Arsenic accumulation in livers of pinnipeds, seabirds and sea turtles: subcellular distribution and interaction between arsenobetaine and glycine betaine

Concentrations of total arsenic and individual arsenic compounds were determined in liver samples of pinnipeds (northern fur seal Callorhinus ursinus and ringed seal Pusa hispida), seabirds (black-footed albatross Diomedea nigripes and black-tailed gull Larus crassirostris) and sea turtles (hawksbill turtle Eretmochelys imbricata and green turtle Chelonia mydas). Among these species, the black-footed albatross contained the highest hepatic arsenic concentration (5.8+/-3.7 microg/g wet mass). Arsenobetaine was the major arsenic species found in the liver of all these higher tropic marine animals. To investigate the cause of high accumulation of arsenobetaine, subcellular distribution of arsenic and relationship between arsenobetaine and glycine betaine concentrations were examined in the livers of these animals. There was no relationship between total arsenic concentration and its subcellular distribution in liver tissues. However, a significant negative correlation was found between arsenobetaine and glycine betaine concentrations in the liver of six species examined. This result may indicate that arsenobetaine is accumulated in these marine animals as an osmolyte along with glycine betaine, which is a predominant osmolyte in marine animals because the chemical structure and properties of arsenobetaine are similar to those of glycine betaine.     

Heavy Metal Accumulation in Four Species of Sea Turtles from the Baja California Peninsula, Mexico

Heavy metals were assessed in four species of sea turtles from the Baja California Peninsula, Mexico, representing the first report of heavy metal concentrations in tissues of post-yearling sea turtles from the Eastern Pacific. Concentrations of Cd measured in C. mydas kidney (653 μg/g dry wt) were the highest ever reported for any sea turtle species. Cd accumulated preferentially in kidney and the ratios of kidney to liver Cd in Baja California turtles were among the highest reported for sea turtles globally. Zn, Ni, and Mn concentrations were also significantly higher in kidney than other tissues, while Cu and Fe were greatest in liver, and all metals were lowest in muscle. With the exception of one value (69.9 μg/g in kidney of C. caretta), Pb was low in all tissues from Baja California. In comparisons across species, kidney of C. mydas had greater Zn and Ni concentrations as compared to other species, although there was no difference in liver metal levels among the species. Positive correlations were detected in the concentrations of Cd, Cu and Ni with the straight carapace length of C. caretta.     

Mercury in the marine food chain in the Southern Ocean at Macquarie Island: an analysis of a top predator,Patagonian toothfish (<Emphasis Type="Italic">Dissostichus eleginoides</Emphasis>) and a mid-trophic species,the warty squid (<Emphasis Type="Italic">Moroteuthis ingens</Emphasis>)

The characteristics and habitat of the Patagonian toothfish (Dissostichus eleginoides) are typical of fish that accumulate high concentrations of mercury. In this study, mercury determinations were made on samples of muscle tissue from Macquarie Island toothfish and the Southern Ocean deepwater warty squid (Moroteuthis ingens). The analysis of mercury in the biological tissues was made by cold vapour-atomic absorption spectrometry following acid digestion. Performance of the analytical procedure was assessed by analysis of certified reference material (DORM-2, dogfish muscle). Mercury concentrations of 16 Macquarie Island toothfish ranged from 0.12 mg kg^–1 (550 g, 381 mm TL) to 0.59 mg kg^–1 (6,100 g, 823 mm TL), with a mean concentration of 0.33±0.12 mg kg^–1. A significant correlation was found between mercury and either toothfish weight or total length. The fish analysed were juveniles, which suggests that larger individuals would have higher mercury concentrations well exceeding food standard code limits for mercury in fish (typically 0.5 mg kg^–1). Warty squid, also from around Macquarie Island, had a low mean mercury concentration of 0.086 mg kg^–1 in mantle tissue; no significant correlation existed between mercury concentration and either squid mantle length or total weight. It is postulated that the squid have a mechanism, possibly involving the digestive gland, that prevents bioaccumulation of mercury in the mantle, and presumably other body tissues.     

