Hypoxia-induced developmental plasticity of larval growth, gill and labyrinth organ morphometrics in two anabantoid fish: The facultative air-breather Siamese fighting fish (Betta splendens) and the obligate air-breather the blue gourami (Trichopodus trichopterus)

Effects of clove oil concentrations on blood chemistry and stress-related gene expression in Siamese fighting fish (Betta splendens) during transportation

Siamese fishing fish (Betta splendens) or betta are usually subjected to a special method of transportation for global trade, where they are individually conveyed in plastic bags containing just enough water to cover their bodies. This study aimed to investigate the effects of transportation on their stress response by measuring hematological values, stress hormone levels, glucose levels, and stress-related gene expression. Betta fish (average body weight 1.91 ± 0.42 g; n = 30) were exposed to simulated transport in a water volume of 40 mL for 12, 24, and 48 h. Baseline levels (pre-transport) were measured prior to the experiment. The control group was transported using water without adding clove oil. Two treatment groups were transported using water with the addition of 1 and 3 mg/L concentrations of clove oil, respectively. The results revealed that transportation can be a factor that affects water quality. The pH and dissolved oxygen levels were significantly lower than baseline, while nitrite and total ammonia concentrations significantly increased. Correlating to the stress responses, significantly increasing total red blood cell counts, plasma cortisol levels, and up-regulating the expression of stress-related genes, including HSP70, GR, MR, and HIF-1α. The addition of 1 mg/L clove oil was found to reduce stress during the transport simulation, as evidenced by a reduction in these stress parameters. Conversely, increasing the concentration of clove oil to 3 mg/L significantly increased plasma cortisol after 12 h of simulated transport, and up-regulated GR, MR, and HIF-1α expression. This study revealed that the transport process can stimulates stress in betta fish but adding a concentration of 1 mg/L clove oil to the transport water could mitigate this stress response and promote animal welfare during their transportation.

Structure and function of the larval teleost fish gill

The fish gill is a multifunctional organ that is important in multiple physiological processes such as gas transfer, ionoregulation, and chemoreception. This characteristic organ of fishes has received much attention, yet an often-overlooked point is that larval fishes in most cases do not have a fully developed gill, and thus larval gills do not function identically as adult gills. In addition, large changes associated with gas exchange and ionoregulation happen in gills during the larval phase, leading to the oxygen and ionoregulatory hypotheses examining the environmental constraint that resulted in the evolution of gills. This review thus focuses exclusively on the larval fish gill of teleosts, summarizing the development of teleost larval fish gills and its function in gas transfer, ionoregulation, and chemoreception, and comparing and contrasting it to adult gills where applicable, while providing some insight into the oxygen vs ionoregulatory hypotheses debate.

Air-breathing behavior in Heterotis niloticus fingerlings: Response to changes in oxygen, temperature, and exercise regimes

Air-breathing in fish is believed to have arisen as an adaptation to aquatic hypoxia. Although air-breathing has been widely studied in numerous fish species, little is known about the obligate air-breathing African bonytongue, Heterotis niloticus. We evaluated if abiotic factors and physical activity affect air-breathing patterns in fingerlings. The air-breathing frequency (fAB ) and behavioral responses of H. niloticus fingerlings were assessed in response to environmental oxygen, temperature, and exhaustion and activity in a series of experiments. The air-breathing behavior of H. niloticus fingerlings under optimum water conditions was characterized by swift excursions lasting less than 1 s to the air-water interface to gulp air. The intervals between air-breaths were highly variable, ranging from 3 to 259 s. Body size only slightly affected fAB , while hypoxia, hyperthermia, and exercise stress significantly increased fAB . Progressive hypoxia from 17.69 to 2.17 kPa caused a ~2.5-fold increase in fAB . Increasing temperatures to 27 and 32°C, from a baseline temperature of 22°C, significantly increased fAB from 0.4 ± 0.2 to 1.3 ± 0.5 and 1.6 ± 0.4 breaths min-1 , respectively. Lastly, following exhaustive exercise, fAB increased up to 3-fold. These observations suggest that H. niloticus fingerlings are very reliant on aerial oxygen, and their air-breathing behavior is sensitive to environmental changes and activity levels.

