Expression and distribution of Na, K-ATPase in gill and kidney of the spotted green pufferfish, Tetraodon nigroviridis, in response to salinity challenge

Freshwater (FW) spotted green pufferfish (Tetraodon nigroviridis) were transferred directly from a local aquarium to fresh water (FW; 0 per thousand ), brackish water (BW; 15 per thousand ), and seawater (SW; 35 per thousand ) conditions in the laboratory and reared for at least two weeks. No mortality was found. To investigate the efficient mechanisms of osmoregulation in the euryhaline teleost, distribution and expression of Na,K-ATPase (NKA) in gill and kidney of the pufferfish were examined and the osmolality, [Na+] and [Cl-] of the blood were assayed. The lowest levels of both relative protein abundance and activity were found to be exhibited in the BW group, and higher levels in the SW group than FW group. In all salinities, branchial NKA immunoreactivity was found in epithelial cells of the interlamellar region of the filament and not on the lamellae. Relative abundance of kidney NKA alpha-subunit, as well as the NKA activity, was found to be higher in the FW pufferfish than fish in BW or SW. Renal NKA appeared in the epithelial cells of distal tubules, proximal tubules, and collecting tubules, but not in glomeruli, in fish groups of various salinities. Plasma osmolality and chloride levels were significantly lower in FW pufferfish than those in BW and SW, whereas plasma sodium did not differ among the groups. Although identical distributions of NKA were found in either gill or kidney of FW-, BW- or SW-acclimated spotted green pufferfish, differential NKA expression in fish of various salinity groups was associated with physiological homeostasis (stable blood osmolality), and illustrated the impressive osmoregulatory ability of this freshwater and estuarine species in response to salinity challenge.     

The expression of gill Na,K-ATPase in milkfish,Chanos chanos,acclimated to seawater,brackish water and fresh water

Juvenile milkfish Chanos chanos (Forssk?l, 1775) were transferred from a local fish farm to fresh water (FW; 0 per thousand ), brackish water (BW; 10 per thousand, 20 per thousand ) and seawater (SW; 35 per thousand ) conditions in the laboratory and reared for at least two weeks. The blood and gill of the fish adapted to various salinities were analyzed to determine the osmoregulatory ability of this euryhaline species. No significant difference was found in plasma osmolality, sodium or chloride concentrations of milkfish adapted to various salinities. In FW, the fish exhibited the highest specific activity of Na, K-ATPase (NKA) in gills, while the SW group was found to have the lowest. Relative abundance of branchial NKA alpha-subunit revealed similar profiles. However, in contrary to other euryhaline teleosts, i.e. tilapia, salmon and eel, the naturally SW-dwelling milkfish expresses higher activity of NKA in BW and FW. Immunocytochemical staining has shown that most Na, K-ATPase immunoreactive (NKIR) cells in fish adapted to BW and SW were localized to the filaments with very few on the lamellae. Moreover, in FW-adapted milkfish, the number of NKIR cells found on the lamellae increased significantly. Such responses as elevated NKIR cell number and NKA activity are thought to improve the osmoregulatory capacity of the milkfish in hyposaline environments.     

Ontogeny of salinity tolerance and evidence for seawater-entry preparation in juvenile green sturgeon, <Emphasis Type="Italic">Acipenser medirostris</Emphasis>

We measured the ontogeny of salinity tolerance and the preparatory hypo-osmoregulatory physiological changes for seawater entry in green sturgeon (Acipenser medirostris), an anadromous species occurring along the Pacific Coast of North America. Salinity tolerance was measured every 2 weeks starting in 40-day post-hatch (dph) juveniles and was repeated until 100% survival at 34‰ was achieved. Fish were subjected to step increases in salinity (5‰ 12 h⁻¹) that culminated in a 72-h exposure to a target salinity, and treatment groups (0, 15, 20, 25, 30, 34‰; and abrupt exposure to 34‰) were adjusted as fish developed. After 100% survival was achieved (134 dph), a second experiment tested two sizes of fish for 28-day seawater (33‰) tolerance, and gill and gastrointestinal tract tissues were sampled. Their salinity tolerance increased and plasma osmolality decreased with increasing size and age, and electron microscopy revealed three types of mitochondria-rich cells: one in fresh water and two in seawater. In addition, fish held on a natural photoperiod in fresh water at 19°C showed peaks in cortisol, thyroid hormones and gill and pyloric ceca Na⁺, K⁺-ATPase activities at body sizes associated with seawater tolerance. Therefore, salinity tolerance in green sturgeon increases during ontogeny (e.g., as these juveniles may move down estuaries to the ocean) with increases in body size. Also, physiological and morphological changes associated with seawater readiness increased in freshwater-reared juveniles and peaked at their seawater-tolerant ages and body sizes. Their seawater-ready body size also matched that described for swimming performance decreases, presumably associated with downstream movements. Therefore, juvenile green sturgeon develop structures and physiological changes appropriate for seawater entry while growing in fresh water, indicating that hypo-osmoregulatory changes may proceed by multiple routes in sturgeons.     