A bloom of Lyngbya majuscula in Shoalwater Bay, Queensland, Australia: An important feeding ground for the green turtle (Chelonia mydas)

Lyngbya majuscula, a toxic cyanobacterium, was observed blooming during June–July (winter) 2002 in Shoalwater Bay, Queensland, Australia, an important feeding area for a large population of green turtles (Chelonia mydas). The bloom was mapped and extensive mats of L. majuscula were observed overgrowing seagrass beds along at least 18 km of coast, and covering a surface area of more than 11 km². Higher than average rainfall preceded the bloom and high water temperatures in the preceding summer may have contributed to the bloom. In bloom samples, lyngbyatoxin A (LA) was found to be present in low concentration (26 μg kg⁻¹_{(dry weight)}), but debromoaplysiatoxin (DAT) was not detected. The diet of 46 green turtles was assessed during the bloom and L. majuscula was found in 51% of the samples, however, overall it contributed only 2% of the animals’ diets. L. majuscula contribution to turtle diet was found to increase as the availability of the cyanobacterium increased. The bloom appeared to have no immediate impact on turtle body condition, however, the presence of a greater proportion of damaged seagrass leaves in diet in conjunction with decreases in plasma concentrations of sodium and glucose could suggest that the turtles may have been exposed to a substandard diet as a result of the bloom. This is the first confirmed report of L. majuscula blooming in winter in Shoalwater Bay, Queensland, Australia and demonstrates that turtles consume the toxic cyanobacterium in the wild, and that they are potentially exposed to tumour promoting compounds produced by this organism.     

Influence of the diet on the bioaccumulation of heavy metals in zooplankton-eating petrels at Kerguelen archipelago,Southern Indian Ocean

Concentrations of cadmium, mercury, copper and zinc were measured in muscle, kidney and liver tissues and in the main prey of five species of zooplankton-eating petrels: blue petrel (Halobaena caerulea), thin-billed (Pachyptila belcheri) and Antarctic (P. desolata) prions, and South Georgian (Pelecanoides georgicus) and common (Pelecanoides urinatrix) diving petrels. Since some of these species are closely related species with respect to body size, timing of moult and life span, their diet has been examined to evaluate its influence on heavy-metal bioaccumulation. Inter-specific differences were significant for Hg concentrations in the liver and for Zn concentrations in both liver and kidney tissues. Blue petrels exhibited the highest Hg concentrations in the liver (3.9±2.02 µg.g^–1 wet weight). No significant differences were found in Cd concentrations between species. Exposure to heavy metals through the most important prey species in the diet during the breeding period was evaluated. The most evident result was the influence of fish prey on Hg levels. Although crustacean species exhibit different cadmium concentrations, the diet composition does not appear to be discriminant for Cd bioaccumulation within the small petrel community at Kerguelen.     

Uptake of arsenic by mushrooms from soil

ABSTRACT

Graphite furnace atomic absorption spectrometry (standard addition method) was used to determine the total arsenic in wild-growing mushrooms after digestion with nitric acid, then with perchloric acid and in associated soils after digestion with mixtures of nitric and hydrofluoric acids in a microwave system. Among 83 species of mushrooms the highest concentrations of arsenic on a dry mass basis were found in Laccaria amethystea (26–125 mg kg^?1), Laccaria laccata (11–33 mg kg^?1), Thelephora terrestris (38 mg kg^?1), Boletus cavipes (11.6 mg kg^?1) and Ramaria botrytis (10 mg kg^?1). Mushroom caps of L. laccata, L. amethystea, and B. cavipes had approximately double the arsenic concentrations found in stems. The arsenic concentrations in caps of L. amethystea and L. laccata were directly proportional to the concentrations in the soils. The concentrations of arsenic in the soils were in the range 6.5–65 mg kg^?1. Among the 19 mushroom caps with arsenic concentrations above the method detection limit of 0.2 mg As/kg dry mass, only L. amethystea and L. laccata had arsenic concentration ratios ‘cap/soil’ higher than 1 (between 1.1 and 1.9). Thelephora terrestris had a ratio of 2.37.     

