Environmental salinity-modified osmoregulatory response in the sub-Antarctic notothenioid fish Eleginops maclovinus

In this study we assessed changes in the osmoregulatory system of juvenile sub-Antarctic Eleginops maclovinus submitted to different environmental salinities (5, 15, 32 and 45 psu) using two different acclimation trials: (1) an end-point experiment (exposure for 14 days) and (2) a time course experiment (specimens were sampled on days 1, 3, 7 and 14 post-transfer). Plasma osmolality, cortisol and metabolites (glucose, lactate and protein) values as well as Na⁺, K⁺-ATPase (NKA) activity were assessed in several osmoregulatory tissues (gills, kidney and intestine). In both trials, acclimation to different environmental salinities for 14 days induced changes in plasma metabolites (glucose, lactate and proteins) as well as cortisol values related to salinity challenges. Plasma osmolality and gill NKA activity presented a direct and positive relationship with respect to environmental salinity, while kidney NKA activity showed a “U-shaped” relationship. Anterior intestinal NKA activity increased in response to environmental salinity and apparently did not change in the middle portion of this organ, while it was enhanced in the posterior portion in environmental salinities different than seawater. Plasma metabolite values increased under hypo- and hypersaline conditions, indicating the importance of these energy substrates in extreme environments. The time course study revealed that specimens of E. maclovinus are able to accommodate their osmotic and metabolic system to respond to osmoregulatory challenges by allostatic changes.     

Effects of low salinity media on growth, condition, and gill ion transporter expression in juvenile Gulf killifish, Fundulus grandis

The Gulf killifish, Fundulus grandis, is a euryhaline teleost which has important ecological roles in the brackish-water marshes of its native range as well as commercial value as live bait for saltwater anglers. Effects of osmoregulation on growth, survival, and body condition at 0.5, 5.0, 8.0 and 12.0‰ salinity were studied in F. grandis juveniles during a 12-week trial. Relative expression of genes encoding the ion transport proteins Na(+)/K(+)-ATPase (NKA), Na(+)/K(+)/2Cl(-) cotransporter(NKCC1), and cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel was analyzed. At 0.5‰, F. grandis showed depressed growth, body condition, and survival relative to higher salinities. NKA relative expression was elevated at 7 days post-transfer but decreased at later time points in fish held at 0.5‰ while other salinities produced no such increase. NKCC1, the isoform associated with expulsion of ions in saltwater, was downregulated from week 1 to week 3 at 0.5‰ while CFTR relative expression produced no significant results across time or salinity. Our results suggest that Gulf killifish have physiological difficulties with osmoregulation at a salinity of 0.5‰ and that this leads to reduced growth performance and survival while salinities in the 5.0-12.0‰ are adequate for normal function.     

Do Patterns of Bacterial Diversity along Salinity Gradients Differ from Those Observed for Macroorganisms?

It is widely accepted that biodiversity is lower in more extreme environments. In this study, we sought to determine whether this trend, well documented for macroorganisms, also holds at the microbial level for bacteria. We used denaturing gradient gel electrophoresis (DGGE) with phylum-specific primers to quantify the taxon richness (i.e., the DGGE band numbers) of the bacterioplankton communities of 32 pristine Tibetan lakes that represent a broad salinity range (freshwater to hypersaline). For the lakes investigated, salinity was found to be the environmental variable with the strongest influence on the bacterial community composition. We found that the bacterial taxon richness in freshwater habitats increased with increasing salinity up to a value of 1‰. In saline systems (systems with >1‰ salinity), the expected decrease of taxon richness along a gradient of further increasing salinity was not observed. These patterns were consistently observed for two sets of samples taken in two different years. A comparison of 16S rRNA gene clone libraries revealed that the bacterial community of the lake with the highest salinity was characterized by a higher recent accelerated diversification than the community of a freshwater lake, whereas the phylogenetic diversity in the hypersaline lake was lower than that in the freshwater lake. These results suggest that different evolutionary forces may act on bacterial populations in freshwater and hypersaline lakes on the Tibetan Plateau, potentially resulting in different community structures and diversity patterns.     