Fluctuating environments during early development can limit adult phenotypic flexibility: insights from an amphibious fish

The interaction between developmental plasticity and the capacity for reversible acclimation (phenotypic flexibility) is poorly understood, particularly in organisms exposed to fluctuating environments. We used an amphibious killifish (Kryptolebias marmoratus) to test the hypotheses that organisms reared in fluctuating environments (i) will make no developmental changes to suit any one environment because fixing traits to suit one environment could be maladaptive for another, and (ii) will be highly phenotypically flexible as adults because their early life experiences predict high environmental variability in the future. We reared fish under constant (water) or fluctuating (water-air) environments until adulthood and assessed a suite of traits along the oxygen cascade (e.g. neuroepithelial cell density and size, cutaneous capillarity, gill morphology, ventricle size, red muscle morphometrics, terrestrial locomotor performance). To evaluate the capacity for phenotypic flexibility, a subset of adult fish from each rearing condition was then air-exposed for 14 days before the same traits were measured. In support of the developmental plasticity hypothesis, traits involved with O2 sensing and uptake were largely unaffected by water-air fluctuations during early life, but we found marked developmental changes in traits related to O2 transport, utilization and locomotor performance. In contrast, we found no evidence supporting the phenotypic flexibility hypothesis. Adult fish from both rearing conditions exhibited the same degree of phenotypic flexibility in various O2 sensing- and uptake-related traits. In other cases, water-air fluctuations attenuated adult phenotypic flexibility despite the fact that phenotypic flexibility is hypothesized to be favoured when environments fluctuate. Overall, we conclude that exposure to environmental fluctuations during development in K. marmoratus can dramatically alter the constitutive adult phenotype, as well as diminish the scope for phenotypic flexibility in later life.

Meta-Transcriptomic Analysis of RNAseq Data Reveals Pacu and Loach Fish with Unusually High Levels of Myoglobin Expression in Skeletal Muscles

Simple Summary

Oxygen-binding proteins that mediate oxygen-binding for storage and consumption, to reduce energy, are very diverse in fish, depending on their habitats. In the present study, oxygen-binding protein gene expression in the skeletal muscle of 25 diverse fish species was examined by a meta-transcriptomic approach. By using RNAseq data, this is the first study to examine the high level of myoglobin, one of the oxygen-binding proteins, transcripts in pacu and loach fish that might be related to their high tolerance for the oxygen-deficient environment. In addition, this study presents the power of the current method to compare the fish oxygen-binding protein expression and its putative gene expansion event.

Abstract

Oxygen-binding proteins, such as myoglobin, hemoglobin, neuroglobin, and cytoglobin, play a role in oxygen binding and delivery to tissues. In icefish, the loss of myoglobin and hemoglobin genes has been reported to be an adaptive evolution event. This interesting finding prompted us to exam oxygen-binding protein expression in diverse fish species. Taking advantage of substantial RNAseq data deposited in the NCBI (National Center for Biotechnology Information) database, we adopted a meta-transcriptomic approach to explore and compare four oxygen-binding protein gene expression levels in the skeletal muscle of 25 diverse fish species for the first time. RNAseq data were downloaded from the NCBI Sequence Read Archive (SRA) database, and de novo assembly was performed to generate transcript contigs. The genes encoding oxygen-binding proteins were then identified by the BLAST search, and the relative expression level of oxygen-binding protein genes was normalized by the RPKM (Reads per Kilobase Million) method. By performing expression profiling, hierarchy clustering, and principal component analysis, pacu and loach fish were noticed by their high myoglobin expression levels in skeletal muscle tissues among 25 diverse fish species. In conclusion, we demonstrated that meta-transcriptomic analysis of RNAseq data is an informative approach to compare the oxygen-binding protein expression and putative gene expansion event in fish.

Evolutionary and cardio-respiratory physiology of air-breathing and amphibious fishes

Air-breathing and amphibious fishes are essential study organisms to shed insight into the required physiological shifts that supported the full transition from aquatic water-breathing fishes to terrestrial air-breathing tetrapods. While the origin of air-breathing in the evolutionary history of the tetrapods has received considerable focus, much less is known about the evolutionary physiology of air-breathing among fishes. This review summarizes recent advances within the field with specific emphasis on the cardiorespiratory regulation associated with air-breathing and terrestrial excursions, and how respiratory physiology of these living transitional forms are affected by development and personality. Finally, we provide a detailed and re-evaluated model of the evolution of air-breathing among fishes that serves as a framework for addressing new questions on the cardiorespiratory changes associated with it. This review highlights the importance of combining detailed studies on piscine air-breathing model species with comparative multi-species studies, to add an additional dimension to our understanding of the evolutionary physiology of air-breathing in vertebrates.