The degradation of arsenobetaine to inorganic arsenic by sedimentary microorganisms

Two growth media containing arsenobetaine [(CH₃)₃ As⁺ CH₂COO^–] were mixed with coastal marine sediments, the latter providing a source of microorganisms. The mixtures were kept at 25 °C in the dark and shaken for several weeks under an atmosphere of air. The disappearance of arsenobetaine and the appearance of two metabolites were followed by HPLC. The HPLC-retention time of the first metabolite agreed with that of trimethylarsine oxide [(CH₃)₃AsO]. The second metabolite was identified as arsenate (As(V)) using hydride generation/cold trap/GC MS analysis and thin layer chromatography. This is the first scientific evidence showing that arsenobetaine is degraded by microorganisms to inorganic arsenic via trimethylarsine oxide. The degradation of arsenobetaine to inorganic arsenic completes the marine arsenic cycle that begins with the methylation of inorganic arsenic on the way to arsenobetaine.     

Bioko: critically important nesting habitat for sea turtles of West Africa

We evaluate the conservation status and threats faced by sea turtle nesting populations at Bioko Island, Equatorial Guinea (Central Africa). Beaches were monitored to obtain a detailed sea turtle nest census and, where possible, tagging of adult females was undertaken. Four sea turtle species were found nesting in the area: the green turtle (Chelonia mydas), the leatherback (Dermochelys coriacea), the olive ridley (Lepidochelys olivacea) and the hawksbill (Eretmochelys imbricata); with the former two species nesting in regionally important numbers. Nesting activity was concentrated between November and February, with a peak in December–January. Tagging and recapture of green turtles in two consecutive seasons suggested an estimated 560 (interquartile range: 420–1,681) and 414 (interquartile range: 190–1,255) nesting females in the area, respectively. Estimated numbers of nesting leatherbacks ranged from 123 to 215 and 243 to 293 in the first and second season, respectively. The other two species were less abundant (olive ridley: 19–29 and 28–43; hawksbill: 4–10 and 2 turtles). Data were compared with more recent surveys in the area and contextualised with information on human related threats. Despite the size of nesting stocks, ongoing permitted and illegal take of adult turtles at the nesting site constitutes a serious threat for these breeding aggregations. Additionally, tag returns from throughout the Gulf of Guinea suggest that the level of take in regional fisheries may also be a major threat.     

Helicobacter pylori lipopolysaccharide provokes iNOS-mediated acute systemic microvascular inflammatory responses in rat cardiac, hepatic, renal and pulmonary tissues

We have examined the effects of intravenous administration of a purified lipopolysaccharide (LPS) from Helicobacter pylori (3 mg kg⁻¹, i.v.) on rat vascular permeability, assessed by the radiolabelled human serum albumin leakage technique in the heart, kidney, liver and lung 4 h after challenge. An increased vascular permeability in cardiac, renal, hepatic and pulmonary tissues after challenge was determined. The albumin leakage observed in all these organs could be prevented by the selective inducible nitric oxide synthase inhibitor, N-(8-(aminomethyl)benzyl)-acetamidine (1400W; 0.2–1 mg kg⁻¹, s.c.) administered concurrently with LPS. Thus, H. pylori LPS can provoke a microvascular inflammatory response in the rat cardiac, renal, hepatic and pulmonary tissues, actions mediated through the activation of the inducible nitric oxide synhase isoenzyme.     

