Gill remodelling and growth rate of striped catfish Pangasianodon hypophthalmus under impacts of hypoxia and temperature

Effects of structural remodelling on gill physiology

The complex relationships between the structure and function of fish gills have been of interest to comparative physiologists for many years. Morphological plasticity of the gill provides a dynamic mechanism to reversibly alter its structure in response to changes in the conditions experienced by the fish. The best known example of gill remodelling is the growth or retraction of cell masses between the lamellae, a rapid process that alters the lamellar surface area that is exposed to the water (i.e. the functional lamellar surface area). Decreases in environmental O2 availability and/or increases in metabolic O2 demand stimulate uncovering of the lamellae, presumably to increase the capacity for O2 uptake. This review addresses four questions about gill remodelling: (1) what types of reversible morphological changes occur; (2) how do these changes affect physiological function from the gill to the whole animal; (3) what factors regulate reversible gill plasticity; and (4) is remodelling phylogenetically widespread among fishes? We address these questions by surveying the current state of knowledge of gill remodelling in fishes, with a focus on identifying gaps in our understanding that future research should consider.

Immunohistochemical and ultrastructural characterization of the inner ear epithelial cells of splitnose rockfish (Sebastes diploproa)

The inner ear of teleost fish regulates the ionic and acid-base chemistry and secretes protein matrix into the endolymph to facilitate otolith biomineralization, which is used to maintain vestibular and auditory functions. The otolith is biomineralized in a concentric ring pattern corresponding to seasonal growth, and this calcium carbonate (CaCO3) polycrystal has become a vital aging and life-history tool for fishery managers, ecologists, and conservation biologists. Moreover, biomineralization patterns are sensitive to environmental variability including climate change, thereby threatening the accuracy and relevance of otolith-reliant toolkits. However, the cellular biology of the inner ear is poorly characterized, which is a hurdle for a mechanistic understanding of the underlying processes. This study provides a systematic characterization of the cell types in the inner ear of splitnose rockfish (Sebastes diploproa). Scanning electron microscopy revealed the apical morphologies of six inner ear cell types. In addition, immunostaining and confocal microscopy characterized the expression and subcellular localization of the proteins Na+-K+-ATPase, carbonic anhydrase, V-type H+-ATPase, Na+-K+-2Cl--cotransporter, otolith matrix protein 1, and otolin-1 in six inner ear cell types bordering the endolymph. This fundamental cytological characterization of the rockfish inner ear epithelium illustrates the intricate physiological processes involved in otolith biomineralization and highlights how greater mechanistic understanding is necessary to predict their multistressor responses to future climate change.

Effect of hypoxia duration and pattern on channel Catfish (Ictalurus punctatus) neuropeptide gene expression and hematology

Commercial aquaculture production of channel catfish (Ictalurus punctatus) occurs in shallow ponds with daily cycling of dissolved oxygen concentration ranging from supersaturation to severe hypoxia. Once daily minimum dissolved oxygen concentration falls below 3.0 mg O2/L, channel catfish have a reduced appetite, leading to reduced growth rates. In other fishes, upregulation of the neuropeptides corticotropin-releasing factor (CRF) and urotensin I (UI) have been implicated as initiating the mechanism responsible for decreasing appetite once an environmental stressor is detected. Channel catfish maintained at 27 °C in aquaria were subjected to varying durations and patterns of hypoxia (1.75 ± 0.07 mg O2/L) to evaluate underlying physiological responses to hypoxia and determine if hypothalamic CRF and UI are responsible for hypoxia-induced anorexia in channel catfish. During a short exposure to hypoxia (12 h), venous PO2 was significantly lower within 6 h and was coupled with an increase of hematocrit and decrease of blood osmolality, yet all responses reversed within 12 h after returning to normoxia. When this pattern of hypoxia and normoxia was repeated cyclically for 5 days, these physiological responses repeated daily. Extended periods of hypoxia (5 days) resulted in similar hematological responses, which did not recover to baseline values during the hypoxia exposure. This study did not find a significant change in hypothalamic transcription of CRF and UI during hypoxia challenges but did identify multiple physiological adaptive responses that work together to reduce the severity of experimentally induced hypoxia in channel catfish.

