Transfer To Fresh Water Research Articles

Plasticity of osmoregulatory function in the killifish intestine:drinking rates, salt and water transport, and gene expression after freshwater transfer

We have explored intestinal function in the euryhaline killifish Fundulus heteroclitus after transfer from brackish water (10% seawater) to fresh water. Plasma Na+ and Cl- concentrations fell at 12 h post-transfer, but recovered by 7 days. Drinking rate decreased substantially at 12 h (32% of control value) and remained suppressed after 3 and 7 days in fresh water (34 and 43%). By contrast, there was a transient increase in the capacity for water absorption measured across isolated intestines in vitro (3.3- and 2.6-fold at 12 h and 3 days), which returned to baseline after 7 days. These changes in water absorption could be entirely accounted for by changes in net ion flux: there was an extremely strong correlation (R2=0.960) between water absorption and the sum of net Na+ and net Cl- fluxes (3.42+/-0.10 microl water micromol(-1) ion). However, enhanced ion transport across the intestine in fresh water would probably not increase water uptake in vivo, because the drinking rate was far less than the capacity for water absorption across the intestine. The increased intestinal ion absorption after freshwater transfer may instead serve to facilitate ion absorption from food when it is present in the gut. Modulation of net ion flux occurred without changes in mRNA levels of many ion transporters (Na+/K+-ATPase alpha(1a), carbonic anhydrase 2, CFTR Cl- channel, Na+/K+/2Cl- cotransporter 2, and the signalling protein 14-3-3a), and before a measured increase in Na+/K+-ATPase activity at 3 days, suggesting that there is some other mechanism responsible for increasing ion transport. Interestingly, net Cl- flux always exceeded net Na+ flux, possibly to help maintain Cl- balance and/or facilitate bicarbonate excretion. Our results suggest that intestinal NaCl absorption from food is important during the period of greatest ionic disturbance after transfer to fresh water, and provide further insight into the mechanisms of euryhalinity in killifish.

Identification of eel ghrelin in plasma and stomach by radioimmunoassay and histochemistry

The structure of ghrelin has been determined in the Japanese eel, Anguilla japonica. In this study, we identified immunoreactive ghrelin in extracts from plasma and stomach of the eel by radioimmunoassay (RIA) using an antiserum against octanoylated rat ghrelin [1–11]. Using the antiserum, we examined localization of ghrelin-immunopositive cells in the eel stomach. Detection of ghrelin mRNA-expressing cells was also attempted in the eel stomach using a cRNA probe specific for the eel ghrelin gene. Furthermore, we examined ghrelin expression patterns in plasma and stomach after transfer of freshwater (FW) eels to seawater (SW). Multiple types of immunoreactive ghrelin were detected using RIA. These were octanoylated eel ghrelin and other ghrelins that may have different fatty acid modifications, suggesting that this RIA can detect acylated ghrelin of eels as seen previously in the case of rat. Ghrelin-immunopositive cells were observed in the mucosal layer of the stomach, especially in the neck of the fundic gland. Ghrelin mRNA-expressing cells showed similar distribution and characteristics to the immunopositive cells. Plasma ghrelin levels in FW eels starved for one week before experimentation were approximately 40 fmol/ml. Plasma ghrelin levels in control-transferred FW eels did not change for 7 days, but significantly increased on day 14. Plasma ghrelin levels transiently increased fivefold 6 h after SW transfer and then declined to the FW level by 24 h after transfer. Ghrelin content and ghrelin mRNA levels in the stomach did not change after SW transfer, except for a transient decrease in ghrelin content seen 24 h after transfer. The present results suggest that ghrelin may participate in osmoregulation in eels.