The effects of contamination of soil with copper,lead and arsenic on the growth and composition of plants

Summary Accumulations of copper, lead and arsenic in soils affected by orchard sprays or mining were investigated in relation to their effects on growth and composition of plants. Seasonal variations in concentrations of the elements in pasture plants sampled from contaminated soils in the field are reported. The effects of soil temperature and applications of the nutrients P, S and N on the composition of plants grown in contaminated soils were investigated in glasshouse experiments.The copper concentrations of pasture species sampled from sites which were formerly orchards were usually high (20 to 60 mg kg^–1) during most of the growing season and may present some risk of toxicity to grazing ruminant animals. Lead (0.8 to 21 mg kg^–1) and arsenic (<0.2 to 5.8 mg kg^–1) concentrations were within, or close to the normal range of concentration in plants. In the glasshouse experiments, soil temperature was found to be an important factor in the uptake of copper, lead and arsenic. There were significant differences in uptake between genotypes. Applications of fertilizers at rates equivalent to those used for commercial vegetable production generally resulted in small decreases in the concentrations of copper, lead and arsenic concentrations in silver beet.     

The effect of prolonged anoxia at 3°C on tissue high energy phosphates and phosphodiesters in turtles: A 31P-NMR study

Selected tissues (skeletal muscle, heart ventrical, and liver), sampled from turtles (Chrysemys picta bellii) at 3°C either under normoxic conditions or after 12 weeks of anoxic submergence were quantiaatively analysed for intracellular pH and phosphorus metabolites using ³¹P-NMR. Plasma was tested for osmolality and for the concentrations of lactate, calcium, and magnesium to confirm anoxic stress. We hypothesized that, in the anoxic animals, tissue ATP levels would be maintained and that the increased osmolality of the body fluids of anoxic turtles would be accounted for by a corresponding increase in the concentrations of phosphodiesters. The responses observed differed among the three tissues. In muscle, ATP was unchanged by anoxia but phosphocreatine was reduced by 80%; in heart, both ATP and phosphocreatine fell by 35–40%. The reduction in phosphocreatine in heart tissue at 3°C was similar to that observed in isolated, perfused working hearts from turtles maintained at 20°C but no decrease in ATP occurred in the latter tissues. In liver, although analyses of several specimens were confounded by line-broadening, neither ATP nor phosphocreatine was detectable in anoxic samples. Phosphosdiesters were detected in amounts sufficient to account for 30% of normoxic cell osmotic concentration in heart and 11% and 12% in liver and muscle, respectively. The phosphodiester levels did not change in anoxia. Heart ventricular phosphodiester levels in turtles at 3°C were significantly higher than those determined for whole hearts from turtles at 20°C. ¹H, ¹³C and ³¹P NMR analyses of perchloric acid extracts of heart and skeletal muscle from 20°C turtles con firmed that the major phosphodiester observed by NMR in these tissues is serine ethanolamine phosphate. We conclude that the three types of tissues studied differ substantially in their ability to maintain levels of ATP during anoxia, and that liver may continue to function despite NMR-undetectable levels of this metabolite. In addition, we conclude that phosphodiesters do not serve as regulated osmolytes during anoxia, and that the functional significance of their high concentrations in turtle tissues remains uncertain.     

Exercise metabolism in two species of cod in arctic waters

The northern range of Atlantic cod (Gadus morhua), overlaps the southern range of the Greenland cod (Gadus ogac), in the coastal waters of Western Greenland. The availability of a temperate water species (G. morhua) in the same area and oceanographic conditions as a polar species (G. ogac) presented us with the ideal circumstances to test the hypothesis of metabolic cold adaptation (MCA) since many of the problems associated with MCA studies (adaptation of the animals beyond their normal temperature range or mathematical extrapolation of data to common temperatures) could thus be avoided. We therefore used a swim tunnel to measure oxygen consumption in fish at 4°C over a range of swimming speeds and following exhaustion, monitored the size of the oxygen debt and time of oxygen debt repayment. There were no significant differences in standard (60–72 mg O₂ kg^–1· hr^–1), routine (76 mg O₂ kg^–1·hr^–1), active (137mg O₂ kg^–1·hr^–1), or maximal (157 mg O₂ kg^–1·hr^–1) metabolic rate, metabolic scope (2.5) or critical swimming speed (2.2 BL·s^–1) between the two species. Following exhaustive swimming, however, the half-time for oxygen debt repayment in G. ogac (43 min) was almost twice that of G. morhua (25 min). Despite its circumpolar distribution, therefore, there was no evidence of MCA in G. ogac.     

