additional cost for regenerating sexual organs. Testes regeneration and body mass, as indicators of regeneration progress, and routine mass-specific V̇O2 as a function of maturity, regeneration, and reproductive mode, were measured over a 22-day regeneration period. Testes growth was highest in sexually mature adults, ~1/2 that in 14-day post-amputation sexual adults, and not detectable in juveniles and hatchlings. Mass-specific routine V̇O2 in sexuals was highest in mature controls at ~23 µl O2/g/h, but only half that in juveniles, hatchlings, and 14 day post-amputation adults. Both intact and 14-day post-amputation asexuals had a mass-specific routine V̇O2 of ~10-12 µl O2/g/h. The sum of V̇O2 of all amputated sections was ~100% higher than pre-amputation levels in the first 6 days of regeneration in asexuals, but not sexuals. There was no significant difference in V̇O2 of head, middle, and tail sections during regeneration. Overall, the highest metabolic costs associated with regeneration occurred during the initial 1-6 days of regeneration in both strains, but regeneration costs for sexual structures were not reflected in major V̇O2 differences between sexual and asexual strains.

Hypoxia- and hyperoxia-related gene expression dynamics during developmental critical windows of the tropical gar Atractosteus tropicus.

Aquatic hypoxia is both a naturally-occurring and anthropogenically-generated event. Fish species have evolved different adaptations to cope with hypoxic environments, including gill modifications and air breathing. However, little is known about the molecular mechanisms involved in the respiration of embryonic and larval fishes during critical windows of development. We assessed expression of the genes hif-1α, fih-1, nhe1, epo, gr and il8 using the developing tropical gar as a piscine model during three developmental periods (fertilization to hatch, 1 to 6 days post hatch (dph) and 7 to 12 dph) when exposed to normoxia (~7.43 mg/L DO), hypoxia (~2.5 mg/L DO) or hyperoxia (~9.15 mg/L DO). All genes had higher expression when fish were exposed to either hypoxia or hyperoxia during the first two developmental periods. However, fish continuously exposed to hypoxia had increased expression of the six genes by hatching and 6 dph, and by 12 dph only hif-1α still had increased expression. The middle developmental period was the most hypoxia-sensitive, coinciding with several changes in physiology and morphology. The oldest larvae were the most resilient to gene expression change, with little variation in expression of the six genes compared. This study is the first to relate the molecular response of an air-breathing fish to oxygen availability to developmental critical windows and contributes to our understanding of some molecular responses of developing fish to changes in oxygen availability.

Putting the August Krogh principle to work in developmental physiology.

The August Krogh principle has guided many comparative physiological studies, being especially useful for developmental physiology. Several attributes of unusual, if not unique, animals enable researchers to understand developmental phenomena more generally - the essence of the Krogh principle. This article provides examples of unusual traits of animals currently being used to understand development and reproduction. 1) Accelerated development greatly minimizes time spent examining how animals develop across time from egg to adult. For example, the tropical gar begins to breath air within as little as 2.5 days after hatching - much faster than other air-breathing fishes - facilitating study of the development of respiratory reflexes in fishes. 2) Transparency of the body wall has been exploited to image cardiac output in near-microscopic larvae of the zebrafish and mahi mahi, and to capitalize on bacterial biosensors to investigate development of in vivo digestive function in Caenorhabditis elegans. 3) Gigantism, as in the chicken-sized embryos of the emu, or the larvae of the paradoxical frog, allows surgeries not otherwise feasible. 4) Reproductive traits such as polyembryony in armadillos and parthenogenesis in planaria have informed us about classic gene vs. environment questions. Finally, 5) large body mass range enables clearer allometric analyses. Insects like the silk moth, show a more than a 1000-fold difference between eggs and adults. The August Krogh principle, then, is not simply to justify the study of exotic animals (as interesting as that is!), but has been used to generate a broader synthesis and understanding of all taxa.

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers.

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Metabolic physiology of the freshwater planaria Girardia dorotocephela and Schmidtea mediterranea: reproductive mode, specific dynamic action, and temperature.