Tracing organic matter sources in riverine suspended sediment: Implications for fine sediment transfers

The study of sediment transport in riverine systems has historically focussed on the inorganic fluxes, although the suspended load is known to incorporate both inorganic and organic material. Research on freshwater flocculation indicates that sediment morphology and behaviour depend on both components, as biologic material can alter the hydrodynamic and charge properties of composite particles, which has implications for fine sediment transport and storage in stream systems. This study evaluates both the supply and the quality of the organic matter comprising the suspended sediment in a salmon-bearing stream in northern British Columbia, Canada. Seasonally changing hydrodynamic and biologic conditions are considered in the context of suspended sediment (seston) size. Seston samples were collected in an event-based design for analysis of organic matter sources, suspended particulate, inorganic and organic matter concentrations (SPM, SIM and SOM) and effective particle size distributions (EPSD). Organic matter sources were assessed using carbon : nitrogen (C : N) ratios and the associated stable isotopic signals. Samples and environmental measurements were taken during spring melt, pre-spawn low flows, active salmon spawning, post-spawn, and select rainstorms. SPM and SIM were elevated during active salmon spawning compared to that expected for the given hydrologic conditions. Seasonal patterns of C : N ratios and stable isotopes indicate that organic matter types can be differentiated in seston samples and these sources of organic material exhibit changing dominance in the seston over time. The isotopic signature of seston increases in salmon-derived nutrients when live and decaying salmon are present within the study stream. The largest suspended particles are observed during the period of active spawning when both live and rotting salmon modify instream conditions. This appears to be a function of the combined influence of high SIM, low shear stress, and low C : N ratios. The generation of increased floc, or seston size is of ecological importance in this stream and riverine systems elsewhere as it regulates the freshwater transfers of locally derived nutrients and sediments out of the fluvial system.

Short-term effects of hyposmotic shock on Na +/K +-ATPase expression in gills of the euryhaline milkfish, Chanos chanos

Changes in expression of gill Na +/K +-ATPase (NKA) on a short-term (96 h) time-course following hyposmotic shock (direct transfer to fresh water) of the euryhaline, marine milkfish were studied on gene, protein, and cell levels in this paper. Plasma osmolality and [Na +] responded with rapid declines in 3 h post-transfer yet, thereafter, remained constant. Plasma [Cl −] gradually fell to a significantly lower level at 6 h post-transfer. Gills responded to hyposmotic shock by a dual phase enhancement of NKA activity and protein abundance; (a) Before 24 h: NKA activity increased as early as 3 h and reached a maximum level from 6 to 12 h post-transfer coincided with the sustained lower levels of plasma osmolality, [Na +], and [Cl −] since 3 h post-transfer. This was followed by a gradual rise in α-subunit protein levels that peaked at 12 h post-transfer. Meanwhile, α-mRNA of NKA did no show significant change. (b) After 24 h: NKA activity as well as the amounts of α-subunit mRNA and protein increased significantly. Direct freshwater transfer induced a prompt and significant decrease of NKA immunoreactive (NKIR) cell abundance in filaments before 24 h, followed by a significant increase after 24 h due to their development in filaments and lamellae. Increased number of NKIR cells after 24 h of hyposmotic shock may occur in conjunction with rise of NKA activity as well as α-subunit mRNA and protein abundance. In conclusion, milkfish is able to avoid an excessive drop in plasma ions immediately upon hyposmotic shock and maintain plasma ions on a marginal lower level in fresh water. Notably, the initial increase in NKA activity (adjustive phase; 3–12 h) and delayed increase in NKA mRNA and protein abundance (regulatory phase; 48–96 h) indicate the importance of a higher level of the gill enzyme in milkfish upon hyposmotic shock.