Thermal dependence of tissue metabolism in the green turtle,Chelonia mydas

To better understand how tissue specific metabolic rates might contribute to the maintenance of elevated body temperatures in green turtles (Chelonia mydas), we determined the effect of temperature on oxygen consumption of green fat, small intestine, nonswimming skeletal muscle, pectoralis muscle, liver, heart, and kidney tissues from 5–35°C. We found a direct relationship between tissue metabolic rate (microliters of O₂/g wet mass per hour) and temperature in all tissues measured except for green fat. The Q₁₀ values ranged from 0.65 to 3.38. There were significant differences in metabolic rate among tissues as well as in how temperature influenced tissue metabolic rates. Tissue metabolic rates were highest in kidney and heart tissues. Green fat and small intestine had the lowest and most temperature-insensitive values. Muscle tissue had a high oxygen consumption relative to other reptiles, and this elevated metabolism may provide a functional advantage for long distance swimming and heat production.     

Arsenic uptake and accumulation in rice (Oryza sativa L.) irrigated with contaminated water

Long-term use of arsenic contaminated groundwater to irrigate crops, especially paddy rice (Oryza sativaL.) has resulted in elevated soil arsenic levels in Bangladesh. There is, therefore, concern regarding accumulation of arsenic in rice grown on these soils. A greenhouse pot experiment was conducted to evaluate the impact of arsenic-contaminated irrigation water on the growth and uptake of arsenic into rice grain, husk, straw and root. There were altogether 10 treatments which were a combination of five arsenate irrigation water concentrations (0–8 mg As l^–1) and two soil phosphate amendments. Use of arsenate containing irrigation water reduced plant height, decreased rice yield and affected development of root growth. Arsenic concentrations in all plant parts increased with increasing arsenate concentration in irrigation water. However, arsenic concentration in rice grain did not exceed the maximum permissible limit of 1.0 mg As kg^–1. Arsenic accumulation in rice straw at very high levels indicates that feeding cattle with such contaminated straw could be a direct threat for their health and also, indirectly, to human health via presumably contaminated bovine meat and milk. Phosphate application neither showed any significant difference in plant growth and development, nor in As concentrations in plant parts.     

Arsenic concentrations in water and fish from Chautauqua Lake,New York

Synopsis Arsenic persists in Chautauqua Lake, New York waters 13 years after cessation of herbicide (sodium arsenite) application and continues to cycle within the lake. Arsenic concentrations in lake water ranged from 22.4–114.81 g l^–1, = 49.0 ag l^–1. Well water samples generally contained less than 10 g l^–1 arsenic. Arsenic concentrations in lake water exceeded U.S. Public Health Service recommended maximum concentrations (10 g l^–1) and many samples exceeded the maximum permissible limit (50 g l^–1). Fish accumulated arsenic from water but did not magnify it. Fish to water arsenic ratios ranged from 0.4–41.6. Black crappie (Pomoxis nigromaculatus) contained the highest arsenic concentrations (0.14–2.04 g g^–1 ), X = 0.7 g g^–1) while perch (Perca flavescens), muskellunge (Esox masquinongy) and largemouth bass (Micropterus salmoides) contained the lowest concentrations (0.02–0.13 g g^–1). Arsenic concentrations in fish do not appear to pose a health hazard for human consumers.     

