Biochemical responses of two erythrinidae fish to environmental ammonia.

The non-ionized form of ammonia is very toxic to many aquatic species. It is especially important in several aspects of fish biology. A large range of organismal strategies for coping with environmental stressors is usually observed in living organisms. Among those, the responses for managing chemical stressors are well studied. The present work compares biochemical responses of two evolutionarily close species, Hoplias malabaricus and Hoplerythrinus unitaeniatus, exposed to environmental ammonia. Adult fish were submitted to 1.0 mg/L of ammonium chloride for 24 hours, and plasma ammonia and urea levels were determined. The activities of OUC enzymes OCT and ARG, and the accessory enzyme GS, were quantified in liver extract and are expressed below in mumol/min/mg of wet tissue. Increases in OUC enzymes (GS from 1.14 to 2.43, OCT from 0.81 to 1.72, and ARG from 3.15 to 4.23), plasma ammonia (from 0.95 to 1.42 mmol/L), and plasma urea (from 0.82 to 1.53 mmol/L) were observed (p < 0.05) in H. malabaricus exposed to 1 mg/L of ammonia chloride. The GS in H. unitaeniatus increased from 1.43 to 1.84, however the OCT, ARG, and plasma urea from H. unitaeniatus did not change. These data indicate that each species responds differently to the same environmental stressor.     

Comparative oocyte morphology and early development in three species of trahiras from the São Francisco River basin, Brazil

Oocyte morphology, embryogenesis and early larval development were compared in Hoplerythrinus unitaeniatus , Hoplias lacerdae and Hoplias malabaricus (Characiformes: Erythrinidae) by macroscopical, histological, histochemical and ultrastructural analyses. The eggs of the three species were yellowish and adhesive, containing carboxyl and sulphate radicals in the glycoconjugates of the zona radiata. A complex surface arrangement was identified in oocytes of H. unitaeniatus and H. lacerdae , while H. malabaricus had a simple oocyte surface pattern. Lectin histochemistry revealed different carbohydrate terminal residues in cortical alveoli, outer zona radiata and follicular cells of the three species. At the animal pole, the oocyte surface topography surrounding the micropyle was species-specific. The micropylar cell was ConA-positive, suggesting the presence of carbohydrates with mannose/glucose terminal residues that could have a role during fertilization. The erythrinids exhibited a prolonged embryonic and larval development compared to other Characiformes, a reproductive strategy used for increasing offspring protection. Early development proceeded most rapidly in H. unitaeniatus , followed by H. malabaricus and then H. lacerdae , which could have more developed parental care behaviour. An adhesive organ composed of secretory prismatic cells protruding from the cephalic region of the three erythrinid larva allowed them to attach to one another during development. Reproductive behaviour and early developmental strategies were similar in the three species, but the oocyte surface morphology suggests a close relationship between H. unitaeniatus and H. lacerdae .     

Comparative chromosome mapping of 5S rDNA and 5SHindIII repetitive sequences in Erythrinidae fishes (Characiformes) with emphasis on the Hoplias malabaricus 'species complex'

Chromosomal localization of 5S rDNA and 5SHindIII repetitive sequences was carried out in several representatives of the Erythrinidae family, namely in karyomorphs A, D, and F of Hoplias malabaricus, and in H. lacerdae, Hoplerythrinusunitaeniatus and Erythrinus erythrinus. The 5S rDNA mapped interstitially in two chromosome pairs in karyomorph A and in one chromosome pair in karyomorphs D and F and in H. lacerdae. The 5SHindIII repetitive DNA mapped to the centromeric region of several chromosomes (18 to 22 chromosomes) with variations related to the different karyomorphs of H. malabaricus. On the other hand, no signal was detected in the chromosomes of H. lacerdae, H. unitaeniatus and E. erythrinus, suggesting that the 5SHindIII-DNA sequences have originated or were lost after the divergence of H. malabaricus from the other erythrinid species. The chromosome distribution of 5S rDNA and 5SHindIII-DNA sequences contributes to a better understanding of the mechanisms of karyotype differentiation among the Erythrinidae members.     