Regulation of the Indonesian throughflow by baroclinic draining of the North Australian Basin

The Indonesian throughflow (ITF) is studied using historic hydrographical data with purpose of obtaining a description of the characteristics of the large-scale flow and an understanding of both the forcing and control of the flow. The investigation makes use of the steric sea level and the contents of freshwater and heat in the water column integrated down to the depth of the deepest connection (effective sill depth) between the Pacific and Indian Oceans within the Indonesian seas. It is shown that it is mainly low-saline water of North Pacific origin that fills the upper part of both the Indonesian seas and a downstream buoyant (surface) pool (DBP) that stretches horizontally over a large part of the North Australian Basin. The long-term mean steric sea level in the Indonesian seas is approximately horizontal and equal to the sea level in the neighboring part of the Pacific Ocean. This indicates small frictional losses against the mean throughflow, which is verified using a coupled multiple basin model of the Indonesian seas. The change of the steric sea level, from the Pacific Ocean level to the Indian Ocean level, occurs essentially at the border between the DBP and the adjacent Indian Ocean. It is found that Darwin is situated inside the DBP, indicating that the sea level recorded in this place is a poor representative of mean and low frequency parts of the sea level variability of the Indian Ocean. It is argued that the control of ITF is set by the baroclinic transport capacity of the DBP relative to the adjacent (Indian Ocean) water. The mean ITF, estimated as the outflow from the DBP to the South Equatorial Current, is about 10 Sv. The integrated ITF imprint is fresh and cold and the net heat transport is estimated to be 0.3 PW. The buffering capacity of the DBP is about 5 years, so variations in the circulation should be modest on shorter time-scales. It is suggested that the atmospheric transfer of freshwater to the North Pacific and vertical mixing in the North Pacific provide the driving of the mean ITF and that ITF is a major branch of the estuarine-type vertical circulation of the North Pacific.

Effects of spironolactone and RU486 on gene expression and cell proliferation after freshwater transfer in the euryhaline killifish

We have explored the possible mechanisms by which mineralocorticoid (MR) and glucocorticoid (GR) receptors regulate the response to freshwater transfer in the gills of the euryhaline killifish Fundulus heteroclitus. Killifish were implanted with RU486 (GR antagonist) or spironolactone (MR antagonist) at doses of 0.1-1.0 mg g(-1), and subsequently transferred from 10 per thousand brackish water to freshwater. Compared to brackish water sham fish, mRNA expression of CFTR and NKCC1 decreased in the gills of sham fish transferred to freshwater, whereas Na(+), K(+)-ATPase alpha(1a) mRNA expression and alpha protein abundance, as well as cell proliferation (detected using BrdU) increased. Spironolactone inhibited the normal increase in cell proliferation and Na(+), K(+)-ATPase expression after freshwater transfer. RU486 increased plasma cortisol levels and may have slightly inhibited Na(+), K(+)-ATPase activity, but did not change alpha(1a ) expression. RU486 had no effect on cell proliferation in the non-lamellar region of the gills, but increased proliferation in the lamellar region. Neither antagonist inhibited the suppression of CFTR or NKCC1 expression after freshwater transfer. Glucocorticoid receptor expression was reduced in all sham and antagonist treatments compared to untreated controls, but no other consistent differences were observed. The effects of spironolactone suggest that MR is important for regulating ion transport in killifish gills after freshwater transfer.

Gene expression after freshwater transfer in gills and opercular epithelia of killifish: insight into divergent mechanisms of ion transport

We have explored the molecular basis for differences in physiological function between the gills and opercular epithelium of the euryhaline killifish Fundulus heteroclitus. These tissues are functionally similar in seawater, but in freshwater the gills actively absorb Na+ but not Cl-, whereas the opercular epithelium actively absorbs Cl- but not Na+. These differences in freshwater physiology are likely due to differences in absolute levels of gene expression (measured using real-time PCR), as several proteins important for Na+ transport, namely Na+,H+-exchanger 2 (NHE2), carbonic anhydrase 2 (CA2), Na+,HCO3- cotransporter 1, and V-type H+-ATPase, were expressed at 3- to over 30-fold higher absolute levels in the gills. In gills, transfer from 10% seawater to freshwater increased the activity of Na+,K+-ATPase by twofold (from 12 h to 7 days), increased the expression of NHE2 (at 12 h) and CA2 (from 12 h to 7 days), and decreased the expression of NHE3 (from 12 h to 3 days). In opercular epithelium, NHE2 was not expressed; furthermore, Na+,K+-ATPase activity was unchanged after transfer to freshwater, CA2 mRNA levels decreased, and NHE3 levels increased. Consistent with their functional similarities in seawater, killifish gills and opercular epithelium expressed Na+,K+-ATPase alpha 1a, Na+,K+,2Cl- cotransporter 1 (NKCC1), cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and the signalling protein 14-3-3a at similar absolute levels. Furthermore, NKCC1 and CFTR were suppressed equally in each tissue after freshwater transfer, and 14-3-3a mRNA increased in both. These results provide insight into the mechanisms of ion transport by killifish gills and opercular epithelia, and demonstrate a potential molecular basis for the differences in physiological function between these two organs.