Effects of hypoxia on the gill morphological structure, apoptosis and hypoxia-related gene expression in blunt snout bream (Megalobrama amblycephala)

The blunt snout bream (Megalobrama amblycephala) is a typical hypoxia-sensitive fish, and hypoxia stress leads to reduced vitality and yield during aquaculture. To explore the specific adaptation mechanism under hypoxia, the blunt snout bream was treated with hypoxia (DO = 2.0 ± 0.1 mg/L) for 24 h, followed by 3 h of recovery. Our results depicted that the gill filament structure of blunt snout bream changed after hypoxia. During hypoxia for 24 h, the gill filament structure was altered, including a more than 80% expansion of the lamellar respiratory surface area and a proportionate apoptosis decrease in interlamellar cell mass (ILCM) volume. Thus, the water-blood diffusion distance was shortened to less than 46%. During hypoxia for 24 h, the activity of ROS in gill tissue increased significantly (p < 0.05), while the mitochondrial membrane potential decreased significantly (p < 0.05). During hypoxia, mRNA expression level of anti-apoptotic gene Bcl-2 in the gills of blunt snout bream decreased significantly (p < 0.05), while the expression of pro-apoptotic gene Bax mRNA increased significantly (p < 0.05). Thus, the ratio of Bax/Bcl-2 mRNA increased in the gills of blunt snout bream to promote the activity of Caspase-3. Together, our results indicated hypoxia-induced apoptosis in the gills of blunt snout bream through the mitochondrial pathway. In addition, a decreased expression of Phd1 and an increased expression of Hif-1α in gills under hypoxia stress indicates that blunt snout bream may cope with hypoxia-induced apoptosis by enhancing the HIF pathway. These results provide new insights into fish's adaptation strategies and mechanisms of hypoxia.

Do air-breathing fish suffer branchial oxygen loss in hypoxic water?

In hypoxia, air-breathing fish obtain O2 from the air but continue to excrete CO2 into the water. Consequently, it is believed that some O2 obtained by air-breathing is lost at the gills in hypoxic water. Pangasionodon hypophthalmus is an air-breathing catfish with very large gills from the Mekong River basin where it is cultured in hypoxic ponds. To understand how P. hypophthalmus can maintain high growth in hypoxia with the presumed O2 loss, we quantified respiratory gas exchange in air and water. In severe hypoxia (PO2: ≈ 1.5 mmHg), it lost a mere 4.9% of its aerial O2 uptake, while maintaining aquatic CO2 excretion at 91% of the total. Further, even small elevations in water PO2 rapidly reduced this minor loss. Charting the cardiovascular bauplan across the branchial basket showed four ventral aortas leaving the bulbus arteriosus, with the first and second gill arches draining into the dorsal aorta while the third and fourth gill arches drain into the coeliacomesenteric artery supplying the gut and the highly trabeculated respiratory swim-bladder. Substantial flow changes across these two arterial systems from normoxic to hypoxic water were not found. We conclude that the proposed branchial oxygen loss in air-breathing fish is likely only a minor inefficiency.

Functional exploration of SNP mutations in HIF2αb gene correlated with hypoxia tolerance in blunt snout bream (Megalobrama amblycephala)

Blunt snout bream (Megalobrama amblycephala) is sensitive to hypoxia environment. Hypoxia-inducible factor (HIF) is the most critical factor in the HIF pathway, which strictly regulates the hypoxia stress process of fish. In this study, we found six hifα genes in blunt snout bream that demonstrated different expressions under hypoxia conditions. In HEK293T cells, all six hifαs were detected to activate the HRE region by luciferase reporter assay. More importantly, we identified two linkage-disequilibrium SNP sites at exon 203 and 752 of the hif2αb gene in blunt snout bream. Haplotype II (A²⁰³A⁷⁵²) and its homozygous diplotype II (A²⁰³A²⁰³A⁷⁵²A⁷⁵²) appeared frequently in a selected strain of blunt snout bream with hypoxia tolerance. Diplotype II has a lower oxygen tension threshold for loss of equilibrium (LOE_crit) over a similar range of temperatures. Moreover, its erythrocyte number increased significantly (p < 0.05) than those in diplotype I and diplotype III strains at 48 h of hypoxia. The enzymes related with hypoxia tolerant traits, i.e., reduced glutathione, superoxide dismutase, and catalase, were also significantly (p < 0.05) induced in diplotype II than in diplotype I or III. In addition, the expression of epo in the liver of diplotype II was significantly (p < 0.01) higher than that in the diplotype I or III strains at 48 h of hypoxia. Taken together, our results found that the hypoxia-tolerant-related diplotype II of hif2αb has the potential to be used as a molecular marker in future genetic breeding of hypoxia-tolerant strain.