Planarians are widely used animal models for studies in regeneration, developmental biology, neurobiology, and behavior. However, surprisingly little is known about other aspects of their basic biology, even though such information might help validate these flatworms as a general animal model. We hypothesized that planaria, although dependent on simple diffusion of O2 across the integument for O2 uptake, would nonetheless show changes in oxygen consumption (V̇o2) associated with reproductive mode (sexual or asexual), feeding (specific dynamic action; SDA), temperature (Q10 values), and photoperiod typical of those responses of more complex invertebrates. In the current experiments, routine V̇o2 was measured over the range of 13-28°C in Schmidtea mediterranea and Girardia dorotocephala. At the long-term maintenance temperature of 18°C, routine V̇o2 was ~13 µL O2·g-1·h-1 in the two asexual strains, but approximately twice as high (27 µL O2·g-1·h-1) in the sexual strain of S. mediterranea, suggesting a metabolic cost for sexual reproduction. Metabolic temperature sensitivity, measured by Q10, was about one to three for all three groups. All three groups showed a large (~2- to 3-fold) increase in V̇o2 within a day following feeding, suggesting a large SDA effect. Starvation, causing "degrowth" in some planaria, resulted in a loss of one-third of body mass in sexual S. mediterranea but no body mass loss in either asexual strains. Collectively, these data indicate that, while being a relatively simple flatworm with no dedicated respiratory or circulatory system, their metabolic physiological responses are quite similar to those shown by more complex invertebrates and vertebrates, contributing to their validation as an animal model.

Remodeling the epigenome and (epi)cytoskeleton: a new paradigm for co-regulation by methylation.

The epigenome determines heritable patterns of gene expression in the absence of changes in DNA sequence. The result is programming of different cellular-, tissue- and organ-specific phenotypes from a single organismic genome. Epigenetic marks that comprise the epigenome (e.g. methylation) are placed upon or removed from chromatin (histones and DNA) to direct the activity of effectors that regulate gene expression and chromatin structure. Recently, the cytoskeleton has been identified as a second target for the cell's epigenetic machinery. Several epigenetic 'readers, writers and erasers' that remodel chromatin have been discovered to also remodel the cytoskeleton, regulating structure and function of microtubules and actin filaments. This points to an emerging paradigm for dual-function remodelers with 'chromatocytoskeletal' activity that can integrate cytoplasmic and nuclear functions. For example, the SET domain-containing 2 methyltransferase (SETD2) has chromatocytoskeletal activity, methylating both histones and microtubules. The SETD2 methyl mark on chromatin is required for efficient DNA repair, and its microtubule methyl mark is required for proper chromosome segregation during mitosis. This unexpected convergence of SETD2 activity on histones and microtubules to maintain genomic stability suggests the intriguing possibility of an expanded role in the cell for chromatocytoskeletal proteins that read, write and erase methyl marks on the cytoskeleton as well as chromatin. Coordinated use of methyl marks to remodel both the epigenome and the (epi)cytoskeleton opens the possibility for integrated regulation (which we refer to as 'epiregulation') of other higher-level functions, such as muscle contraction or learning and memory, and could even have evolutionary implications.

Parental transgenerational epigenetic inheritance related to dietary crude oil exposure in Danio rerio.

Transgenerational inheritance from both parental lines can occur by genetic and epigenetic inheritance. Maternal effects substantially influence offspring survival and fitness. However, investigation of the paternal contribution to offspring success has been somewhat neglected. In the present study, adult zebrafish were separated into female and male groups exposed for 21 days to either a control diet or to a diet containing water accommodated fractions of crude oil. Four F1 offspring groups were obtained: (1) control (non-exposed parents), (2) paternally exposed, (3) maternally exposed and (4) dual-parent-exposed. To determine the maternal and paternal influence on their offspring, we evaluated responses from molecular to whole organismal levels in both generations. Growth rate, hypoxia resistance and heart rate did not differ among parental groups. However, global DNA methylation in heart tissue was decreased in oil-exposed fish compared with control parents. This decrease was accompanied by an upregulation of glycine N-methyltransferase. Unexpectedly, maternal, paternal and dual exposure all enhanced survival of F1 offspring raised in oiled conditions. Regardless of parental exposure, however, F1 offspring exposed to oil exhibited bradycardia. Compared with offspring from control parents, global DNA methylation was decreased in the three offspring groups derived from oil-exposed parents. However, no difference between groups was observed in gene regulation involved in methylation transfer, suggesting that the changes observed in the F1 populations may have been inherited from both parental lines. Phenotypic responses during exposure to persistent environmental stressors in F1 offspring appear to be influenced by maternal and paternal exposure, potentially benefitting offspring populations to survive in challenging environments.