Characterization of halotolerant Bicosoecida and Placididea (Stramenopila) that are distinct from marine forms,and the phylogenetic pattern of salinity preference in heterotrophic stramenopiles

Recent culture‐based studies demonstrate the distinctiveness of the microbial eukaryote biota of very hypersaline environments. In contrast, microscopy‐based faunistic studies suggest that the biota of habitats of more moderate hypersalinity (60–150‰) overlaps substantially with that of marine environments, but this has barely been studied with modern techniques. To investigate the diversity and salinity tolerance range of these organisms, eight cultures of heterotrophic stramenopiles were established from (or from nearby) moderately hypersaline locations. These isolates represent five independent groups; Groups A, B and C are bicosoecids; Groups D and E belong to Placididea. One isolate (Group A) is a strain of the widespread marine species Cafeteria roenbergensis, and cannot grow above 100‰ salinity. The other isolates – Groups B–E – can all grow at 150–175‰ salinities and are probably moderate halophiles. Groups B–E all represent previously unsequenced species or even genera, although Group B is the sister group of the borderline extreme halophile Halocafeteria. The high level of novelty en countered suggests that moderately hypersaline environments may harbour a heterotrophic stramenopile biota distinct from that of marine environments. Interestingly, our new isolates are all most closely related to marine or halophilic forms, and our phylogenies show large clades defined by saline/non‐saline habitats within bicosoecids, placidomonads and related lineages. In particular, most freshwater/soil bicosoecids form one well‐supported clade. The sole major exception is Bicosoeca, which is intermixed with marine environmental sequences originally referred to as ‘MAST‐13’, which are from brackish water, not typical seawater. It seems that the freshwater/marine barrier has been crossed very few times in the evolutionary history of these heterotrophic stramenopile flagellates.     

Prolonged apoptosis in mitochondria-rich cells of tilapia (<Emphasis Type="Italic">Oreochromis mossambicus</Emphasis>) exposed to elevated salinity

The time-course of programmed cell death (apoptosis) during reorganization of gill epithelium in salinity-stressed tilapia was analyzed using a recently developed method based on laser scanning cytometry (LSC) of dissociated gill cells. Apoptosis in mitochondria-rich cells (MRC) was distinguished from that in other cell types using Na⁺/K⁺ ATPase (NKA) as a cell-specific marker. Caspase 3/7 activity in MRC, assessed using LSC and microplate assays, increased significantly starting at 6 h of salinity stress and remained elevated for at least 5 days. This time-course of apoptosis in MRC during acute salinity stress was reflected in elevated apoptotic DNA fragmentation. In parallel to induction of apoptosis, MRC showed a pronounced shift to G2 phase of the cell cycle, which is indicative of G2/M cell cycle arrest, and an increase in NKA abundance per MRC. Unlike in MRC, apoptosis was not significantly increased in other gill cell types, although there was a small transient increase in DNA fragmentation at 6 h. G2 arrest was also observed. Overall, we interpret our data as evidence for a significant role of apoptosis in the extensive reorganization of MRC populations that takes place during salinity acclimation, perhaps similar to its well-established role during organismal development.     