Plastic loss of motile cilia in the gills of Polypterus in response to high CO2 or terrestrial environments

The evolutionary transition of vertebrates from water to land during the Devonian period was accompanied by major changes in animal respiratory systems in terms of physiology and morphology. Indeed, the fossil record of the early tetrapods has revealed the existence of internal gills, which are vestigial fish-like traits used underwater. However, the fossil record provides only limited data on the process of the evolutionary transition of gills from fish to early tetrapods. This study investigated the gills of Polypterus senegalus, a basal ray-finned/amphibious fish which shows many ancestral features of stem Osteichthyes. Based on scanning electron microscopy observations and transcriptome analysis, the existence of motile cilia in the gills was revealed which may create a flow on the gill surface leading to efficient ventilation or remove particles from the surface. Interestingly, these cilia were observed to disappear after rearing in terrestrial or high CO₂ environments, which mimics the environmental changes in the Devonian period. The cilia re-appeared after being returned to the original aquatic environment. The ability of plastic changes of gills in Polypterus revealed in this study may allow them to survive in fluctuating environments, such as shallow swamps. The ancestor of Osteichthyes is expected to have possessed such plasticity in the gills, which may be one of the driving forces behind the transition of vertebrates from water to land.

Arapaima gigas maintains gas exchange separation in severe aquatic hypoxia but does not suffer branchial oxygen loss

One of the most air-reliant obligate air-breathing fish is the South American Arapaima gigas, with substantially reduced gills impeding gas diffusion, thought to be a result of recurring aquatic hypoxia in its habitat. In normoxic water, A. gigas is reported to satisfy 70-80% of its O2 requirement from the air while excreting 60-90% of its CO2 to the water. If this pattern of gas exchange were to continue in severely hypoxic water, O2 loss at the gills would be expected. We hypothesized therefore that partitioning of CO2 would shift to the air phase in severe aquatic hypoxia eliminating the risk of branchial O2 loss. By adapting a respirometer designed to measure aquatic MO2/MCO2 we were able to run intermittent closed respirometry on both water and air phase for both of these gasses as well as sample water for N-waste measurements (ammonia-N, urea-N) so as to calculate metabolic fuel utilization. In contrast to our prediction, we found that partitioning of CO2 excretion changed little between normoxia and severe hypoxia (83% vs 77% aquatic excretion respectively) and at the same time there was no evidence of branchial O2 loss in hypoxia. This indicates that A. gigas can utilize distinct transfer pathways for O2 and CO2. Routine and standard MO2, N-waste excretion, and metabolic fuel utilization did not change with water oxygenation. Metabolism was fueled mostly by protein oxidation (53%) while carbohydrates and lipids accounted for 27% and 20% respectively.

Effects of hypoxia and reoxygenation on gill remodeling, apoptosis, and oxidative stress in hypoxia-tolerant new variety blunt snout bream (Megalobrama amblycephala)

Blunt snout bream plays an important role in freshwater aquaculture in China, but the development of its culture industry has been restricted by increasing hypoxia problem. Through the breeding of wild blunt snout bream populations (F0), a hypoxia-tolerant new variety (F6) was obtained. In this study, the new variety was stressed under low oxygen concentration (2.0 mg·L^-1) for 4 and 7 days, the morphological structure of the gill tissue showed a striking change, the interlamellar cell mass (ILCM) volume reduced significantly (P < 0.05), and the lamellar respiratory surface area enlarged significantly (P < 0.05), compared to normoxic controls. After 7 days of oxygen recovery, gill remodeling was completely reversed. Additionally, the TUNEL-positive apoptotic fluorescence signals increased in the gills exposed to hypoxia up to 4 and 7 days; the apoptosis rate also increased significantly (P < 0.05). Under 4 and 7 days of hypoxia stress, the expression of anti-apoptotic gene Bcl-2 in the gills downregulated significantly (P < 0.05), with the significantly (P < 0.05) upregulated expression of pro-apoptotic gene Bad. Furthermore, under hypoxia stress, the activity or content of oxidative stress-related enzymes (superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA), and glutathione (GSH)) in gill tissue increased to varying degrees compared to normoxic controls. These results offer a new perspective into the cellular and molecular mechanism of hypoxia-induced gill remodeling in blunt snout bream and a theoretical basis for its hypoxia adaptation mechanism.