Exposure to Crude Oil Induces Retinal Apoptosis and Impairs Visual Function in Fish.

Polycyclic aromatic hydrocarbons (PAHs) present in crude oil are known to impair visual development in fish. However, the underlying mechanism of PAH-induced toxicity to the visual system of fish is not understood. Embryonic zebrafish (Danio rerio) at 4 h post fertilization were exposed to weathered crude oil and assessed for visual function using an optokinetic response, with subsequent samples taken for immunohistochemistry and gene expression analysis. Cardiotoxicity was also assessed by measuring the heart rate, stroke volume, and cardiac output, as cardiac performance has been proposed to be a contributing factor to eye-associated malformations following oil exposure. Larvae exposed to the highest concentrations of crude oil (89.8 µg/L) exhibited an increased occurrence of bradycardia, though no changes in stroke volume or cardiac output were observed. However, genes important in eye development and phototransduction were downregulated in oil-exposed larvae, with an increased occurrence of cellular apoptosis, reduced neuronal connection, and reduced optokinetic behavioral response in zebrafish larvae.

Metabolic rate and hypoxia tolerance in Girardinichthys multiradiatus (Pisces: Goodeidae), an endemic fish at high altitude in tropical Mexico.

The darkedged splitfin (Amarillo fish), Girardinichthys multiradiatus is a vulnerable endemic fish species inhabiting central Mexico's high altitude Upper Lerma Basin, where aquatic hypoxia is exacerbated by low barometric pressures (lower PO2s), large aquatic oxygen changes, poor aquatic systems management and urban, agricultural and industrial pollution. The respiratory physiology of G. multiradiatus under such challenging conditions is unknown - therefore the main goal of the present study was to determine metabolic rates and hypoxia tolerance to elucidate possible physiological adaptations allowing this fish to survive high altitude and increasingly eutrophic conditions. Fish came from two artificial reservoirs - San Elías and Ex Hacienda - considered refuges for this species. Both reservoirs showed high dial PO2 variation, with hypoxic conditions before midday and after 20:00 h, ~4 h of normoxia (15 kPa) from 16:00-20:00, and ~4 h of hyperoxia (16-33 kPa) from 12:00-16:00. Standard metabolic rate at 20 ±â€¯0.5 °C of larvae from Ex Hacienda was significantly higher than those from San Elías, but these differences disappeared in juveniles and adults. Metabolic rate at 20 ±â€¯0.5 °C for adults was 9.8 ±â€¯0.1 SEM µmol O2/g/h. The metabolic scaling exponent for adults was 0.58 for San Elías fish and 0.83 for Ex Hacienda fish, indicating possible ecological effects on this variable. Post-larval fish in Ex Hacienda and all stages in San Elias site showed considerable hypoxia tolerance, with PCrit mean values ranging from 1.9-3.1 kPa, lower than those of many tropical fish at comparable temperatures. Collectively, these data indicate that G. multiradiatus is well adapted for the hypoxia associated with their high-altitude habitat.

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.

Inadequacy of typical physiological experimental protocols for investigating consequences of stochastic weather events emerging from global warming.