Computer-based identification and tracking of Antarctic icebergs in SAR images

Icebergs play an important role in climate through the transfer of fresh water and heat between ice sheets and the oceans. Synthetic aperture radar (SAR) images offer a robust way of observing icebergs in the often dark and cloud-covered polar areas. A computer-based technique is proposed to identify icebergs at higher resolution than in the past, allowing the shape of small to medium icebergs to be retained and used for tracking between images acquired at different times. The identification consists of (i) automatic segmentation, followed by (ii) classification of these objects as icebergs or nonicebergs. The classification stage requires user intervention as the icebergs are not always a separable class from other objects, most notably sea ice. The icebergs are then matched between images using their size and shape similarities, and their position tracked. The system performance was assessed by applying the technique to three wintertime ERS-1 PRI images, around Kapp Norvegia (10°W), on the Eastern Weddell Sea coast. The results show both the identification and the tracking to be effective.

Changes in gene expression in gills of the euryhaline killifish Fundulus heteroclitus after abrupt salinity transfer.

Maintenance of ion balance requires that ionoregulatory epithelia modulate ion flux in response to internal or environmental osmotic challenges. We have explored the basis of this functional plasticity in the gills of the euryhaline killifish Fundulus heteroclitus. The expression patterns of several genes encoding ion transport proteins were quantified after transfer from near-isosmotic brackish water [10 parts/thousand (ppt)] to either freshwater (FW) or seawater (SW). Many changes in response to SW transfer were transient. Increased mRNA expression occurred 1 day after transfer for Na(+)-K(+)-ATPase-alpha(1a) (3-fold), Na(+)-K(+)-2Cl(-)-cotransporter 1 (NKCC1) (3-fold), and glucocorticoid receptor (1.3-fold) and was paralleled by elevated Na(+)-K(+)-ATPase activity (2-fold). The transient increase in NKCC1 mRNA expression was followed by a later 2-fold rise in NKCC protein abundance. In contrast to the other genes studied in the present work, mRNA expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel generally remained elevated (2-fold) in SW. No change in protein abundance was detected, however, suggesting posttranscriptional regulation. The responses to FW transfer were quite different from those to SW transfer. In particular, FW transfer increased Na(+)-K(+)-ATPase-alpha(1a) mRNA expression and Na(+)-K(+)-ATPase activity to a greater extent than did SW transfer but had no effect on V-type H(+)-ATPase expression, supporting the current suggestion that killifish gills transport Na(+) via Na(+)/H(+) exchange. These findings demonstrate unique patterns of ion transporter expression in killifish gills after salinity transfer and illustrate important mechanisms of functional plasticity in ion-transporting epithelia.

Effects of transfer from seawater to fresh water on the growth hormone/insulin-like growth factor-I axis and prolactin in the Tilapia, Oreochromis mossambicus