Lophiosilurus alexandri, a sedentary bottom fish, adjusts its physiological parameters to survive in hypoxia condition

We investigated blood gas, hematological and biochemical parameters, and gill morphology of Lophiosilurus alexandri juveniles submitted to hypoxia for 48 h, followed by recovery for 48 h. A total of 48 juveniles (360.0 ± 141.6 g) were distributed among eight tanks (120 L) and subjected to hypoxia condition (water with dissolved oxygen at 2.12 ± 0.90 mg L^-1) or normoxia (at 5.60 ± 0.31 mg L^-1). Blood gas values (pH, PvCO₂, PvO₂, sO₂, HCO₃^-, stHCO₃^-, and base excess) in hypoxia were significantly different from normoxia, while for lactate and the electrolytes (K⁺, Na⁺, Cl^-, and Ca²⁺) there was no significant change among treatments. The erythrocytes differed significantly between normoxia and hypoxia at 72 h (24 h of recovery), while for hemoglobin and hematocrit there were no significant differences. There was a significant difference in glucose, triglycerides, and cholesterol for both normoxia and hypoxia, while plasma protein remained unchanged. All gill components (epithelial cells, erythrocytes, pillar cells, mucous cells, ionocytes, undifferentiated cells, and blood capillary lumen) differed significantly between normoxia and hypoxia. A reduction in the length of the primary lamella was observed in the hypoxia and recovery treatments, when compared to normoxia. The secondary branchial lamella showed no significant difference for both treatments. Juveniles of Lophiosilurus alexandri adapted well to hypoxia for 48 h, as they were able to adjust most of their physiological variables to survive this stress condition. After 48 h of hypoxia recovery, fish showed parameters similar to animals in normoxia. Thus, the present study shows that the tolerance to hypoxia conditions of L. alexandri, together with other important beneficial characteristics of the species, such as the high meat quality and high commercial value, demonstrates its great potential for production among regional species.

Histopathological, hematological, and biochemical changes in high-latitude fish Phoxinus lagowskii exposed to hypoxia

Hypoxia is one of the most significant threats to biodiversity in aquatic systems. The ability of high-latitude fish to tolerate hypoxia with histological and physiological responses is mostly unknown. We address this knowledge gap by investigating the effects of exposures to different oxygen levels using Phoxinus lagowskii (a high-latitude, cold-water fish) as a model. Fish were exposed to different oxygen levels (0.5 mg/L and 3 mg/L) for 24 h. The loss of equilibrium (LOE), an indicator of acute hypoxia tolerance, was 0.21 ± 0.01 mg/L, revealing the ability of fish to tolerate low-oxygen conditions. We sought to determine if, in P. lagowskii, the histology of gills and liver, blood indicators, enzyme activities of carbohydrate and lipid metabolism, and antioxidants changed to relieve stress in response to acute hypoxia. Notably, changes in vigorous jumping behavior under low oxygen revealed the exceptional hypoxia acclimation response compared with other low-latitude fish. A decrease in blood parameters, including RBC, WBC, and Hb, as well as an increase in MCV was observed compared to the controls. The increased total area in lamella and decreased ILCM volume in P. lagowskii gills were detected in the present study. Our results also showed the size of vacuoles in the livers of the hypoxic fish shrunk. Interestingly, an increase in the enzyme activity of lipid metabolism but not glucose metabolism was observed in the groups exposed to hypoxia at 6 h and 24 h. After combining histology and physiology results, our findings provide evidence that lipid metabolism plays a crucial role in enhancing hypoxia acclimation in P. lagowskii. Additionally, SOD activity significantly increased during hypoxia, suggesting the presence of an antioxidant response of P. lagowskii during hypoxia. High expression levels of lipogenesis and lipolysis-related genes were detected in the 6 h 3 mg/L and 24 h 3 mg/L hypoxia group. Enhanced expression of lipid-metabolism genes (ALS4, PGC-1, and FASN) was detected during hypoxia exposure. Together, these data suggest that P. lagowskii's ability to tolerate hypoxic events is likely mediated by a comprehensive strategy.

High environmental temperature and low pH stress alter the gill phenotypic plasticity of Hoven's carp Leptobarbus hoevenii

Climate warming and low pH environment are known to negatively impact all levels of aquatic organism from cellular to organism and population levels. For ammonotelic freshwater species, any abiotic factor fluctuation will cause disturbance to the fish, specifically at the gills which act as a multifunctional organ to support all biological processes. Therefore, this study was designed to investigate the effect of temperature (28 vs. 32°C) and pH (7.0 vs. 5.0) stress on the gill plasticity of Hoven's carp after 20 days of continuous exposure. The results demonstrated that high temperature and low pH caused severe changes on the primary and secondary lamellae as well as the cells within lamellae. An increasing trend of the proportion available for gas exchange was noticed at high temperature in both pH exposures, which resulted from a reduction of the primary lamellae width with elongated and thinner secondary lamellae compared to fishes at ambient temperature. Following exposure to high temperature and acidic pH, Hoven's carp experienced gill modifications including aneurysm, oedema, hypertrophy, curling of secondary lamellae, epithelial lifting, hyperplasia and lamellae fusion. These modifications are indicators of the coping mechanism of Hoven's carp to the changing environment in order to survive.