Osmotic adjustment in cyanobacteria from hypersaline environments

The intracellular concentrations of the monovalent inorganic cations K⁺ and Na⁺, low molecular weight carbohydrates and quaternary ammonium compounds have been determined for 4 strains of cyanobacteria (Aphanothece halophytica, Coccochloris elabens, Dactylococcopsis salina and Synechocystis DUN52) originally isolated from hypersaline habitats (i.e. habitats with a salinity greater than that of seawater) over a range of external salt concentration (from 50% to 400% seawater). Intracellular cation levels (Na⁺ and K⁺) were determined to be within the range 80–320 mmol · dm^-3 (cell volume), showing only minor changes in response to salinity. Intracellular carbohydrates were found to comprise a negligible component of the intracellular osmotic potential [at 2–19 mmol · dm^-3 (cell volume)], throughout the salinity range. Quaternary ammonium compounds, however, were recorded in osmotically significant quantities [up to 1,640 mmol · dm^-3 (cell volume)] in these strains, showing major variation in response to salinity. Thus Synechocystis DUN 52 showed an increase in quaternary ammonium compounds in the oder of 1,200 mmol · dm^-3 between 50% and 400% seawater medium, accounting for a significant proportion of the change in external osmotic potential.Examination of intact cells and cell extracts using ¹³C and ¹H nuclear magnetic resonance (NMR) spectroscopy confirmed the presence of the quaternary ammonium compound glycine betaine as the major osmoticum in the 4 strains; no other compounds were detected during NMR assays. These results suggest a common mechanism of osmotic adjustment, involving quaternary ammonium compounds, in cyanobacteria from hypersaline environments.     

Concentration of MgSO4 in the intestinal lumen of Opsanus beta limits osmoregulation in response to acute hypersalinity stress

Marine teleosts constantly lose water to their surrounding environment, a problem exacerbated in fish exposed to salinity higher than normal seawater. Some fish undergo hypersaline exposures in their natural environments, such as short- and long-term increases in salinity occurring in small tidal pools and other isolated basins, lakes, or entire estuaries. Regardless of the degree of hypersalinity in the ambient water, intestinal absorption of monovalent ions drives water uptake to compensate for water loss, concentrating impermeable MgSO(4) in the lumen. This study considers the potential of luminal [MgSO(4)] to limit intestinal water absorption, and therefore osmoregulation, in hypersalinity. The overall tolerance and physiological response of toadfish (Opsanus beta) to hypersalinity exposure were examined. In vivo, fish in hypersaline waters containing artificially low [MgSO(4)] displayed significantly lower osmolality in both plasma and intestinal fluids, and increased survival at 85 parts per thousand, indicating improved osmoregulatory ability than in fish exposed to hypersalinity with ionic ratios similar to naturally occurring ratios. Intestinal sac preparations revealed that in addition to the osmotic pressure difference across the epithelium, the luminal ionic composition influenced the absorption of Na(+), Cl(-), and water. Hypersalinity exposure increased urine flow rates in fish fitted with ureteral catheters regardless of ionic composition of the ambient seawater, but it had no effect on urine osmolality or pH. Overall, concentrated MgSO(4) within the intestinal lumen, rather than renal or branchial factors, is the primary limitation for osmoregulation by toadfish in hypersaline environments.     

Trade-offs between ontogenetic changes and food consumption in Brazilian silverside Atherinella brasiliensis from two tropical estuaries

Salinity variation in estuarine environments influences the distribution of fish species as well as the availability of food resources to be used by them. This study examines the effect of the range of salinity on the trade-off between growth and feeding intensity of Atherinella brasiliensis from two tropical estuaries (positive and hypersaline). To investigate the effects of salinity, we hypothesized that hypersalinity negatively affects foraging intensity, consumption and prey selection by the Brazilian silverside, leading to differences in body condition. Sampling was carried out using the beach seine method in two areas of the estuaries (upper and lower zone) during rainy and dry periods. A total of 2549 stomachs (1124 for the positive estuary and 1425 for the hypersaline estuary) were examined, and the results indicated a dissimilarity of 92.7% of the diet between environments. In the positive estuary, there was more predation on Calanoida, Gastropoda, Hymenoptera, Ceratopogonidae larvae and Decapoda larvae, while Alga and plant-material characterized the diet in the hypersaline estuary. Significant correlations between the volume of food and salinity were observed in both estuaries. The vacuity index indicated that hypersaline environments presented higher contributions of semifull stomachs, indicating an intense consumption of algae. On the other hand,in the positive estuary, these values were less intense, but the stomachs were always with animal items. The variation found for both environments reinforces the effect of salinity on the physiological mechanism of the populations once the higher proportions of filled stomachs in the hypersaline environment indicate the need for constant and high ingestion of prey to guarantee the pronounced energy expenditure with osmoregulation.     