Chloride cell responses to ion challenge in two tropical freshwater fish,the erythrinids Hoplias malabaricus and Hoplerythrinus unitaeniatus

Chloride cell (CC) responses to ion challenge and plasma ion concentration were evaluated in two ecologically distinct erythrinids, Hoplias malabaricus, an exclusively water-breathing species, and Hoplerythrinus unitaeniatus, a facultative air-breathing fish, at one, two, seven, and 15 days of exposure to deionized water and to ion-rich water. H. malabaricus displayed high CC proliferation on filament and lamellar epithelium during exposure to deionized water and significant CC proliferation in the filament epithelium on the first day of exposure to water rich in NaCl and Ca2+ and in the lamellar epithelium on the first, second, and seventh day of exposure to such water. CC proliferation in H. unitaeniatus occurred only in the lamellar epithelium of fish exposed to deionized water. CC proliferation on both species was not accompanied by significant increase of CC density in contact with the external medium. The increase in the CC fractional area (CCFA) resulted from the increase of individual CC apical surface area on the first and second days of exposure to deionized water in H. malabaricus and only on the first day in H. unitaeniatus. Plasma ions in both erythrinid species showed transitory changes and, on the fifteenth day of exposure to the two types of experimental water, the plasma ion concentration was similar to the control fish. The CC responses of these erythrinid fish showed that CC proliferation depends on previous CC density in the gill and is not related solely to exposure to ion-poor water. Furthermore, CC proliferation in gill epithelium did not always involve an increase of CC density in contact with the external medium.     

Respiratory responses to hypoxia in relation to mode of life of two erythrinid species (Hoplias malabaricus and Hoplias lacerdae)

Predominantly, Hoplias malabaricus inhabits stagnant O₂ poor environments, whereas Hoplias lacerdae occurs in well-aerated streams. The present study evaluates the influence of mode of life on O₂ uptake and gill ventilation in equally-sized (300 g) specimens of this genus at 25° C. Comparing the species, H. lacerdae was characterized by the highest O₂ uptake and gill ventilation combined with a relatively higher cost of breathing and a lower O₂ extraction. Both species substantially increased ventilation in response to hypoxia with the difference that H. malabaricus exclusively augmented tidal volume, whereas H. lacerdae also increased breathing frequency.     

Mast cell heterogeneity between two different species of Hoplias sp. (Characiformes: Erythrinidae): response to fixatives, anatomical distribution, histochemical contents and ultrastructural features

Mast cells from two erythrinid species: Hoplias malabaricus and Hoplias lacerdae, were studied in several tissues throughout the body using light and electron microscopy. Mast cells were found in all organs studied, but were especially abundant in the gastrointestinal tract, and were always in association with connective tissue. These cells showed different characteristics between the two species studied, like varied morphology, anatomical distribution, density, basophilic/eosinophilic staining and heparin content. In H. malabaricus, the tissues fixed with Helly's solution contained mast cells that were basophilic, metachromatic and had heparin in their cytoplasmic granules, while the tissues fixed with Karnovsky's solution contained eosinophilic and orthochromatic mast cells in which heparin was not detected. In H. lacerdae, the use of both fixatives resulted in mast cells that were eosinophilic, orthochromatic, with no identifiable heparin content. Exclusively in H. malabaricus oesophagus, the mast cells were additionally seen among the epithelial cells. The ultrastructural studies performed in hindgut fixed with Karnovsky's solution revealed that the cytoplasmic granules seen in H. lacerdae mast cells were better preserved than in H. malabaricus mast cells. The latter had electron-lucent granules that were often merged, forming channels. The present study demonstrated that mast cells from two species belonging to the same genus or even mast cells from the same species but under different fixatives can present heterogeneous characteristics, possibly due to their functional properties or to their sensitivity to fixatives.     