A practical guide to unbiased quantitative morphological analyses of the gills of rainbow trout (Oncorhynchus mykiss) in ecotoxicological studies

Rainbow trout (Oncorhynchus mykiss) are frequently used as experimental animals in ecotoxicological studies, in which they are experimentally exposed to defined concentrations of test substances, such as heavy metals, pesticides, or pharmaceuticals. Following exposure to a broad variety of aquatic pollutants, early morphologically detectable toxic effects often manifest in alterations of the gills. Suitable methods for an accurate and unbiased quantitative characterization of the type and the extent of morphological gill alterations are therefore essential prerequisites for recognition, objective evaluation and comparison of the severity of gill lesions. The aim of the present guidelines is to provide practicable, standardized and detailed protocols for the application of unbiased quantitative stereological analyses of relevant morphological parameters of the gills of rainbow trout. These gill parameters inter alia include the total volume of the primary and secondary gill lamellae, the surface area of the secondary gill lamellae epithelium (i.e., the respiratory surface) and the thickness of the diffusion barrier. The featured protocols are adapted to fish of frequently used body size classes (300-2000 g). They include well-established, conventional sampling methods, probes and test systems for unbiased quantitative stereological analyses of light- and electron microscopic 2-D gill sections, as well as the application of modern 3-D light sheet fluorescence microscopy (LSFM) of optically cleared gill samples as an innovative, fast and efficient quantitative morphological analysis approach. The methods shown here provide a basis for standardized and representative state-of-the-art quantitative morphological analyses of trout gills, ensuring the unbiasedness and reproducibility, as well as the intra- and inter-study comparability of analyses results. Their broad implementation will therefore significantly contribute to the reliable identification of no observed effect concentration (NOEC) limits in ecotoxicological studies and, moreover, to limit the number of experimental animals by reduction of unnecessary repetition of experiments.

Extreme warm acclimation temperature alters oxygen consumption, micronucleus formation in erythrocytes, and gill morphology of rohu (Labeo rohita) fingerlings

Experiencing the seasonal variation and rapid global warming in the tropical climate is a common phenomenon which challenged the aquatic organisms to adapt the physiology and behavior. To investigate the effect of high-temperature acclimation, we selected Indian major carp, rohu (Labeo rohita), a commercially important freshwater aquaculture species. Oxygen consumptions, micronucleus formation in erythrocytes, and gill histopathology were observed in L. rohita fingerlings acclimated at three temperatures (30, 33, and 36 °C) for 30 days. Results showed that the highest acclimated temperature (36 °C) induced higher oxygen consumption and increased frequency of micronucleus formation in erythrocytes. Severity of different histological alterations (hyperplasia, epithelial necrosis, telangiectasis, epithelial lifting, and hypertrophy of chloride cells) in the gills was found to be increased in the highest acclimated temperature (36 °C). These findings indicate the temperature induced adaptive responses and climate vulnerability in a changing environment.

Toxicity and bioaccumulation of Cadmium, Copper and Zinc in a direct comparison at equitoxic concentrations in common carp (Cyprinus carpio) juveniles

The individual toxicity and bioaccumulation of cadmium, copper and zinc for common carp juveniles was evaluated in a direct comparison in two experimental setups. First, fish were exposed for 10 days to different metal concentrations in order to link metal bioaccumulation to LC₅₀ values (concentration lethal to 50% of the animals) and incipient lethal levels (ILL, concentration where 50% survives indefinitely). Accumulated metals showed a positive dose dependent uptake for cadmium and copper, but not for zinc. Toxicity was in the order cadmium>copper>zinc with 96h LC₅₀ values for cadmium at 0.20±0.16 μM, for copper at 0.77±0.03 μM, and for zinc at 29.89±9.03 μM respectively. For copper, the 96h exposure was sufficient to calculate the incipient lethal level and therefore 96h LC₅₀ and ILL levels were the same, while for cadmium and zinc 5 to 6 days were needed to reach ILL resulting in slightly lower values at 0.16 μM and 28.33 μM respectively. Subsequently, a subacute exposure experiment was conducted, where carp juveniles were exposed to 2 equitoxic concentrations (10% and 50% of LC₅₀ 96 h) of the three metals for 1, 3 and 7 days. Again a significant dose-dependent increase in gill cadmium and copper, but not in zinc, was observed during the 7-day exposure. Copper clearly affected sodium levels in gill tissue, while zinc and cadmium did not significantly alter any of the gill electrolytes. The overall histopathological effects (e.g. hyperemia and hypertrophy) of the metal exposures were mild for most of the alterations. Our study showed that copper an cadmium (but not zinc) showed dose dependent metal accumulation, however this bioaccumulation was only correlated with mortality for cadmium. Metal specific alterations were reduced gill sodium levels in copper exposed fish and oedema of the primary epithelium which typically occurred in both levels of zinc exposure.