Environmental hypoxia causes growth retardation, osteoclast differentiation and calcium dyshomeostasis in juvenile rainbow trout (Oncorhynchus mykiss)

Hypoxia generally refers to a dissolved oxygen (DO) level that is less than 2-3 mg/L. With ongoing global warming and environment pollution, environmental or geological studies showed hypoxia frequently occurs in global aquatic systems including ocean, river, estuaries and coasts. A preliminary study was performed to evaluate hypoxia tolerant of rainbow trout (Oncorhynchus mykiss) with parameters of mortality, behavior, endocrine and metabolite, identifying three DO levels including normoxia (Ctrl, 7.0 mg/L), non-lethal hypoxia (NH, 4.5 mg/L) and lethal hypoxia (LH, 3.0 mg/L). Furthermore, trout was treated by Ctrl, NH and LH for six hours to mimic the acute hypoxia in wild and/or farming conditions. A significantly higher mortality was observed in LH group. Trout of NH and LH showed stressful responses with unnormal swimming, increased serum cortisol and up-regulated gill hif1α transcription. Despite trout of NH and LH increased the oxygen delivery abilities by increasing the serum hemoglobin levels, the anerobic metabolism were inevitably observed with increased lactate. This study also showed a prolonged influence of NH and LH on growth after 30-days' recovery. Based on RNA-Seq data, different expression genes (DEGs) associated with stress, apoptosis, antioxidant, chaperone, growth, calcium and vitamin D metabolism were identified. Enrichment analysis showed DEGs were clustered in osteoclast differentiation, apoptosis and intracellular signaling transduction pathways. Results further showed NH and LH significantly decreased bone calcium content and disrupted the growth hormone-insulin-like growth factor (GH-IGF) axis. Our study might contribute to a better understanding of the effects of hypoxia on rainbow trout.

Acute Exposure to key Aquaculture Environmental Stressors Impaired the Aerobic Metabolism of Carassius auratus gibelio

Carassius auratus gibelio is an omnivore favored for its flavor and is commonly used as a benthic species in traditional pond polyculture. This study investigated the effects of common aquaculture stressors, such as high ammonia, high nitrite, high pH, and hypoxia on the aerobic metabolism of C. auratus gibelio. The results showed that the standard metabolic rate (SMR) was positively correlated with ammonia, nitrite, and pH, while the maximum metabolic rate (MMR) was negatively correlated with all four stressors. Thus, aerobic scope (AS) was reduced when C. auratus gibelio was exposed to high ammonia, high nitrite, high pH, and hypoxia. The peak of post-prandial O₂ consumption was positively correlated with nitrite, pH, and the occurrence of the peak metabolic rate post-prandial was delayed in high ammonia, high nitrite, hypoxia, and high pH conditions. These findings indicated that, in experimental conditions, exposure to these environmental stressors can influence aerobic metabolism in C. auratus gibelio. With more energy required to maintain standard metabolic rates, less will be available for growth. While the C. auratus gibelio is one of the most hypoxia tolerance species, the reduction we observed in AS caused by stressors that commonly occur in ponds and in nature will likely affect growth in ponds and fitness in nature. These data have provided insight into the optimal, fitness-maximizing thresholds for these common stressors in this species of interest.

Phenotypic Switching Resulting From Developmental Plasticity: Fixed or Reversible?

The prevalent view of developmental phenotypic switching holds that phenotype modifications occurring during critical windows of development are "irreversible" - that is, once produced by environmental perturbation, the consequent juvenile and/or adult phenotypes are indelibly modified. Certainly, many such changes appear to be non-reversible later in life. Yet, whether animals with switched phenotypes during early development are unable to return to a normal range of adult phenotypes, or whether they do not experience the specific environmental conditions necessary for them to switch back to the normal range of adult phenotypes, remains an open question. Moreover, developmental critical windows are typically brief, early periods punctuating a much longer period of overall development. This leaves open additional developmental time for reversal (correction) of a switched phenotype resulting from an adverse environment early in development. Such reversal could occur from right after the critical window "closes," all the way into adulthood. In fact, examples abound of the capacity to return to normal adult phenotypes following phenotypic changes enabled by earlier developmental plasticity. Such examples include cold tolerance in the fruit fly, developmental switching of mouth formation in a nematode, organization of the spinal cord of larval zebrafish, camouflage pigmentation formation in larval newts, respiratory chemosensitivity in frogs, temperature-metabolism relations in turtles, development of vascular smooth muscle and kidney tissue in mammals, hatching/birth weight in numerous vertebrates,. More extreme cases of actual reversal (not just correction) occur in invertebrates (e.g., hydrozoans, barnacles) that actually 'backtrack' along normal developmental trajectories from adults back to earlier developmental stages. While developmental phenotypic switching is often viewed as a permanent deviation from the normal range of developmental plans, the concept of developmental phenotypic switching should be expanded to include sufficient plasticity allowing subsequent correction resulting in the normal adult phenotype.

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 P_O2s), 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 P_O2 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 ± 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 ± 0.5 °C for adults was 9.8 ± 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 P_Crit 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.