Effect of alkalinity (pH 10) on ureogenesis in the air-breathing walking catfish,Clarias batrachus

Exposure of fish to alkaline conditions inhibits the rate of ammonia excretion, leading to ammonia accumulation and toxicity. The purpose of this study was to determine the role of ureogenesis via the urea cycle, to avoid the accumulation of ammonia to a toxic level during chronic exposure to alkaline conditions, for the air-breathing walking catfish, Clarias batrachus, where a full complement of urea cycle enzyme activity has been documented. The walking catfish can survive in water with a pH up to 10. At a pH of 10 the ammonia excretion rate by the walking catfish decreased by approximately 75% within 6 h. Although there was a gradual improvement of ammonia excretion rate by the alkaline-exposed fish, the rate remained 50% lower, even after 7 days. This decrease of ammonia excretion was accompanied by a significant accumulation of ammonia in plasma and body tissues (except in the brain). Urea-N excretion for alkaline-exposed fish increased 2.5-fold within the first day, which was maintained until day 3 and was then followed by a slight decrease to maintain a 2-fold increase in the urea-N excretion rate, even after 7 days. There was also a higher accumulation of urea in plasma and other body tissues (liver, kidney, muscle and brain). The activity of glutamine synthetase and three enzymes operating in the urea cycle (carbamyl phosphate synthetase, argininosuccinate synthetase, argininosuccinate lyase) increased significantly in hepatic and extra-hepatic tissue, such as the kidney and muscle in C. batrachus, during exposure to alkaline water. A significant increase in plasma lactate concentration noticed during alkaline exposure possibly helped in the maintenance of the acid-base balance. It is apparent that the stimulation of ureogenesis via the induced urea cycle is one of the major physiological strategies adopted by the walking catfish (C. batrachus) during chronic exposure to alkaline water, to avoid the in vivo accumulation of ammonia to a toxic level in body tissues and for the maintenance of pH homeostasis.     

Ammonia excretion and urea handling by fish gills: present understanding and future research challenges

In fresh water fishes, ammonia is excreted across the branchial epithelium via passive NH(3) diffusion. This NH(3) is subsequently trapped as NH(4)(+) in an acidic unstirred boundary layer lying next to the gill, which maintains the blood-to-gill water NH(3) partial pressure gradient. Whole animal, in situ, ultrastructural and molecular approaches suggest that boundary layer acidification results from the hydration of CO(2) in the expired gill water, and to a lesser extent H(+) excretion mediated by apical H(+)-ATPases. Boundary layer acidification is insignificant in highly buffered sea water, where ammonia excretion proceeds via NH(3) diffusion, as well as passive NH(4)(+) diffusion due to the greater ionic permeability of marine fish gills. Although Na(+)/H(+) exchangers (NHE) have been isolated in marine fish gills, possible Na(+)/NH(4)(+) exchange via these proteins awaits evaluation using modern electrophysiological and molecular techniques. Although urea excretion (J(Urea)) was thought to be via passive diffusion, it is now clear that branchial urea handling requires specialized urea transporters. Four urea transporters have been cloned in fishes, including the shark kidney urea transporter (shUT), which is a facilitated urea transporter similar to the mammalian renal UT-A2 transporter. Another urea transporter, characterized but not yet cloned, is the basolateral, Na(+) dependent urea antiporter of the dogfish gill, which is essential for urea retention in ureosmotic elasmobranchs. In ureotelic teleosts such as the Lake Magadi tilapia and the gulf toadfish, the cloned mtUT and tUT are facilitated urea transporters involved in J(Urea). A basolateral urea transporter recently cloned from the gill of the Japanese eel (eUT) may actually be important for urea retention during salt water acclimation. A multi-faceted approach, incorporating whole animal, histological, biochemical, pharmacological, and molecular techniques is required to learn more about the location, mechanism of action, and functional significance of urea transporters in fishes.     