Elevated Na+/K+-ATPase responses and its potential role in triggering ion reabsorption in kidneys for homeostasis of marine euryhaline milkfish (Chanos chanos) when acclimated to hypotonic fresh water

The milkfish (Chanos chanos) is an economic species in Southeast Asia. In Taiwan, the milkfish are commercially cultured in environments of various salinities. Na⁺/K⁺-ATPase (NKA) is a key enzyme for fish iono- and osmoregulation. When compared with gills, NKA and its potential role were less examined by different approaches in the other osmoregulatory organs (e.g., kidney) of euryhaline teleosts. The objective of this study was to investigate the correlation between osmoregulatory plasticity and renal NKA in this euryhaline species. Muscle water contents (MWC), plasma, and urine osmolality, kidney histology, as well as distribution, expression (mRNA and protein), and specific activity of renal NKA were examined in juvenile milkfish acclimated to fresh water (FW), seawater (SW 35‰), and hypersaline water (HSW 60‰) for at least two weeks before experiments. MWC showed no significant difference among all groups. Plasma osmolality was maintained within the range of physiological homeostasis in milkfish acclimated to different salinities, while, urine osmolality of FW-acclimated fish was evidently lower than SW- and HSW-acclimated individuals. The renal tubules were identified by staining with periodic acid Schiff’s reagent and hematoxylin. Moreover, immunohistochemical staining showed that NKA was distributed in the epithelial cells of proximal tubules, distal tubules, and collecting tubules, but not in glomeruli, of milkfish exposed to different ambient salinities. The highest abundance of relative NKA α subunit mRNA was found in FW-acclimated milkfish rather than SW- and HSW-acclimated individuals. Furthermore, relative protein amounts of renal NKA α and β subunits as well as NKA-specific activity were also found to be higher in the FW group than SW and the HSW groups. This study integrated diverse levels (i.e., histological distribution, gene, protein, and specific activity) of renal NKA expression and illustrated the potential role of NKA in triggering ion reabsorption in kidneys of the marine euryhaline milkfish when acclimated to a hypotonic FW environment.     

Mechanisms of seawater acclimation in a primitive,anadromous fish,the green sturgeon

Relatively little is known about salinity acclimation in the primitive groups of fishes. To test whether physiological preparative changes occur and to investigate the mechanisms of salinity acclimation, anadromous green sturgeon, Acipenser medirostris (Chondrostei) of three different ages (100, 170, and 533 dph) were acclimated for 7 weeks to three different salinities (<3, 10, and 33 ppt). Gill, kidney, pyloric caeca, and spiral intestine tissues were assayed for Na⁺, K⁺-ATPase activity; and gills were analyzed for mitochondria-rich cell (MRC) size, abundance, localization and Na⁺, K⁺-ATPase content. Kidneys were analyzed for Na⁺, K⁺-ATPase localization and the gastro-intestinal tract (GIT) was assessed for changes in ion and base content. Na⁺, K⁺-ATPase activities increased in the gills and decreased in the kidneys with increasing salinity. Gill MRCs increased in size and decreased in relative abundance with fish size/age. Gill MRC Na⁺, K⁺-ATPase content (e.g., ion-pumping capacity) was proportional to MRC size, indicating greater abilities to regulate ions with size/age. Developmental/ontogenetic changes were seen in the rapid increases in gill MRC size and lamellar length between 100 and 170 dph. Na⁺, K⁺-ATPase activities increased fourfold in the pyloric caeca in 33 ppt, presumably due to increased salt and water absorption as indicated by GIT fluids, solids, and ion concentrations. In contrast to teleosts, a greater proportion of base (HCO₃ ⁻ and 2CO₃ ²⁻) was found in intestinal precipitates than fluids. Green sturgeon osmo- and ionoregulate with similar mechanisms to more-derived teleosts, indicating the importance of these mechanisms during the evolution of fishes, although salinity acclimation may be more dependent on body size.     