The effect of freshwater (FW) transfer on growth and on the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis was examined in the tilapia, Oreochromis mossambicus. Tilapia were raised in seawater (SW) for 5 months and then transferred to FW for an additional 40 days. The growth rate of the fish transferred to FW was significantly reduced compared with the growth rate of fish that remained in SW. Plasma levels of GH were significantly elevated in FW-transferred fish, as were plasma IGF-I levels. Pituitary GH and liver IGF-I mRNA levels, on the other hand, were significantly reduced in the fish transferred to FW. There was a significant correlation between body mass and mRNA levels of GH and IGF-I, but not with plasma levels of GH and IGF-I. Fish transferred to FW had significantly higher prolactin (PRL) 177 levels than the SW control fish, although there was no difference in plasma PRL 188 levels. Consistent with the hyperosmoregulatory effects of PRL, mRNA levels of both PRL 177 and PRL 188 were significantly higher in FW-transferred fish than in the fish in SW. These results suggest that transferring tilapia from SW to FW activates the GH/IGF-I axis, but growth is still inhibited, possibly due to the greater metabolic cost of osmoregulation in FW than in SW.

Temporal and Spatial Variability of Fluid Flow: Submarine Groundwater Discharge

The coastal zones belong to the most intensively populated and fast developing regions of the world. High and continually increasing requirements for groundwater for domestic, agricultural, and industrial needs is a consequence. Strategies for sustainable use of fresh water in this area are required. In several regions, submarine groundwater discharge (SGD) is a significant pathway for the transfer of freshwater, nutrients, trace gases, and pollutants to coastal environments. Assessment of SGD is a requirement for sustainable management of ecosystem and resources. For the study area Eckernforde Bay (Western Baltic Sea), seepage of freshwater from sub-seafloor aquifers was observed by video observations, sediment sampling, and by high resolution bathymetric and side scan sonar surveys by an Autonomous Underwater Vehicle (AUV). Discharge occurs as dispersed flow from muddy sediments and from well-defined morphological features, so called pockmarks. The relation of these features to groundwater seepage was proven by pore water investigations of the methane and chloride concentration. Through a submarine well system, we were able to sample the sub-seafloor aquifer directly, allowing age dating of the groundwater. The spatial distribution of freshwater discharge was derived and a budget was computed for the entire bay.

The typological approach to submarine groundwater discharge (SGD)

Coastal zone managers need to factor submarine groundwater discharge (SGD) in their integration. SGD provides a pathway for the transfer of freshwater, and its dissolved chemical burden, from the land to the coastal ocean. SGD reduces salinities and provides nutrients to specialized coastal habitats. It also can be a pollutant source, often undetected, causing eutrophication and triggering nuisance algal blooms. Despite its importance, SGD remains somewhat of a mystery in most places because it is usually unseen and difficult to measure. SGD has been directly measured at only about a hundred sites worldwide. A typology generated by the Land–Ocean Interaction in the Coastal Zone (LOICZ) Project is one of the few tools globally available to coastal resource managers for identifying areas in their jurisdiction where SGD may be a confounding process. (LOICZ is a core project of the International Geosphere/Biosphere Programme.) Of the hundreds of globally distributed parameters in the LOICZ typology, a SGD subset of potentially relevant parameters may be culled. A quantitative combination of the relevant hydrological parameters can serve as a proxy for the SGD conditions not directly measured. Web-LOICZ View, geospatial software then provides an automated approach to clustering these data into groups of locations that have similar characteristics. It permits selection of variables, of the number of clusters desired, and of the clustering criteria, and provides means of testing predictive results against independent variables. Information on the occurrence of a variety of SGD indicators can then be incorporated into regional clustering analysis. With such tools, coastal managers can focus attention on the most likely sites of SGD in their jurisdiction and design the necessary measurement and modeling programs needed for integrated management.

Plasma Concentrations of Arginine Vasotocin and Urotensin II Are Reduced Following Transfer of the Euryhaline Flounder (Platichthys flesus) from Seawater to Fresh Water