Unusual physiology of scale-less carp,Gymnocypris przewalskii,in Lake Qinghai: a high altitude alkaline saline lake

The scale-less carp (Gymnocypris przewalskii) inhabits Lake Qinghai located on the Qinghai-Tibet plateau (elevation, 3200 m) in western China. The lake waters are alkaline (pH 9.4, titratable alkalinity=30 mmol l(-1)), Mg(2+)-rich (18.7 mmol l(-1)), Ca(2+)-poor (0.30 mmol l(-1)) and saline (9 per thousand ). These fish make annual spawning migrations into freshwater rivers. We investigated the physiology of nitrogen excretion and ionoregulation of fish from the lake and river. Fish from both waters were ammonotelic, although ammonia-N excretion rates were lower in lake fish (175 vs. 344 micromol kg(-1) h(-1), P<0.05) resulting in unusually high levels of ammonia in blood plasma (2.23 vs. 0.32 mmol l(-1)), bile, liver, muscle and brain. Exposure to 0.4 mmol l(-1) total ammonia in lake water ([NH(3)]=0.16 mmol l(-1)) killed fish within 8 h. River fish survived exposure to 1.0 mmol l(-1) total ammonia in river water at pH 8.0 ([NH(3)]=0.023 mmol l(-1)) for 24 h suggesting high ammonia tolerance in lake fish. High glutamate dehydrogenase and glutamine synthetase activities in tissues probably allow the fish to alleviate ammonia toxicity by amino acid accumulation. Neither lake nor river fish relied on urea excretion to remove excess N. Urea-N excretion rates were below 20 micromol kg(-1) h(-1) for both groups, and levels of urea in plasma and tissues were moderate. When exposed to elevated ammonia, urea-N excretion increased slightly (approximately 50 micromol kg(-1) h(-1)) and liver and muscle urea levels increased in the river fish. Plasma ion levels were within the range typical of cyprinids, but river fish had significantly higher plasma [Na(+)] and [Cl(-)] and lower [K(+)] than fish from the lake. During 48-h lake-to-river water transfer, plasma Na(+) and Cl(-) levels rose significantly. Significantly higher Na(+)/K(+)-ATPase activity in the gills of river fish may be related to the higher plasma ion levels. Plasma [Mg(2+)] and [Ca(2+)] were tightly regulated despite the great differences in the lake and river water levels.     

Gill chloride cell proliferation and respiratory responses to hypoxia of the neotropical erythrinid fish Hoplias malabaricus

The effect of chloride cell proliferation on the respiratory function was evaluated by measuring oxygen consumption (VO2) and ventilatory parameters during normoxia and gradual hypoxia in the tropical fish Hoplias malabaricus. Chloride cell proliferation was induced by keeping fish in deionized water, and the effect on the respiratory function was measured on the 1st, 2nd, and 7th day in this water using a flow-through respirometry system. Plasma osmolarity and the partial pressure of arterial oxygen (PaO2) and carbon dioxide (PaCO2) were measured under conditions of normoxia and severe hypoxia. Chloride cell proliferation on the lamellae significantly increased the water-blood diffusion distance on the 2nd and 7th day in deionized water. VO2 was kept constant until the critical oxygen pressure (PcO2) of 21.6+/-0.9 mmHg in both the control and deionized water fish was reached. The ventilatory parameters were higher in deionized water fish in normoxia, and increased during hypoxia, matching decreases in the water's partial O2 pressure. Impairment of the respiratory function was evidenced by the decrease of PaO2 of deionized water fish in normoxic condition. However, despite the changes in the epithelial morphology of gills in fish kept in deionized water, H. malabaricus proved be a hypoxic-tolerant tropical species.     