Soil washing of Pb, Zn and Cd using biodegradable chelator and permeable barriers and induced phytoextraction by Cannabis sativa

The feasibility of combined phytoextraction and in situ washing of soil contaminated with Pb (1750 mg kg^–1), Zn (1300 mg kg^–1), and Cd (7.2 mg kg^–1), induced by the addition of biodegradable chelator, [S,S] stereoisomere of ethylenediamine discuccinate ([S,S]-EDDS), was tested in soil columns with hemp (Cannabis sativa) as the phytoextracting plant. The addition of [S,S]-EDDS (10 mmol kg^–1 dry soil) yielded concentrations of 1026±442, 330.3±114.7 and 3.84±1.55 mg kg^–1 of Pb, Zn and Cd in the dry above-ground plant biomass, respectively. These concentrations were 1926, 7.5, and 11 times higher, respectively, compared to treatments with no chelator addition. Horizontal permeable barriers, composed of a 3 cm high layer of nutrient enriched sawdust and vermiculite mixture, and a 3 cm layer of soil, vermiculite and apatite mixture, were positioned 20, 30 and 40 cm deep in the soil. In chelator treatments, barriers placed 30 cm deep reduced leaching of Pb, Zn and Cd by 435, 4 and 53 times, respectively, compared to columns with no barrier, where 3.0, 4.3 and 3.3% of total initial Pb, Zn and Cd, respectively, was leached during 6-weeks water irrigation after chelator addition. Lower positioned barriers were almost equally effective in preventing leaching of Pb than barriers positioned closer to the soil surface, less effective for Cd, and did not prevent leaching of Zn. 2.53% of total initial Pb and 2.83% of Cd was washed from the contaminated soil and accumulated into the barrier. Combined method was less effective than simulated ex situ soil washing, where 14.2, 5.5 and 24.5% of Pb, Zn and Cd, respectively, were removed after 1-h extraction, but comparable effective to 48-h extraction. Abbreviations: BCF – bioconcentration factor; EDTA – ethylene diaminetetraacetate; HM – heavy metal; PP – phytoextraction potential; [S,S]-EDDS – [S,S]-ethylenediamine disuccinate.     

Effects of soil cadmium on Pinus sylvestris L. seedlings

The effects of Cd on the growth and distribution of Cd and mineral nutrients within plant tissues were investigated for Pinus sylvestris L. seedlings grown in mineral forest soil with increasing levels of Cd addition (0–100 mg kg^–1). Approximately 20% of added Cd was found to be extractable from sandy loam forest soil. Root growth was less affected by Cd than shoot growth, which showed a significant reduction in the 100 mg Cd kg^–1 treatment. Cadmium accumulated in roots up to 325 mg kg^–1. Decreased concentrations of K in needles and Ca in stems with increasing Cd levels suggest a disturbance of mineral nutrition as a result of Cd addition.     

Vegetative approach for improving the quality of water produced from soils in the westside of central California

Water reuse is a proposed strategy for utilizing or disposing of poor quality drainage water produced in the westside of central California. This 2-year field study evaluated the ability of two potential forage species to tolerate irrigation with water high in salinity, boron (B), and selenium (Se). The species used were: Sporobulus airoides var. salado (alkali sacaton) and Medicago sativa var. salado (alfalfa). After first year establishment with good quality water (<1 dS m^–1), the two species were furrow-irrigated with drainage effluent that had an average composition of sulfate-dominated salinity ((electrical conductivity (EC) of 6.2 dS m^–1)) B (5 mg l^–1), and Se (0.245 mg l^–1). Both crops were clipped monthly from June to October of each year. Total dry matter yields averaged between 11 and 12 mg ha^–1 for both crops irrigated with effluent for two growing seasons. Plant concentrations of Se ranged from a low of 1.3 mg kg^–1 in alkali sacaton to a high of 2.5 mg kg^–1 in alfalfa, while B concentrations ranged from a low of 60 mg kg^–1 in alkali sacaton to a high of 170 mg kg^–1 in alfalfa. Chemical composition of the soil changed as follows from preplant to post-irrigation after two seasons with drainage effluent: EC from 2.78 to 6.5 dS m^–1, extractable B from 1.9 to 5.6 mg l^–1, and no change in extractable Se at 0.012 mg l^–1 between 0 and 45 cm. Between 45 and 90 cm, EC values increased from 4.95 to 6.79 dS m^–1, extractable B from 2.5 to 4.8 mg l^–1, and no change in extractable Se at 0.016 mg l^–1. Increased salinity and extractable B levels in the soil indicate that management of soil salinity and B will be necessary over time to sustain long term reuse with poor quality water.     