Morphological responses of mitochondria-rich cells to hypersaline environment in the Australian mudskipper, Periophthalmus minutus

A population of the Australian mudskipper, Periophthalmus minutus, was found to inhabit mudflat that remained uncovered by tide for more than 20 days in some neap tides. During these prolonged emersion periods, P. minutus retreated into burrows containing little water, with a highest recorded salinity of 84 ± 7.4 psu (practical salinity unit). To explore the mechanical basis for this salinity tolerance in P. minutus, we determined the densities of mitochondria-rich cells (MRCs) in the inner and outer opercula and the pectoral fin skin, in comparison with P. novaeguineaensis, from an adjacent lower intertidal habitat, and studied morphological responses of MRCs to exposure to freshwater (FW), and 100% (34-35 psu) and 200% seawater (SW). Periophthalmus minutus showed a higher density of MRCs in the inner operculum (3365 ± 821 cells mm(-2)) than in the pectoral fin skin (1428 ± 161) or the outer operculum (1100 ± 986), all of which were higher than the MRC densities in p. novaeguineaensis. No mortality occurred in 100% or 200% SW, but half of the fish died within four days in FW. Neither 200% SW nor FW exposure affected MRC density. Transfer to 200% SW doubled MRC size after 9-14 days with no change in the proportion of MRCs with apical pits or plasma sodium concentration. In contrast, transfer to FW resulted in a rapid closing of pits and a significant reduction in plasma sodium concentration. These results suggest that P. minutus has evolved morphological and physiological mechanisms to withstand hypersaline conditions that they may encounter in their habitat.     

Abundance, distribution, and activity of Fe(II)-oxidizing and Fe(III)-reducing microorganisms in hypersaline sediments of Lake Kasin, southern Russia

The extreme osmotic conditions prevailing in hypersaline environments result in decreasing metabolic diversity with increasing salinity. Various microbial metabolisms have been shown to occur even at high salinity, including photosynthesis as well as sulfate and nitrate reduction. However, information about anaerobic microbial iron metabolism in hypersaline environments is scarce. We studied the phylogenetic diversity, distribution, and metabolic activity of iron(II)-oxidizing and iron(III)-reducing Bacteria and Archaea in pH-neutral, iron-rich salt lake sediments (Lake Kasin, southern Russia; salinity, 348.6 g liter(-1)) using a combination of culture-dependent and -independent techniques. 16S rRNA gene clone libraries for Bacteria and Archaea revealed a microbial community composition typical for hypersaline sediments. Most-probable-number counts confirmed the presence of 4.26 × 10(2) to 8.32 × 10(3) iron(II)-oxidizing Bacteria and 4.16 × 10(2) to 2.13 × 10(3) iron(III)-reducing microorganisms per gram dry sediment. Microbial iron(III) reduction was detected in the presence of 5 M NaCl, extending the natural habitat boundaries for this important microbial process. Quantitative real-time PCR showed that 16S rRNA gene copy numbers of total Bacteria, total Archaea, and species dominating the iron(III)-reducing enrichment cultures (relatives of Halobaculum gomorrense, Desulfosporosinus lacus, and members of the Bacilli) were highest in an iron oxide-rich sediment layer. Combined with the presented geochemical and mineralogical data, our findings suggest the presence of an active microbial iron cycle at salt concentrations close to the solubility limit of NaCl.     