Impact and tissue metabolism of nitrite at two acclimation temperatures in striped catfish (Pangasianodon hypophthalmus)

Elevated concentrations of nitrite develop occasionally in various aquatic habitats and aquaculture facilities, providing a potential danger for freshwater fish that take up nitrite via the gill chloride uptake mechanism. We studied the uptake, effects and metabolism of nitrite in blood, heart and skeletal muscle at two temperatures in striped catfish Pangasianodon hypophthalmus, a facultative air-breathing fish that is heavily cultivated in Southeast Asia. Exposure to 0.8 mM ambient nitrite increased blood [nitrite] and [methaemoglobin] (metHb) to high values at day 1, but values subsequently decreased towards controls at day 7. Blood [nitrite] and metHb content were unexpectedly higher at 27 °C (∼1.2 mM; 69% at day 1) than at 33 °C (∼0.9 mM; 55%), reflecting a lower nitrite uptake at the highest temperature, possibly via an increased reliance on air-breathing relative to water-breathing with temperature increase. A large fraction of the nitrite taken up was effectively eliminated by being detoxified to nitrate. Further, erythrocyte metHb reductase activity was increased during nitrite exposure, efficiently reducing metHb to functional haemoglobin. The uptake of nitrite into white skeletal musculature (main part of the fish) was much lower than into heart tissue. While heart [nitrite] was close to blood plasma levels, muscle [nitrite] peaked at ∼0.2 mM at day 1 and subsequently declined to ∼0.05 mM at day 7, which is below levels reported in various commercial cured meat products. Nitrite was partly metabolized to iron-nitrosyl, S-nitroso and N-nitroso compounds. The increase in nitros(yl)ated compounds was marginal in skeletal muscle and more pronounced in heart tissue.

Gill remodelling during terrestrial acclimation in the amphibious fish Polypterus senegalus

Fishes are effectively weightless in water due to the buoyant support of the environment, but amphibious fishes must cope with increased effective weight when on land. Delicate structures such as gills are especially vulnerable to collapse and loss of surface area out of water. We tested the 'structural support' hypothesis that amphibious Polypterus senegalus solve this problem using phenotypically plastic changes that provide mechanical support and increase stiffness at the level of the gill lamellae, the filaments, and the whole arches. After 7 d in terrestrial conditions, enlargement of an inter-lamellar cell mass filled the water channels between gill lamellae, possibly to provide structural support and/or reduce evaporative water loss. Similar gill remodelling has been described in several other actinopterygian fishes, suggesting this may be an ancestral trait. There was no change in the mechanical properties or collagen composition of filaments or arches after 7 days out of water, but 8 months of terrestrial acclimation caused a reduction in gill arch length and mineralized bone volume. Thus, rather than increasing the size and stiffness of the gill skeleton, P. senegalus may instead reduce investment in supportive gill tissue while on land. These results are strikingly similar to the evolutionary trend of gill loss that occurred during the tetrapod invasion of land, raising the possibility that genetic assimilation of gill plasticity was an underlying mechanism.

Learning to Air-Breathe: The First Steps

Air-breathing in vertebrates has evolved many times among the bony fish while in water. Its appearance has had a fundamental impact on the regulation of ventilation and acid-base status. We review the physico-chemical constraints imposed by water and air, place the extant air-breathing fish into this framework, and show how that the advantages of combining control of ventilation and acid-base status are only available to the most obligate of air-breathing fish, thus highlighting promising avenues for research.

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)