Uptake and accumulation of zinc in juvenile rainbow trout,Salmo gairdneri

Synopsis The toxicity of zinc to rainbow trout was determined and the 72 h median lethal concentration was found to be 2.00 mg l^–1 in freshwater, hardness 7.50 mg l^–1 as calcium. An insignificant increase in zinc concentration of internal tissues occurred in fish exposed to 1.52 mg l^–1 in freshwater for 72 h. However, there was a significant uptake of zinc by gills and the body surface. Fish exposed to 10 mg l^–1 zinc for 72 h in two-thirds sea water showed significant zinc uptake by liver, rectum and muscle, when compared to control fish. Drinking rate decreased from 1.43 to 0.26 ml kg^–1 h^–1 when zinc sulphate was added to freshwater. Trout adapted to two-thirds sea water showed no decrease in drinking, about 7 ml kg^–1 h^–1 when zinc was added to the water.     

Freshwater to seawater acclimation of juvenile bull sharks (<Emphasis Type="Italic">Carcharhinus leucas</Emphasis>): plasma osmolytes and Na<Superscript>+</Superscript>/K<Superscript>+</Superscript>-ATPase activity in gill,rectal gland,kidney and intestine

This study examined the osmoregulatory status of the euryhaline elasmobranch Carcharhinus leucas acclimated to freshwater (FW) and seawater (SW). Juvenile C. leucas captured in FW (3 mOsm l^–1 kg^–1) were acclimated to SW (980–1,000 mOsm l^–1 kg^–1) over 16 days. A FW group was maintained in captivity over a similar time period. In FW, bull sharks were hyper-osmotic regulators, having a plasma osmolarity of 595 mOsm l^–1 kg^–1. In SW, bull sharks had significantly higher plasma osmolarities (940 mOsm l^–1 kg^–1) than FW-acclimated animals and were slightly hypo-osmotic to the environment. Plasma Na⁺, Cl^–, K⁺, Mg²⁺, Ca²⁺, urea and trimethylamine oxide (TMAO) concentrations were all significantly higher in bull sharks acclimated to SW, with urea and TMAO showing the greatest increase. Gill, rectal gland, kidney and intestinal tissue were taken from animals acclimated to FW and SW and analysed for maximal Na⁺/K⁺-ATPase activity. Na⁺/K⁺-ATPase activity in the gills and intestine was less than 1 mmol Pi mg^–1 protein h^–1 and there was no difference in activity between FW- and SW-acclimated animals. In contrast Na⁺/K⁺-ATPase activity in the rectal gland and kidney were significantly higher than gill and intestine and showed significant differences between the FW- and SW-acclimated groups. In FW and SW, rectal gland Na⁺/K⁺-ATPase activity was 5.6±0.8 and 9.2±0.6 mmol Pi mg^–1 protein h^–1, respectively. Na⁺/K⁺-ATPase activity in the kidney of FW and SW acclimated animals was 8.4±1.1 and 3.3±1.1 Pi mg^–1 protein h^–1, respectively. Thus juvenile bull sharks have the osmoregulatory plasticity to acclimate to SW; their preference for the upper reaches of rivers where salinity is low is therefore likely to be for predator avoidance and/or increased food abundance rather than because of a physiological constraint.