Salinity effects on oxygen consumption, gill Na+, K+-ATPase and ion regulation in juvenile coho salmon

The metabolic response of juvenile coho salmon Oncorhynchus kisutch to different salinities was examined, using whole-animal oxygen consumption rates and gill Na⁺, K⁺-ATPase activities as indicators of osmoregulatory energetics. Coho salmon smolts were acclimated to fresh water (FW), isosmotic salinity (ISO, 10‰) and sea water (SW, 28‰) and were sampled for up to 6 weeks for plasma levels of cortisol, glucose and ions (Na⁺, K⁺, Cl⁻), gill Na⁺, K⁺-ATPase activity and oxygen consumption rates. Following an initial adjustment period, plasma constituents in SW fish returned to near-FW values, indicating that the fish were acclimated to SW by day 21. Gill Na⁺, K⁺-ATPase activities on days 21 and 42 were lowest in ISO, higher in FW and highest in SW. This result is consistent with the idea that less energy would be required to maintain ion balance in an isosmotic environment, where the ionic gradients between extracellular fluid and water would be minimal. Oxygen consumption rates of swimming fish (1 body length s⁻¹), however, did not differ significantly between the three test salinities after 6 weeks. The results of this study suggest that the metabolic response of juvenile salmonids to changes in salinity is dependent on life-history stage (e.g. fry v . smolt), and that oxygen consumption rates do not necessarily reflect osmoregulatory costs.     

The ClC-3 chloride channel and osmoregulation in the European Sea Bass, Dicentrarchus labrax

Dicentrarchus labrax migrates between sea (SW), brackish and fresh water (FW) where chloride concentrations and requirements for chloride handling change: in FW, fish absorb chloride and restrict renal losses; in SW, they excrete chloride. In this study, the expression and localization of ClC-3 and Na⁺/K⁺-ATPase (NKA) were studied in fish adapted to SW, or exposed to FW from 10 min to 30 days. In gills, NKA-α1 subunit expression transiently increased from 10 min and reached a stabilized intermediate expression level after 24 h in FW. ClC-3 co-localized with NKA in the basolateral membrane of mitochondria-rich cells (MRCs) at all conditions. The intensity of MRC ClC-3 immunostaining was significantly higher (by 50 %) 1 h after the transfer to FW, whereas the branchial ClC-3 protein expression was 30 % higher 7 days after the transfer as compared to SW. This is consistent with the increased number of immunopositive MRCs (immunostained for NKA and ClC-3). However, the ClC-3 mRNA expression was significantly lower in FW gills. In the kidney, after FW transfer, a transient decrease in NKA-α1 subunit expression was followed by significantly higher stable levels from 24 h. The low ClC-3 protein expression detected at both salinities was not observed by immunocytochemistry in the SW kidney; ClC-3 was localized in the basal membrane of the collecting ducts and tubules 7 and 30 days after transfer to FW. Renal ClC-3 mRNA expression, however, seemed higher in SW than in FW. The potential role of this chloride channel ClC-3 in osmoregulatory and osmosensing mechanisms is discussed.     

Physiological preparedness and performance of Atlantic salmon Salmo salar smolts in relation to behavioural salinity preferences and thresholds