Increasingly variable, extreme, and nonpredictable weather events are predicted to accompany climate change, and such weather events will especially affect temperate, terrestrial environments. Yet, typical protocols in comparative physiology that examine environmental change typically employ simple step-wise changes in the experimental stressor of interest (e.g., temperature, water availability, oxygen, nutrition). Such protocols fall short of mimicking actual natural environments and may be inadequate for fully exploring the physiological effects of stochastic, extreme weather events. Indeed, numerous studies from the field of thermal biology, especially, indicate nonlinear and sometimes counterintuitive findings associated with variable and fluctuating (but rarely truly stochastic) protocols for temperature change. This Perspective article suggests that alternative experimental protocols should be employed that go beyond step-wise protocols and even beyond variable protocols employing circadian rhythms, for example, to those that actually embrace nonpredictable elements. Such protocols, though admittedly more difficult to implement, are more likely to reveal the capabilities (and, importantly, the limitations) of animals experiencing weather, as distinct from climate. While some possible protocols involving stochasticity are described as examples to stimulate additional thought on experimental design, the overall goal of this Perspective article is to encourage comparative physiologists to entertain incorporation of nonpredictable experimental conditions as they design future experimental protocols.

Parental stressor exposure simultaneously conveys both adaptive and maladaptive larval phenotypes through epigenetic inheritance in the zebrafish (Danio rerio).

Genomic modifications occur slowly across generations, whereas short-term epigenetic inheritance of adaptive phenotypes may be immediately beneficial to large numbers of individuals, acting as a bridge for survival when adverse environments occur. In the present study, crude oil was used as an example of an environmental stressor. Adult zebrafish (P0) were dietarily exposed for 3 weeks to no, low, medium or high concentrations of crude oil. The F1 offspring obtained from the P0 groups were then assessed for transgenerational epigenetic transfer of oil-induced phenotypes. The exposure did not alter body length, body and organ mass or condition factor in the P0 groups. However, the P0 fecundity of both sexes decreased in proportion to the amount of oil fed. The F1 larvae from each P0 were then exposed from 3 hpf to 5 dpf to oil in their ambient water. Remarkably, F1 larvae derived from oil-exposed parents, when reared in oiled water, showed a 30% enhanced survival compared with controls (P<0.001). Unexpectedly, from day 3 to 5 of exposure, F1 larvae from oil-exposed parents showed poorer survival in clean water (up to 55% decreased survival). Additionally, parental oil exposure induced bradycardia (presumably maladaptive) in F1 larvae in both clean and oiled water. We conclude that epigenetic transgenerational inheritance can lead to an immediate and simultaneous inheritance of both beneficial and maladaptive traits in a large proportion of the F1 larvae. The adaptive responses may help fish populations survive when facing transient environmental stressors.

Invited review: Development of acid-base regulation in vertebrates.

Acid-base regulation at intra- and extracellular levels is of fundamental importance for developing animals, as it is for adults. Although adult animals exhibit well established acid-base regulatory capacities to offset acid-base disturbances, early developmental stages must develop some of the mechanisms to compensate for these disturbances. Thus, in this review we consider the multiple mechanisms that are involved in acid-base regulation in the early developmental stages of vertebrates. We start with a brief general review of the three basics mechanisms that adult animals use for acid-base regulation: 1) rapidly acting bicarbonate, phosphate and protein buffer systems; 2) respiratory elimination of metabolically produced CO2 with a time-course of minutes; and 3) excretion of HCO3- and H+ via gills, kidneys or other excretory organs, the slowest of the mechanisms with a time-course of hours or days. We then consider the ability of developing fishes, amphibians, reptiles and birds to regulate acid-base balance using these three mechanisms, as well as the timing of the onset of these various mechanisms during development. Overall, the ability to regulate acid-base balance in the face of environmental stressors becomes apparent early in developing vertebrates, especially when the early stages are free living (e.g. embryos in eggs) and have no parental support via placental structures. However, we still have only a poor understanding of the dynamics and actual partitioning of acid-base regulation between buffer systems, respiratory elimination of CO2, and urinary excretion of HCO3- and H+ excretion in developing or adult fishes, amphibians, reptiles and birds. Yet, such studies could help understand key questions ranging from the evolution of endothermy to the relative importance α-stat and pH-stat acid-base regulation.

Metabolic physiology of the Mayan cichlid fish (Mayaheros uropthalmus): Re-examination of classification as an oxyconformer.