Salinity-dependent changes in Na+/K+-ATPase content of mitochondria-rich cells contribute to differences in thermal tolerance of Mozambique tilapia

The Mozambique tilapia (Oreochromis mossambicus) is prone to osmoregulatory disturbances when faced with fluctuating ambient temperatures. To investigate the underlying causes of this phenomenon, freshwater (FW)- and seawater (SW)-acclimated tilapia were transferred to 15, 25, or 35°C for 2 weeks, and along with typically used indicators of osmoregulatory status [plasma osmolality and branchial and intestinal specific Na⁺, K⁺-ATPase (NKA) activity], we used tissue microarrays (TMA) and laser-scanning cytometry (LSC) to characterize the effects of temperature acclimation. Tissue microarrays were stained with fluorescently labeled anti-Na⁺, K⁺-ATPase antibodies that allowed for the quantification of NKA abundance per unit area within individual branchial mitochondria-rich cells (MRCs) as well as sections of renal tissue. Mitochondria-rich cell counts and estimates of size were carried out for each treatment by the detection of DASPMI fluorescence. The combined analyses showed that SW fish have larger but fewer MRCs that contain more NKA per unit area. After a 2-week acclimation to 15°C tilapia experienced osmotic imbalances in both FW and SW that were likely due to low NKA activity. SW-acclimated fish compensated for the low activity by increasing MRC size and subsequently the concentration of NKA within MRCs. Although there were no signs of osmotic stress in FW-acclimated tilapia at 25°C, there was an increased NKA capacity that was most likely mediated by a higher MRC count. We conclude on the basis of the different responses to temperature acclimation that salinity-induced changes in the NKA concentration of MRCs alter thermal tolerance limits of tilapia.     

Morphological study of Cyclotella choctawhatcheeana Prasad (Stephanodiscaceae) from a saline Mexican lake

Background

Extracellular dissolved DNA has been demonstrated to be present in many terrestrial and aquatic environments, actively secreted, or released by decaying cells. Free DNA has the genetic potential to be acquired by living competent cells by horizontal gene transfer mediated by natural transformation. The aim of this work is to study the persistence of extracellular DNA and its biological transforming activity in extreme environments like the deep hypersaline anoxic lakes of the Mediterranean Sea. The brine lakes are separated from the upper seawater by a steep chemocline inhabited by stratified prokaryotic networks, where cells sinking through the depth profile encounter increasing salinity values and osmotic stress.

Results

Seven strains belonging to different taxonomic groups isolated from the seawater-brine interface of four hypersaline lakes were grown at medium salinity and then incubated in the brines. The osmotic stress induced the death of all the inoculated cells in variable time periods, between 2 hours and 144 days, depending on the type of brine rather than the taxonomic group of the strains, i.e. Bacillaceae or gamma-proteobacteria. The Discovery lake confirmed to be the most aggressive environment toward living cells. In all the brines and in deep seawater dissolved plasmid DNA was substantially preserved for a period of 32 days in axenic conditions. L'Atalante and Bannock brines induced a decrease of the supercoiled form up to 70 and 40% respectively; in the other brines only minor changes in plasmid conformation were observed. Plasmid DNA after incubation in the brines maintained the capacity to transform naturally competent cells of Acinetobacter baylii strain BD413.

Conclusion

Free dissolved DNA is likely to be released by the lysis of cells induced by osmotic stress in the deep hypersaline anoxic lakes. Naked DNA was demonstrated to be preserved and biologically active in these extreme environments, and hence could constitute a genetic reservoir of traits acquirable by horizontal gene transfer.     

高盐环境可培养细菌噬菌体的研究进展

病毒广泛存在于各类环境中并担负着重要的生态功能,其中包括高盐环境。对高盐环境病毒的研究已成为极端环境微生物研究领域的新热点,目前已被报道的100多株盐病毒中,90多株感染古菌,仅有14株感染细菌。本文综述了目前已知的14株高盐环境细菌噬菌体的形态特征、盐度响应及基因组学的研究进展,并分析了高盐噬菌体的形态多样性、生存策略以及包含在基因组中的进化和起源信息,分析结果表明:高盐噬菌体以有尾噬菌体为主;它们具有广盐性(Euryhaline)的特征,盐度极大地影响其吸附和增殖;它们与非高盐环境噬菌体可能具有共同的起源。高盐噬菌体虽然历经近30年的研究历程,但仅有14株被分离与培养,所以其分离纯化是今后重要工作之一,且结合免培养技术揭示高盐噬菌体的多样性与生态功能是其研究的发展方向。     