This study investigated the relationships between behavioural responses of Atlantic salmon Salmo salar smolts to saltwater (SW) exposure and physiological characteristics of smolts in laboratory experiments. It concurrently described the behaviour of acoustically tagged smolts with respect to SW and tidal cycles during estuary migration. Salmo salar smolts increased their use of SW relative to fresh water (FW) from April to June in laboratory experiments. Mean preference for SW never exceeded 50% of time in any group. Preference for SW increased throughout the course of smolt development. Maximum continuous time spent in SW was positively related to gill Na⁺, K⁺‐ATPase (NKA) activity and osmoregulatory performance in full‐strength SW (measured as change in gill NKA activity and plasma osmolality). Smolts decreased depth upon reaching areas of the Penobscot Estuary where SW was present, and all fish became more surface oriented during passage from head of tide to the ocean. Acoustically tagged, migrating smolts with low gill NKA activity moved faster in FW reaches of the estuary than those with higher gill NKA activity. There was no difference in movement rate through SW reaches of the estuary based on gill NKA activity. Migrating fish moved with tidal flow during the passage of the lower estuary based on the observed patterns in both vertical and horizontal movements. The results indicate that smolts select low‐salinity water during estuary migration and use tidal currents to minimize energetic investment in seaward migration. Seasonal changes in osmoregulatory ability highlight the importance of the timing of stocking and estuary arrival.     

Differential expression of branchial Na+/K(+)-ATPase of two medaka species, Oryzias latipes and Oryzias dancena, with different salinity tolerances acclimated to fresh water, brackish water and seawater

Previous studies on non-diadromous euryhaline teleosts introduced a hypothesis that the lowest level of gill Na⁺/K⁺-ATPase (NKA) activity occurs in the environments with salinity close to the primary natural habitats of the studied species. To provide more evidence of the hypothesis, two medaka species, Oryzias latipes and O. dancena, whose primary natural habitats are fresh water (FW) and brackish water (BW) environments, respectively, were compared from levels of mRNA to cells in this study. The plasma osmolalities of O. latipes and O. dancena were lowest in the FW individuals. The muscle water contents of O. latipes decreased with elevated external salinities, but were constant among FW-, BW-, and seawater (SW)-acclimated O. dancena. Expression of NKA, the primary driving force of ion transporters in gill ionocytes, revealed different patterns in the two Oryzias species. The highest NKA α-subunit mRNA abundances were found in the gills of the SW O. latipes and the FW O. dancena, respectively. The pattern of NKA activity and α-subunit protein abundance in the gills of O. latipes revealed that the FW group was the lowest, while the pattern in O. dancena revealed that the BW group was the lowest. Immunohistochemical staining showed similar profiles of NKA immunoreactive (NKIR) cell activities (NKIR cell number × cell size) in the gills of these two species among FW, BW, and SW groups. Taken together, O. latipes exhibited better hyposmoregulatory ability, while O. dancena exhibited better hyperosmoregulatory ability. Our results corresponding to the hypothesis indicated that the lowest branchial NKA activities of these two medaka species were found in the environments with salinities similar to their natural habitats.     

Natural growth and diet of known‐age pallid sturgeon (Scaphirhynchus albus) early life stages in the upper Missouri River basin,Montana and North Dakota

Prior to anthropogenic modifications, the historic Missouri River provided ecological conditions suitable for reproduction, growth, and survival of pallid sturgeon Scaphirhynchus albus. However, little information is available to discern whether altered conditions in the contemporary Missouri River are suitable for feeding, growth and survival of endangered pallid sturgeon during the early life stages. In 2004 and 2007, nearly 600 000 pallid sturgeon free embryos and larvae were released in the upper Missouri River and survivors from these releases were collected during 2004–2010 to quantify natural growth rates and diet composition. Based on genetic analysis and known‐age at release (1–17 days post‐hatch, dph), age at capture (dph, years) could be determined for each survivor. Totals of 23 and 28 survivors from the 2004 and 2007 releases, respectively, were sampled. Growth of pallid sturgeon was rapid (1.91 mm day^?1) during the initial 13–48 dph, then slowed as fish approached maximum length (120–140 mm) towards the end of the first growing season. The diet of young‐of‐year pallid sturgeon was comprised of Diptera larvae, Diptera pupae, and Ephemeroptera nymphs. Growth of pallid sturgeon from ages 1–6 years was about 48.0 mm year^?1. This study provides the first assessment of natural growth and diet of young pallid sturgeon in the wild. Results depict pallid sturgeon growth trajectories that may be expected for naturally produced wild stocks under contemporary habitat conditions in the Missouri River and Yellowstone River.     