A morphological and cytochemical study of erythrocytes, thrombocytes and leukocytes in four freshwater teleosts

Erythrocytes, thrombocytes and leukocytes morphology and cytochemical staining were studied in big head carp Aristichthys nobilis , oscar Astronotus ocellatus , traíra Hoplias malabaricus and lambari Astyanax bimaculatus . Reticulocytes contained a granular material similar to residual RNA following staining with brilliant cresyl blue. Neutrophils, lymphocytes and monocytes were morphologically similar in all the four species. Thrombocytes were present in all the four species and were predominantly fusiform, whereas eosinophils occurred only in A. ocellatus . Aristichthys nobilis contained a leukocyte with unstained granules following Romanowsky-type staining, which stained intensely with periodic acid-Schiff (PAS). Glycogen granules were present in thrombocytes, neutrophils and eosinophils but not in monocytes or lymphocytes. Peroxidase staining was observed in neutrophils of A. ocellatus , H. malabaricus and A. bimaculatus but not in A. nobilis . Monocytes of A. ocellatus , H. malabaricus and A. bimaculatus stained positively for non-specific esterase, whereas those of A. nobilis did not stain. Thrombocytes and leukocytes in all four species were negative for alkaline phosphatase. Neutrophils of A. ocellatus and H. malabaricus may be involved in respiratory burst and may play an important microbicidal role.     

Nitrogen metabolism and excretion in Allenbatrachus grunniens(L): effects of variable salinity, confinement, high pH and ammonia loading

The nitrogen metabolism and excretion patterns of the grunting toadfish Allenbatrachus grunniens and the effects of salinity on these processes were examined. Individuals of A. grunniens were subjected to several experimental treatments, including variable salinity (2 to 30), high pH (8·5 compared to 7·0 for controls), high environmental ammonia (10 mM) and confinement to small water volumes, and measurements were made of activities of selected enzymes of nitrogen metabolism, ammonia and urea excretion rates, and tissue and plasma contents of ammonia, urea and amino acids. Activities of key ornithine‐urea cycle enzymes were rather low ( e.g . liver carbamoyl phosphate synthetase III activity was 0·001 μmols min⁻¹ g⁻¹), and A. grunniens consistently demonstrated a low capacity for urea excretion despite significant elevations of plasma and tissue ammonia contents by the high pH and high ammonia treatments. This species could thus be categorized as ammoniotelic. Total free amino acid contents in plasma and tissues were increased by the high pH and high ammonia treatments, but no patterns were discerned in individual amino acids that would indicate any preferential accumulation ( e.g . alanine and glutamine) as has been noted previously in several semi‐terrestrial fish species. Thus, it appeared that A. grunniens was not unusual in its patterns of nitrogen metabolism and excretion in comparison to other 'typical' teleosts. Furthermore, manipulation of salinity had no major effects on nitrogen excretion in either this species or in comparative studies with the ureotelic gulf toadfish Opsanus beta . The results are discussed in the context of the broader pattern of nitrogen metabolism and excretion in the Batrachoididae.     

Nitrogen metabolism and excretion in the aquatic chinese soft-shelled turtle, Pelodiscus sinensis, exposed to a progressive increase in ambient salinity

This study aimed to determine effects of 6-day progressive increase in salinity from 1 per thousand to 15 per thousand on nitrogen metabolism and excretion in the soft-shelled turtle, Pelodiscus sinensis. For turtles exposed to 15 per thousand water on day 6, the plasma osmolality and concentrations of Na+, Cl- and urea increased significantly, which presumably decreased the osmotic loss of water. Simultaneously, there were significant increases in contents of urea, certain free amino acids (FAAs) and water-soluble proteins that were involved in cell volume regulation in various tissues. There was an apparent increase in proteolysis, releasing FAAs as osmolytes. In addition, there might be an increase in catabolism of certain amino acids, producing more ammonia. The excess ammonia was retained as indicated by a significant decrease in the rate of ammonia excretion on day 4 in 15 per thousand water, and a major portion of it was converted to urea. The rate of urea synthesis increased 1.4-fold during the 6-day period, although the capacity of the hepatic ornithine urea cycle remained unchanged. Urea was retained for osmoregulation because there was a significant decrease in urea excretion on day 4. Increased protein degradation and urea synthesis implies greater metabolic demands, and indeed turtles exposed to 15 per thousand water had significantly higher O2 consumption rate than the freshwater (FW) control. When turtles were returned from 15 per thousand water to FW on day 7, there were significant increases in ammonia (probably released through increased amino acid catabolism) and urea excretion, confirming that FAAs and urea were retained for osmoregulatory purposes in brackish water.     