急性盐度胁迫对军曹鱼稚鱼渗透压调节的影响

研究了环境盐度急性胁迫对军曹鱼(Rachycentron canadum)稚鱼鳃Na⁺-K⁺ATPase(NKA)活性及血清渗透压、Na⁺、K⁺和Cl^-离子调节的影响.结果表明:将稚鱼从盐度37中直接转移至盐度0、5、15、25、37（对照）和45的水体中,12 h后仅盐度0处理出现死亡(死亡率100％).各处理鳃NKA活性和血清渗透压在最初3 h内出现一定波动,随后变化平稳.试验结束时(12 h), NKA活性与盐度梯度呈“U”型分布,盐度5处理酶活性显著高于其它处理(P<0.05),盐度15处理活性最低,而各处理的血清渗透压大小(293～399 mOsmol·kg^-1)与盐度呈正相关;在3～12 h内稚鱼血清Na⁺和Cl^-浓度随盐度升高而升高,但增幅较小,血清K⁺浓度则与盐度呈负相关;12 h稚鱼的等渗点为328.2 mOsm·kg^-1,相当于盐度11.48,而Na⁺、K⁺和Cl^-等离子点分别为155.2、6.16和137.1 mmol·L^-1,分别相当于盐度10.68、20.44及8.41.军曹鱼在生理上具有广盐性鱼类的“低渗环境高NKA活性”特征,有较强及迅速的渗透压和离子调节与平衡能力.     

Freshening effect on the osmotic response of the Antarctic spiny plunderfish Harpagifer antarcticus

Global warming is having a significant impact around the world, modifying environmental conditions in many areas, including in zones that have been thermally stable for thousands of years, such as Antarctica. Stenothermal sedentary intertidal fish species may suffer due to warming, notably if this causes water freshening from increased freshwater inputs. Acute decreases in salinity, from 33 down to 5, were used to assess osmotic responses to environmental salinity fluctuations in Antarctic spiny plunderfish Harpagifer antarcticus, in particular to evaluate if H. antarcticus is able to cope with freshening and to describe osmoregulatory responses at different levels (haematological variables, muscle water content, gene expression, NKA activity). H. antarcticus were acclimated to a range of salinities (33 as control, 20, 15, 10 and 5) for 1 week. At 5, plasma osmolality and calcium concentration were both at their lowest, while plasma cortisol and percentage muscle water content were at their highest. At the same salinity, gill and intestine Na⁺-K⁺-ATPase (NKA) activities were at their lowest and highest, respectively. In kidney, NKA activity was highest at intermediate salinities (15 and 10). The salinity-dependent NKA mRNA expression patterns differed depending on the tissue. Marked changes were also observed in the expression of genes coding membrane proteins associated with ion and water transport, such as NKCC2, CFTR and AQP8, and in the expression of mRNA for the regulatory hormone prolactin (PRL) and its receptor (PRLr). Our results demonstrate that freshening causes osmotic imbalances in H. antarcticus, apparently due to reduced capacity of both transport and regulatory mechanisms of key organs to maintain homeostasis. This has implications for fish species that have evolved in stable environmental conditions in the Antarctic, now threatened by climate change.     

Ancient saltern metagenomics: tracking changes in microbes and their viruses from the underground to the surface

Microbial communities in hypersaline underground waters derive from ancient organisms trapped within the evaporitic salt crystals and are part of the poorly known subterranean biosphere. Here, we characterized the viral and prokaryotic assemblages present in the hypersaline springs that dissolve Triassic-Keuper evaporite rocks and feed the Añana Salt Valley (Araba/Alava, Basque Country, Spain). Four underground water samples (around 23% total salinity) with different levels of exposure to the open air were analysed by means of microscopy and metagenomics. Cells and viruses in the spring water had lower concentrations than what are normally found in hypersaline environments and seemed to be mostly inactive. Upon exposure to the open air, there was an increase in activity of both cells and viruses as well as a selection of phylotypes. The underground water was inhabited by a rich community harbouring a diverse set of genes coding for retinal binding proteins. A total of 35 viral contigs from 15 to 104 kb, representing partial or total viral genomes, were assembled and their evolutionary changes through the spring system were followed by SNP analysis and metagenomic island tracking. Overall, both the viral and the prokaryotic assemblages changed quickly upon exposure to the open air conditions.     