Acute changes in gill Na+-K+-ATPase and creatine kinase in response to salinity changes in the euryhaline teleost, tilapia (Oreochromis mossambicus)

Some freshwater (FW) teleosts are capable of acclimating to seawater (SW) when challenged; however, the related energetic and physiological consequences are still unclear. This study was conducted to examine the changes in expression of gill Na(+)-K(+)-ATPase and creatine kinase (CK) in tilapia (Oreochromis mossambicus) as the acute responses to transfer from FW to SW. After 24 h in 25 ppt SW, gill Na(+)-K(+)-ATPase activities were higher than those of fish in FW. Fish in 35 ppt SW did not increase gill Na(+)-K(+)-ATPase activities until 1.5 h after transfer, and then the activities were not significantly different from those of fish in 25 ppt SW. Compared to FW, the gill CK activities in 35 ppt SW declined within 1.5 h and afterward dramatically elevated at 2 h, as in 25 ppt SW, but the levels in 35 ppt SW were lower than those in 25 ppt SW. The Western blot of muscle-type CK (MM form) was in high association with the salinity change, showing a pattern of changes similar to that in CK activity; however, levels in 35 ppt SW were higher than those in 25 ppt SW. The activity of Na(+)-K(+)-ATPase highly correlated with that of CK in fish gill after transfer from FW to SW, suggesting that phosphocreatine acts as an energy source to meet the osmoregulatory demand during acute transfer.     

The influence of salinity on ionic concentrations and osmolarity of blood serum in lake sturgeon, Acipenser fulvescens

This study examined the extent to which lake sturgeon, Acipenser fulvescens, regulated serum ion concentrations and osmolarity in relation to ambient salinity. Twenty juvenile lake sturgeon, brood stock from the Moose River in the Hudson Bay drainage basin, were gradually transferred from freshwater to more saline waters at a rate of 2 day^-1. Blood was sampled from fish at 0, 10, 15 and 25 following 48 h acclimation periods. Concentrations of Na⁺, K⁺, Cl^-, Ca²⁺ and osmolarity were measured. Experimentation was terminated at 25 due to poor fish health resulting in two mortalities. Within freshwater, lake sturgeon blood serum displays similar osmotic and ionic composition to other North American species of sturgeon. Juvenile lake sturgeon do not appear capable of homeostasis above 15 nor are they tolerant to environments exceeding 25. Some evidence of greater tolerance to elevated salinities with increased size was noted. The ecological potential for resource exploitation in regions of elevated salinities is discussed.     

Ontogeny of osmoregulatory patterns in the South American shrimp Macrobrachium amazonicum: Loss of hypo-regulation in a land-locked population indicates phylogenetic separation from estuarine ancestors

Ontogenetic changes in osmoregulation were compared between two geographically separate populations of a South American shrimp, Macrobrachium amazonicum, originating from the Amazon delta (A) and the Pantanal (P), respectively. Population A lives in coastal rivers and estuaries in northern Brazil, whereas population P may be considered as land-locked, spending its entire life cycle in inland freshwater (FW) habitats in southwestern Brazil. All life-history stages of population A tolerated brackish and seawater (SW) conditions, being hyper-osmoregulators at salinities < 17, iso-osmotic at ca. 17, and hypo-regulators at higher concentrations. The capabilities to survive and osmoregulate in FW were in this population expressed already at hatching (zoea I), but not any longer in the subsequent larval stages (II-IX), which showed complete mortality during an experimental 24 h exposure to fully limnic conditions. FW tolerance re-appeared only in the juvenile and adult life-history stages. Similarly, the ability to hyper-regulate at salinities 1-5 was strong in the zoea I, weaker in the subsequent larval stages, and increasing again after metamorphosis. The function of hypo-regulation in concentrated media including SW was present throughout ontogeny, particularly in late larval and early juvenile stages. These ontogenetic patterns of osmoregulation and FW tolerance are congruent with a diadromous life cycle, which includes larval release in FW and a subsequent downstream transport of the salt-dependent early larvae towards estuarine or coastal marine waters, where development to metamorphosis is possible. The FW-tolerant juveniles can later migrate upstream, recruiting to riverine populations. In the land-locked population P, all life-history stages tolerated FW and brackish conditions up to salinity 25, but mortality was high in SW (100% in adults). All postembryonic stages of this population were hyper-osmoregulators at salinities < 17, with a strong osmoregulatory capacity in FW. Unlike in population A, all stages were osmoconformers at higher salinities, lacking the function of hypo-regulation. In summary, our results show in two hydrologically and genetically isolated shrimp populations close relationships between differential patterns of ontogenetic change in osmoregulatory functions, salinity tolerance, and the ecology of successive life-history stages. In all postembryonic stages of the hololimnetic Pantanal population, the acquisition of an increased ability to hyper-osmoregulate in FW and, in particular, the complete loss of the ability to hypo-osmoregulate at high salt concentrations represent striking differences to the diadromous population from the Amazon estuary. These differences reflect different life styles and reproductive strategies, suggesting an at least incipient phylogenetic separation.     