Larval and juvenile air breathing fish may experience nocturnal and/or seasonal aquatic hypoxia. Yet, whether hypoxia induces respiratory developmental plasticity in larval air breathing fish is uncertain. This study predicted that larvae of two closely related anabantid fish-the facultative air breather the Siamese fighting fish (Betta splendens) and the obligate air breathing blue gourami (Trichopodus trichopterus)-show distinct differences in developmental changes in body, gill, and labyrinth morphology because of their differences in levels of dependency upon air breathing and habitat. Larval populations of both species were reared in normoxia or chronic nocturnal hypoxia from hatching through 35-38 days postfertilization. Gill and labyrinth variables were measured at the onset of air breathing. Betta splendens reared in normoxia possessed larger, more developed gills (~3× greater area) than T. trichopterus at comparable stages. Surface area of the emerging labyrinth, the air breathing organ, was ~ 85% larger in normoxic B. splendens compared to T. trichopterus. Rearing in mild hypoxia stimulated body growth in B. splendens, but neither mild nor severe hypoxia affected growth in T. trichopterus. Condition factor, K (~ 1.3 in B. splendens, 0.7 in T. trichopterus) was unaffected by mild hypoxia in either species, but was reduced by severe hypoxia to <0.9 only in B. splendens. Severe, but not mild, hypoxia decreased branchial surface area in B. splendens by ~40%, but neither hypoxia level affected Trichopodus branchial surface. Mild, but not severe, hypoxia increased labyrinth surface area by 30% in B. splendens. However, as for branchial surface area, labyrinth surface area was not affected in Trichopodus. These differential larval responses to hypoxic rearing suggest that different larval habitats and activity levels are greater factors influencing developmental plasticity than genetic closeness of the two species.

Conflict and Compromise: Using Reversible Remodeling to Manage Competing Physiological Demands at the Fish Gill

The structural features of the fish gill necessary for oxygen uptake also favor undesirable, passive movements of ions and water. Reversible gill remodeling is one solution to this conflict. Cell masses that limit functional surface area are lost when oxygen availability decreases in hypoxia or oxygen demand increases with exercise or high temperature. However, much remains to be learned about how widespread reversible gill remodeling is among fish species, and how and why it occurs.

Cardiac plasticity influences aerobic performance and thermal tolerance in a tropical, freshwater fish at elevated temperatures

Fishes faced with novel thermal conditions often modify physiological functioning to compensate for elevated temperatures. This physiological plasticity (thermal acclimation) has been shown to improve metabolic performance and extend thermal limits in many species. Adjustments in cardiorespiratory function are often invoked as mechanisms underlying thermal plasticity because limitations in oxygen supply have been predicted to define thermal optima in fishes; however, few studies have explicitly linked cardiorespiratory plasticity to metabolic compensation. Here, we quantified thermal acclimation capacity in the commercially harvested Nile perch (Lates niloticus) of East Africa, and investigated mechanisms underlying observed changes. We reared juvenile Nile perch for 3 months under two temperature regimes, and then measured a series of metabolic traits (e.g. aerobic scope) and critical thermal maximum (CT_max) upon acute exposure to a range of experimental temperatures. We also measured morphological traits of heart ventricles, gills and brains to identify potential mechanisms for compensation. We found that long-term (3 month) exposure to elevated temperature induced compensation in upper thermal tolerance (CT_max) and metabolic performance (standard and maximum metabolic rate, and aerobic scope), and induced cardiac remodeling in Nile perch. Furthermore, variation in heart morphology influenced variations in metabolic function and thermal tolerance. These results indicate that plastic changes enacted over longer exposures lead to differences in metabolic flexibility when organisms are acutely exposed to temperature variation. Furthermore, we established functional links between cardiac plasticity, metabolic performance and thermal tolerance, providing evidence that plasticity in cardiac capacity may be one mechanism for coping with climate change.

The effects of elevated environmental CO2 on nitrite uptake in the air-breathing clown knifefish, Chitala ornata

Nitrite and carbon dioxide are common environmental contaminants in the intensive aquaculture ponds used to farm clown knifefish (Chitala ornata) in the Mekong delta, Vietnam. Here we tested the hypothesis that hypercapnia reduces nitrite uptake across the gills, because pH regulation will reduce chloride uptake and hence nitrite uptake as the two ions compete for the same transport route via the branchial HCO₃^-/Cl^- exchanger. Fish fitted with arterial catheters were exposed to normocapnic/normoxic water (control), nitrite (1 mM), hypercapnia (21 mmHg CO₂), or combined hypercapnia (acclimated hypercapnia) and nitrite for 96 h. Blood was sampled to measure acid-base status, haemoglobin derivatives and plasma ions. Plasma nitrite increased for 48 h, but levels stayed below the exposure concentration, and subsequently decreased as a result of nitrite detoxification to nitrate. The total uptake of nitrite (evaluated as [NO₂^-] + [NO₃^-]) was significantly decreased in hypercapnia, in accordance with the hypothesis. Methemoglobin and nitrosylhemoglobin levels were similarly lower during hypercapnic compared to normocapnic nitrite exposure. The respiratory acidosis induced by hypercapnia was half-compensated by bicarbonate accumulation in 96 h, which was mainly chloride-mediated (i.e. reduced Cl^- influx via the branchial HCO₃^-/Cl^- exchanger). Plasma osmolality and main ions (Na⁺, Cl^-) were significantly decreased by hypercapnia and by nitrite exposure, consistent with inhibition of active transport. We conclude that hypercapnia induces a long-lasting, and mainly chloride-mediated acid-base regulation that reduces the uptake of nitrite across the gills.