Urea production and transport in teleost fishes

Teleosts appear to have retained the genes for the urea cycle enzymes. A few species express the full complement of enzymes and are ureotelic (e.g., Lake Magadi tilapia) or ammoniotelic (e.g., largemouth bass), whereas most species have low or non-detectable enzyme activities in liver tissue and excrete little urea (e.g., adult rainbow trout). It was surprising, therefore, to find the expression of four urea cycle enzymes during early life stages of rainbow trout. The urea cycle may play a role in ammonia detoxification during a critical time of development. Exposure to alkaline water (pH 9.0-9.5) or NH4Cl (0.2 mmol/l) increased urea excretion by several-fold in trout embryos, free embryos and alevin. Urea transport is either by passive simple diffusion or via carried-mediated transport proteins. Molecular studies have revealed that a specialised urea transport protein is present in kidney tissue of elasmobranchs, similar to the facilitated urea transporter found in the mammalian inner medulla of the kidney.     

Chromosomal distribution of repetitive DNA sequences highlights the independent differentiation of multiple sex chromosomes in two closely related fish species

The arrangement of 6 repetitive DNA sequences in the mitotic and meiotic sex chromosomes of 2 Erythrinidae fish, namely Hoplias malabaricus and Erythrinus erythrinus, both with a multiple X(1)X(1)X(2)X(2)/X(1)X(2)Y sex chromosome system, was analyzed using fluorescence in situ hybridization. The distribution patterns of the repetitive sequences were distinct for each species. While some DNA repeats were species-specific, others were present in the sex chromosomes of both species at different locations. These data, together with the different morphological types of sex chromosomes and the distinct chromosomal rearrangements associated with the formation of the neo-Y chromosomes, support the plasticity of sex chromosome differentiation in the Erythrinidae family. Our present data highlight that the sex chromosomes in fish species may follow diverse differentiation patterns, even in the same type of sex chromosome system present in cofamiliar species.     

The interplay of increased urea synthesis and reduced ammonia production in the African lungfish Protopterus aethiopicus during 46 days of aestivation in a mucus cocoon

This study was undertaken to test the hypothesis that the rate of urea synthesis in Protopterus aethiopicus was up-regulated to detoxify ammonia during the initial phase of aestivation in air (day 1-day 12), and that a profound suppression of ammonia production occurred at a later phase of aestivation (day 35-day 46) which eliminated the need to sustain the increased rate of urea synthesis. Fasting apparently led to a greater rate of nitrogenous waste excretion in P. aethiopicus in water, which is an indication of increases in production of endogenous ammonia and urea probably as a result of increased proteolysis and amino acid catabolism for energy production. However, 46 days of fasting had no significant effects on the ammonia or urea contents in the muscle, liver, plasma and brain. In contrast, there were significant decreases in the muscle ammonia content in fish after 12, 34 or 46 days of aestivation in air when compared with fish fasting in water. Ammonia was apparently detoxified to urea because urea contents in the muscle, liver, plasma and brain of P. aethiopicus aestivated for 12, 34 or 46 days were significantly greater than the corresponding fasting control; the greatest increases in urea contents occurred during the initial 12 days. There were also significant increases in activities of some of the hepatic ornithine-urea cycle enzymes from fish aestivated for 12 or 46 days. Therefore, contrary to a previous report on P. aethiopicus, our results demonstrated an increase in the estimated rate of urea synthesis (2.8-fold greater than the day 0 fish) in this lungfish during the initial 12 days of aestivation. However, the estimated rate of urea synthesis decreased significantly during the next 34 days. Between day 35 and day 46 (12 days), urea synthesis apparently decreased to 42% of the day 0 control value, and this is the first report of such a phenomenon in African lungfish undergoing aestivation. On the other hand, the estimated rate of ammonia production in P. aethiopicus increased slightly (14.7%) during the initial 12 days of aestivation as compared with that in the day 0 fish. By contrast, the estimated rate of ammonia production decreased by 84% during the final 12 days of aestivation (day 35-day 46) compared with the day 0 value. Therefore, it can be concluded that P. aethiopicus depended mainly on increased urea synthesis to ameliorate ammonia toxicity during the initial phase of aestivation, but during prolonged aestivation, it suppressed ammonia production profoundly, eliminating the need to increase urea synthesis which is energy-intensive.     