Plasma concentrations and stored levels of the neuroendocrine peptides arginine vasotocin (AVT) and urotensin II (UII) were measured in the euryhaline flounder (Platichthys flesus) following the acute hypo-osmotic challenge of direct seawater (SW) to fresh water (FW) transfer. Hormone measures, plasma osmolality, and ion concentrations and tissue water content were determined 1, 4, 8, 24, 72, and 144 h after transfer. Plasma AVT concentration fell initially following FW transfer but then returned toward pretransfer levels by day 6. Plasma UII concentration decreased while urophysial UII content was increased following hypo-osmotic challenge relative to SW time-matched controls, suggesting down regulation of the UII system during the initial stages after FW transfer. These changes in neuroendocrine activity were associated with a significant fall in plasma osmolality and major plasma ions. Positive correlations were observed between plasma AVT and osmolality and Cl− and Mg2+ concentrations, suggesting functional association of these plasma parameters with AVT action and/or control of AVT secretion. The initial response to hypotonic challenge involves reduced plasma AVT and UII levels consistent with the proposed role for these hormones, supporting flounder osmoregulation in hypertonic media.

Distributional Changes in Branchial Chloride Cells during Freshwater Adaptation in Japanese Sea Bass Lateolabrax japonicus

Abstract Distributional changes in branchial chloride cells were examined in Japanese sea bass (Lateolabrax japonicus) juveniles transferred from seawater (SW) to fresh water (FW) during their migration season toward low salinity habitat in nature. Chloride cells were identified by immunocytochemistry with a specific antiserum for Na+,K+-ATPase. In fish reared in SW as controls, branchial chloride cells were localized exclusively in the filaments and absent in the lamellae. When sea bass were transferred from SW to FW, chloride cells emerged in gill lamellae, starting at the proximal part of the lamellae and thereafter spread over the lamellar epithelium. On 7th and 15th days after FW transfer, chloride cells were mostly found on the lamellae, whereas the number of filament chloride cells was decreased. These results suggest that, in Japanese sea bass juveniles, chloride cells in the gill lamellae are important in FW adaptation, and that lamellar chloride cells originated from the filaments and migrated t...

The Arctic ocean-ice system studied by contamination modelling

The Arctic represents a relatively pristine frontier that is vulnerable to pollution. Substances originating at mid latitudes are transported to the Arctic by atmospheric processes, ocean currents and rivers. These pollutants can accumulate in the Arctic environment. Testing of nuclear weapons, dumping of waste and operation of ships, and nuclear power plants also pose threats.To investigate possible pollutant pathways we used a multi-level primitive-equation ocean model, coupled to a dynamic-thermodynamic sea-ice model. Coupling included conservative transfer of momentum, heat and fresh water. Atmospheric forcing (wind stress, temperature, humidity, radiation and heat fresh-water fluxes) was supplied by datasets or bulk formulae. Open lateral-boundary conditions for the ocean model were supplied either by datasets (temperature and salinity) or from a larger-scale ocean model (momentum). Two integrations were compared — one used a centred-difference advection scheme and viscous damping, while the other used a better representation of an advection scheme and a sub-gridscale eddy parameterization.Tracer simulations showed (1) the importance of good representation of numerical advection, and (2) the role of eddy interacting with sea-floor topography (the neptune effect).

Differential response of plasma prolactin to freshwater transfer of smolts and postsmolts of seawater-adapted coho salmon ( Oncorhynchus kisutch)

Prolactin (PRL) was purified from chum salmon, Oncorhynchus keta, pituitary glands and was used to develop a homologous radioimmunoassay for the measurement of PRL from salmon. The plasma PRL response to freshwater (FW) transfer differed in seawater (SW)-adapted postsmolt (250 g) and smolts (15 g) of coho salmon. Postsmolts had a pronounced and prolonged elevation of plasma titers of PRL with hypercalcemia and stable plasma sodium levels. The FW-transferred postsmolts had significantly lower pituitary gland PRL only at 0.5 and 2 hr post-transfer as compared to SW-SW. Smaller smolts showed stable plasma PRL levels after FW transfer, hypocalcemia 48 post-transfer, depressed plasma sodium concentrations, and lowered plasma osmotic pressure. This different response may be due to an increased osmoionic regulatory challenge encountered by the smaller smolts or possibly due to some other developmental change between the two different age classes.