Lipid and fatty acid variations in muscle tissues of the ‘yellow’ stage of the European eel (Anguilla anguilla) during short-term adaptation to freshwater and seawater under food deprivation

Changes in body mass and water content (WC), lipids, and fatty acid (FA) composition of muscle were determined in European eels (Anguilla anguilla) in the ‘yellow’ stage (sexually immature). The animals were held in either freshwater (FW) or seawater (SW) and deprived of food for four weeks. During this period, body mass decreased steadily. This decrease was accompanied by reduced muscle WC and total lipid in both SW and FW. ‘Yellow’ eels appeared to use lipids in muscle to provide metabolic and osmoregulatory energy. However, they maintained relative constancy in the FA profiles, so that total FA, saturated, monounsaturated, and polyunsaturated FA, were similar between salinities during short-term starvation.     

The acute temperature tolerance of green sturgeon (Acipenser medirostris) and the effect of environmental salinity

We investigated the effect of environmental salinity on the upper thermal tolerance of green sturgeon (Acipenser medirostris), a threatened species whose natural habitat is vulnerable to temperature and salinity variation as a result of global climate change. Freshwater (FW)-reared sturgeon were gradually acclimated to salinities representing FW, estuary water (EST), or San Francisco Bay water (BAY) at 18 degrees C, and their critical thermal maximum (CTMax) was measured by increasing temperature 0.3 degrees C/min until branchial ventilation ceased. CTMax was 34.2+/-0.09 degrees C in EST-acclimated fish, with FW- and BAY-acclimated fish CTMax at 33.7+/-0.08 and 33.7+/-0.1 degrees C, respectively. Despite the higher CTMax in EST-acclimated fish, FW-acclimated sturgeon ventilation rate reached a peak that was 2 degrees C higher than EST- and BAY-acclimated groups and had a greater range of temperatures within which they exhibited normal ventilatory function as assessed by Q(10) calculation. The osmoregulatory consequences of exposure to near-lethal temperatures were assessed by measuring plasma osmolality and hematocrit, as well as white muscle, brain, and heart tissue water contents. Hematocrit was increased following CTMax exposure, most likely owing to the elevated metabolic demands of temperature increase, and plasma osmolality was significantly increased in EST- and BAY-acclimated fish, which was likely the result of a greater osmotic gradient across the gill as metabolism increased. To our knowledge, this represents the first evidence for an effect of salinity on the upper thermal tolerance of sturgeon, as well as the first investigation of the osmoregulatory consequences of exposure to near-lethal temperatures. J. Exp. Zool. 309A:477-483, 2008. (c) 2008 Wiley-Liss, Inc.     

Plasma and erythrocyte solute properties of juvenile bull sharks, Carcharhinus leucas, acutely exposed to increasing environmental salinity

Plasma and erythrocyte solute properties were examined in freshwater (FW) acclimated juvenile Carcharhinus leucas following acute transfer to 75% seawater (SW), and 100% SW. Blood samples were taken at 0, 12 and 96 h following transfer to 75% SW and 24 and 72 h after transfer to 100% SW. A control group in FW was subjected to the same sampling regime. Upon transfer of C. leucas to 75% and 100% SW, plasma Na⁺, Cl⁻, K⁺, Mg²⁺, Ca²⁺, urea and TMAO concentrations all increased significantly but disproportionately. Plasma Na⁺ and Cl⁻ increased immediately, followed by an increase in plasma urea. Erythrocyte urea and TMAO concentrations increased significantly following transfer to 75% and 100% SW; however, changes in erythrocyte inorganic ion concentrations were insignificant. Haematocrit, haemoglobin and mean cell haematocrit did not differ significantly after transfer to seawater; however, plasma water was slightly reduced after 24 and 72 h in 100% SW. Red blood cell (RBC) water content was elevated 24 h after transfer to 100% SW but returned to FW levels after 72 h. These results demonstrate that the transfer from 75% to 100% SW presents C. leucas with a greater osmoregulatory challenge than transfer from FW to 75% SW, despite the larger concentration gradient in the latter. In summary, C. leucas tolerate rapid and significant increases in salinity by rapidly increasing plasma osmolality to be hyperosmotic to the environment whilst maintaining a tight regulation of their intracellular fluid environment.     