The fish fauna in tropical rivers: the case of the Sorocaba River basin, São Paulo, Brazil

A survey was carried out on the fish species in the Sorocaba River basin, the main tributary of the left margin of the Tietê River, located in the State of S?o Paulo, Brazil. The species were collected with gill nets. After identification of the specimens, their relative abundance, weight and standard length were determined. Up to the present moment there are not any studies that focus this subject in this hydrographic basin. Fifty-three species, distributed in eighteen families and six orders were collected. Characiformes were represented by twenty-eight species, Siluriformes by seventeen species, the Gymnotiformes by three species, Perciformes and Cyprinodontiformes by two species, and the Synbranchiformes by one species. Among the collected species there were two exotic. The most abundant species were Astyanax fasciatus and Hypostomus ancistroides. In relation to total weight the most representative species were Hoplias malabaricus and Hypostomus ancistroides. Cyprinus carpio, Prochilodus lineatus, Schizodon nasutus and Hoplias malabaricus were the most representative species in relation to average weight. Largest standard length were recorded for Sternopygus macrurus, Steindachnerina insculpta, Eigenmannia aff. virescens and Cyprinus carpio.     

Molecular cytogenetics and its contribution to the understanding of the chromosomal diversification in Hoplias malabaricus (Characiformes)

Cytogenetic markers were used to compare the karyotypes of an isolated population of Hoplias malabaricus with others previously described. The results revealed peculiar characteristics that indicate a new independent evolutionary unit within the H. malabaricus complex.     

Role of ureogenesis in the mud-dwelled Singhi catfish (Heteropneustes fossilis) under condition of water shortage

The air-breathing Singhi catfish Heteropneustes fossilis was kept inside moist peat for 1 month mimicking their normal habitat in summer and the role of ureogenesis for their survival in a water-restricted condition was studied. The ammonia excretion rate by the mud-dwelled fish increased transiently between 6 and 12 h of re-immersion in water to approximately between eight and 10-fold, followed by a sharp decrease almost to the normal level at the later part of re-immersion. The urea-N excretion by the mud-dwelled fish increased to approximately 11-fold within 0-3 h of re-immersion, followed by a gradual decrease from 9 h onwards. The rate of urea-N excretion by the mud-dwelled fish, however, remained significantly higher (approx. threefold more) than the control fish even after 36-48 h of re-immersion. Although there was a significant increase of both ammonia and urea levels in the plasma and other tissues (except ammonia in the brain), the level of accumulation of urea was higher than ammonia in the mud-dwelled fish as indicated by the decrease in the ratio of ammonia: urea level in different tissues including the plasma. The activities (units/g tissue and /mg protein) of glutamine synthetase and three enzymes of the urea cycle, carbamyl phosphate synthetase, argininosuccinate synthetase and argininosuccinate lyase increased significantly in most of the tissues (except the brain) of the mud-dwelled fish as compared to the control fish. Higher accumulation of ammonia in vivo in the mud-dwelled Singhi catfish is suggested to be one of the major factors contributing to stimulation of ureogenesis. Due to this physiological adaptive strategy of ureogenesis, possibly along with other physiological adaptation(s), this air-breathing amphibious Singhi catfish is able to survive inside the moist peat for months in a water-restricted condition.