Ontogeny and morphometrics of the gills and swim bladder of air-breathing striped catfish Pangasianodon hypophthalmus

The air-breathing fish Pangasianodon hypophthalmus has been shown to have highly plastic branchial surfaces whose area (SA) increases with temperature and aquatic hypoxia. This modulation occurs through development of inter-lamellar cell mass (ILCM). Paradoxically, in conditions where this fish has been shown capable of covering its entire aerobic scope from the water phase, it has been shown to have a very small branchial SA. To address this paradox, we measured the SA, harmonic mean diffusion distance (τ_h) and calculated the anatomic diffusion factor (ADF) of the branchial and swim bladder surfaces in fish ranging from 3 to 1900 g at 27°C in normoxia. Since the lamellae were distinguishable from the ILCM, we measured the actual SA as well as the potential SA if ILCM were lost. As a result of low τ_h, P. hypophthalmus has a high capacity for branchial oxygen uptake with or without ILCM. Actual and potential gill ADF were 361 and 1002 cm² µm^-1 kg^-1, respectively, for a 100 g fish and the ADF of the swim bladder was found to be 308 cm² µm^-1 kg^-1 By swimming fish to exhaustion at different temperatures, we show that modulation of this SA is rapid, indicating that the apparent paradox between previous studies is eliminated. Regression analysis of log-log plots of respiratory SA in relation to body mass shows that the gill scales with mass similarly to the SA in active water-breathing fish, whereas the swim bladder scales with mass more like the mammalian lung does. This fish presents a combination of respiratory surfaces not previously seen in air-breathing fish.

Differential Gene Expression Profiles and Alternative Isoform Regulations in Gill of Nile Tilapia in Response to Acute Hypoxia

Fish often encounters exposures to acute environmental hypoxia either spatially or temporally. Gill organ plays important roles in response to hypoxic stress in fish. Few studies focus on the molecular regulation mechanisms of gills under hypoxic stress. In this study, we investigated the transcriptomic response to 12-h acute hypoxia in gill of a hypoxia tolerant fish, Nile tilapia Oreochromis niloticus through RNA sequencing (RNA-Seq). We sequenced messenger RNA from three control samples and three hypoxia-treated samples. Bioinformatics analysis identified 239 differentially expressed genes (DEG) and 34 genes (DUES) that had significant differential alternative isoform regulation events in at least one exonic region in gill in response to acute hypoxia. The spatiotemporal expression analysis in five tissues (heart, liver, brain, gill, and spleen) sampled at three time points (6, 12, and 24 h) under hypoxia treatment confirmed the significant association of differential exon usages in two DUES genes (TLDC2 and SSX2IPA) with hypoxia conditions. Further functional analysis suggested several energy and immune response-related pathways, e.g., metabolic pathway and antigen processing and presentation, contained the most abundant DEG genes. We found that some GO biological processes for DEG genes were significantly enriched under hypoxic stress, such as glycolysis, metabolic process, generation of precursor metabolites and energy, and cholesterol metabolic process. Our findings suggest abundant differential gene expression changes and alternative isoform regulation events in genes involved in the hypoxia response in gill. Our results provide a basis for exploring the gene regulation mechanism under hypoxic stress in fish.

Lactate provides a strong pH-independent ventilatory signal in the facultative air-breathing teleost Pangasianodon hypophthalmus

Fish regulate ventilation primarily by sensing O₂-levels in the water and arterial blood. It is well established that this sensory process involves several steps, but the underlying mechanisms remain frustratingly elusive. Here we examine the effect of increasing lactate ions at constant pH on ventilation in a teleost; specifically the facultative air-breathing catfish Pangasianodon hypophthalmus. At lactate levels within the physiological range obtained by Na-Lactate injections (3.5 ± 0.8 to 10.9 ± 0.7 mmol L⁻¹), gill ventilation increased in a dose-dependent manner to levels comparable to those elicited by NaCN injections (2.0 µmol kg⁻¹), which induces a hypoxic response and higher than those observed in any level of ambient hypoxia (lowest PO₂ = 20 mmHg). High lactate concentrations also stimulated air-breathing. Denervation of the first gill arch reduced the ventilatory response to lactate suggesting that part of the sensory mechanism for lactate is located at the first gill arch. However, since a residual response remained after this denervation, the other gill arches or extrabranchial locations must also be important for lactate sensing. We propose that lactate plays a role as a signalling molecule in the hypoxic ventilatory response in fish.