The effect of elevated salinity on 'California' Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) metabolism

California Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) are extremely saline tolerant and have been previously shown to reduce whole-animal oxygen consumption rate (MO(2)) upon exposures to salinities greater than that of seawater (SW). In this study tilapia were acclimated to 15, 30, 45, 60 and 75 g/L salinity for 1, 5, 14, or 28 days. There was little change in plasma osmolality or muscle water content in salinities below 60 g/L, and branchial Na(+), K(+)-ATPase (NKA) activity was low in 15 and 30 g/L relative to 60 and 75 g/L. When tilapia were exposed to 75 g/L, plasma osmolality and NKA activity were significantly increased within 5 days of exposure relative to those in 15 and 30 g/L, and remained elevated over the entire 28 days acclimation, indicating that short term salinity challenges (i.e., 5 days) are predictive of longer exposure durations in this species. MO(2) following transfer to 15 and 30 g/L was elevated, reflecting the high energy demand required for switching from a hyper- to a hypo-osmoregulatory strategy. The MO(2) of 60 g/L-exposed fish was significantly reduced at 1, 5, and 14 days, relative to 30 g/L-exposed fish; however by 28 days there were no significant differences. We investigated the potential for a metabolic basis for the salinity-induced MO(2) reduction, using forward stepwise linear regression to correlate enzyme activities of brain, liver, and kidney with MO(2). Brain NKA was correlated with MO(2) after 5 days (p<0.01, r(2)=0.944) and both brain NKA and hepatic total ATPase were correlated with the reduced MO(2) at 14 days (p=0.027, r(2)=0.980 and p=0.025, r(2)=0.780, respectively). These results may indicate a tissue-level metabolic suppression, which has not been previously described as a response to hypersaline exposure in fishes.     

Morphological and physiological traits of Mediterranean sticklebacks living in the Camargue wetland (Rhone river delta)

Three-spined sticklebacks (Gasterosteus aculeatus L.) living at the southern limit of the species distribution range could possess specific morphological and physiological traits that enable these fish to live at the threshold of their physiological capacities. Morphological analysis was carried out on samples of sticklebacks living in different saline habitats of the Camargue area (Rhone delta, northern Mediterranean coast) obtained from 1993 to 2017. Salinity acclimation capacities were also investigated using individuals from freshwater-low salinity drainage canals and from mesohaline–euryhaline lagoons. Fish were maintained in laboratory conditions at salinity values close to those of their respective habitats: low salinity (LS, 5‰) or seawater (SW, 30‰). Fish obtained from a mesohaline brackish water lagoon (BW, 15‰) were acclimated to SW or LS. Oxygen consumption rates and branchial Na⁺/K⁺-ATPase (NKA) activity (indicator of fish osmoregulatory capacity) were measured in these LS or SW control fish and in individuals subjected to abrupt SW or LS transfers. At all the studied locations, only the low-plated “leiurus” morphotype showed no spatial or temporal variations in their body morphology. Gill rakers were only longer and denser in fish sampled from the LS–freshwater (FW) drainage canals. All fish presented similar physiological capacities. Oxygen consumption rates were not influenced by salinity challenge except in SW fish transferred to LS immediately and 1 h after transfer. However, and as expected, gill NKA activity was salinity dependent. Sticklebacks of the Camargue area sampled from habitats with contrasted saline conditions are homogenously euryhaline, have low oxygen consumption rates and do not appear to experience significantly greater metabolic costs when challenged with salinity. However, an observed difference in gill raker length and density is most probably related to the nutritional condition of their habitat, indicating that individuals can rapidly acclimatize to different diets.