The Mayan cichlid (Mayaheros uropthalmus) is a freshwater fish inhabiting warm, potentially hypoxic and/or brackish waters, in Mexico and Central America. Despite its description as highly hypoxia tolerant, M. uropthalmus has been classified physiologically as an 'oxyconformer', which would place it in a very small (and shrinking) category of fishes that purportedly cannot maintain oxygen consumption (MO2) as ambient PO2 falls. However, hypoxia tolerance is often associated with strong oxyregulation, not oxyconformation as described for M. uropthalmus. To resolve these inconsistencies, we measured MO2, the ambient PO2 at which MO2 begins to decline as PO2 falls (PCrit), and gill ventilation rate (fG) in the Mayan cichlid. Variables were measured at 23o, 28 o and 33 °C and temperature sensitivity (Q10) calculated for each function. MO2 at air saturation was 2.9 ±â€¯0.2, 4.3 ±â€¯0.4, and 5.9 ±â€¯0.3 µmol O2/g/h at 23o, 28o and 33 °C, respectively. PCrits were low at 2.6 ±â€¯0.8 kPa, 3.2 ±â€¯0.8 kPa and 4.7 ±â€¯0.9 kPa at 23o, 28o and 33 °C, respectively. Q10 values for MO2 were 2.56 ±â€¯0.21 (23-28 °C), 1.89 ±â€¯0.15 (28-33 °C) and 2.2 ±â€¯0.1 (full temperature range of 23-33 °C), suggesting overall Q10s typical for tropical freshwater fish. fG was 39 ±â€¯3, 45 ±â€¯4, and 53 ±â€¯6 breaths/min at 23o, 28o and 33 °C, respectively, and increase 2-3 fold in severe hypoxia at each temperature. Experiments employing hyperoxia up to 35 kPa indicate a strong 'hypoxic drive' for gill ventilation. Collectively, these data show that, in contrast to a previous characterization, the Mayan cichlid is a strong oxyregulator exhibiting attributes (e.g. very low PCrit) typical of very hypoxia-tolerant fishes.

Developmental changes in oxygen consumption and hypoxia tolerance in the heat and hypoxia-adapted tabasco line of the Nile tilapia Oreochromis niloticus, with a survey of the metabolic literature for the genus Oreochromis.

The genus Oreochromis is among the most popular of the tilapiine cichlid tribe for aquaculture. However, their temperature and hypoxia tolerance, if tested at all, is usually tested at temperatures of 20-25°C, rather than at the considerably higher temperatures of 30-35°C typical of tropical aquaculture. We hypothesized that both larvae and adults of the heat and hypoxia-adapted Tabasco-line of the Nile tilapia Oreochromis niloticus would be relatively hypoxia-tolerant. Oxygen consumption rate ( M ˙ O 2 ), Q10 and aquatic surface respiration (ASR) was measured using closed respirometry at 2 (c. 0.2 g), 30 (c. 2-5 g), 105 c. (10-15 g) and 240 (c. 250 g) days of development, at 25°C, 30°C and 35°C. M ˙ O 2 at 30°C was inversely related to body mass: c. 90 µM O2 g-1 /h in larvae down to c. 1 µM O2 g-1 /h in young adults. Q10 for M ˙ O 2 was typical for fish over the range 25-35°C of 1.5-2.0. ASR was exhibited by 50% of the fish at pO2 of 15-50 mmHg in a temperature-dependent fashion. However, the largest adults showed notable ASR only when pO2 fell to below 10 mmHg. Remarkably, pcrit for M ˙ O 2 was 12-17 mmHg at 25-30°C and still only 20-25 mmHg across development at 35°C. These values are among the lowest measured for teleost fishes. Noteworthy is that all fish maintain equilibrium, ventilated their gills and showed routine locomotor action for 10-20 min after M ˙ O 2 ceased at near anoxia and when then returned to oxygenated waters, all fish survived, further indicating a remarkable hypoxic tolerance. Remarkably, data assembled for M ˙ O 2 from >30 studies showed a > x2000 difference, which we attribute to calculation or conversion errors. Nonetheless, pcrit was very low for all Oreochromis sp. and lowest in the heat and hypoxia-adapted Tabasco line.