Osmoregulatory and metabolic costs of salt excretion in the Rufous-collared sparrow Zonotrichia capensis

Recent experiments on shorebirds have demonstrated that maintaining an active osmoregulatory machinery is energetically expensive. This may, in part, explain diet and habitat selection in birds with salt glands. However little is known about the osmoregulatory costs in birds lacking functional salt glands. In these birds, osmotic work is done almost exclusively by the kidneys. We investigated the osmoregulatory cost in a bird species lacking functional salt glands, the passerine Zonotrichia capensis. After 20 days of acclimation to fresh water (FW) and salt water (200 mM NaCl, SW), SW birds tended to be heavier than FW birds. However, this difference was not statistically significant. Total basal metabolic rate was higher in SW birds as compared with FW birds. Renal and heart masses were also higher in the SW group. We also found greater medullary development and an increase in urine osmolality in the SW group. In spite of Z. capensis' ability to tolerate a moderate salt load in the laboratory, we hypothesize that increased cost of maintenance produced by salt consumption may significantly affect energy budget, dietary, and habitat choices in the field.     

Divergence in physiological factors affecting swimming performance between anadromous and resident populations of brook charr Salvelinus fontinalis

In this study, an anadromous strain (L) and a freshwater‐resident (R) strain of brook charr Salvelinus fontinalis as well as their reciprocal hybrids, were reared in a common environment and submitted to swimming tests combined with salinity challenges. The critical swimming speeds (U_crit) of the different crosses were measured in both fresh (FW) and salt water (SW) and the variations in several physiological traits (osmotic, energetic and metabolic capacities) that are predicted to influence swimming performance were documented. Anadromous and resident fish reached the same U_crit in both FW and SW, with U_crit being 14% lower in SW compared with FW. The strains, however, seemed to use different underlying strategies: the anadromous strain relied on its streamlined body shape and higher osmoregulatory capacity, while the resident strain had greater citrate synthase (FW) and lactate dehydrogenase (FW, SW) capacity and either greater initial stores or more efficient use of liver (FW, SW) and muscle (FW) glycogen during exercise. Compared with R_♀L_♂ hybrids, L_♀R_♂ hybrids had a 20% lower swimming speed, which was associated with a 24% smaller cardio‐somatic index and higher physiological costs. Thus swimming performance depends on cross direction (i.e. which parental line was used as dam or sire). The study thus suggests that divergent physiological factors between anadromous and resident S. fontinalis may result in similar swimming capacities that are adapted to their respective lifestyles.     

Stable isotope analyses provide new insights into ecological plasticity in a mixohaline population of European eel

Recent studies have shown that anguillid eel populations in habitats spanning the marine–freshwater ecotone can display extreme plasticity in the range of catadromy expressed by individual fishes. Carbon and nitrogen stable isotope analysis was used to differentiate between European eels (Anguilla anguilla) collected along a short (2 km) salinity gradient ranging from <1‰ to ~30‰ in Lough Ahalia, a tidal Atlantic lake system. Significant differences were recorded in mean δ¹³C, δ¹⁵N and C:N values from eels collected from fresh, brackish and marine-dominated basins. A discriminant analysis using these predictor variables correctly classified ca. 85% of eels to salinity zone, allowing eels to be classified as freshwater (FW), brackish (BW) or marine (MW) residents. The results of the discriminant analysis also suggested that a significant proportion of eels moved between habitats (especially between FW and BW). Comparisons of several key population parameters showed significant variation between eels resident in different salinity zones. Mean condition and estimated age was significantly lower in MW eels, whilst observed length at age (a correlate of growth) was significantly higher in MW eels, intermediate in BW and lowest in FW eels. This study has demonstrated that the ecology of eels found along a short salinity gradient can be extremely plastic and that stable isotope analysis has considerable utility in demonstrating intra-population variation in diadromous fishes.