Time course of extracellular acid‐base adjustments under hypo‐ or hyperosmotic conditions in the euryhaline fish Platichthys flesus

In order to better understand the basis for the euryhalinity of the flounder, Platichthys flesus, which tolerates large variations in water salinity, experiments have been designed to characterize the time course of extracellular ionic and acid‐base adjustments under hypo‐ or hyperosmotic conditions. Abrupt transfer from sea water (SW) to fresh water (FW) provokes a transient decrease in the plasma osmolality (Posm) and a concomitant transient metabolic alkalosis (whole blood pH 7.78 in SW and 8.04 five days after FW transfer) associated with a marked, persistent hypercapnia. After 33 days in FW, Posm and whole blood pH are not significantly different from those in SW, but whole blood Pco2 and plasma bicarbonate concentration are always higher than SW values. Opposite transitory fluctuations, i.e. a metabolic acidosis associated with a respiratory alkalosis, occur when flounder long‐acclimated to FW are again exposed to SW. The mechanisms involved in these salinity‐dependent acid‐base disturbances are rather complex and remain to be elucidated. These observations attest to the importance of the extracellular acid‐base changes that may be (i) linked to extracellular anisosmotic regulation and/or to cellular metabolic adjustments, and (ii) compensated partially by ventilatory adjustments.

Renal function in migrating adult atlantic salmon, Salmo salar L.

1. 1. Renal function in migrating adult Atlantic salmon was studied in sea-water (SW) and following abrupt transfer to fresh water (FW). 2. 2. Urine flow rate of SW-adapted fish, 0.72 ml/kg/hr, increased 6.3-fold to 4.55 ml/kg/hr after 2–3 days in FW, later decreasing to around 1 ml/kg/hr. 3. 3. Changes in glomerular filtration rate and ion filtration rates largely paralleled changes in urine flow. In SW-adapted salmon about 4% of excreted magnesium is filtered. Tubular magnesium secretion declined within 1 day of FW transfer. 4. 4. During the period of maximum diuresis, urinary sodium loss is 77% of the branchial sodium uptake rate. This falls to less than 20% in FW-adapted fish.

Osmoregulatory ability of chum salmon,Oncorhynchus keta, reared in fresh water for prolonged periods

The osmoregulatory ability of chum salmon (Oncorhynchus keta), reared in fresh water for a prolonged period, was examined by transferring them directly to seawater and then back to fresh water. When fry and juveniles weighing 0.3-125g, reared in fresh water for 1.5-13 months, were transferred directly to seawater, they adjusted their plasma Na(+) concentration to the seawater-adapted level within 12-24h. When they were transferred back to fresh water after having been adapted to seawater for 2 weeks, the plasma Na(+) level gradually decreased during the first 12-24h, and then increased to reattain the initial freshwater level after 5-7 days. No mortality was observed during the experiment except among the smallest fry weighing about 0.3g after transfer to seawater (2.1%). The maintenance of good osmoregulatory ability of the chum salmon for a prolonged period in fresh water seems to be unique among Pacific salmon, with the possible exception of the pink salmon.Changes in plasma levels of hormones during the transfer experiments were recorded in juveniles reared in fresh water for 13 months. Prolactin levels increased maximally 3 days after transfer from seawater to fresh water, as would be expected from its well-established role in freshwater adaptation in several euryhaline teleosts. In addition, an increase in plasma growth hormone was observed during the first 12h after seawater transfer, along with a tendency towards a decrease during freshwater transfer, suggesting an important role for this hormone in seawater adaptation. There were no consistent changes in plasma levels of thyroxine and cortisol during freshwater to seawater or seawater to freshwater transfer.

Changes in urinary bladder and kidney function in the starry flounder ( Platichthys stellatus) in response to prolactin and to freshwater transfer

The effect of ovine prolactin treatment and of acclimation to freshwater on renal hydromineral metabolism were investigated in the seawater starry flounder, Platichthys stellatus. Isolated urinary bladders of seawater, freshwater, and prolactin-injected seawater fish readily transport sodium and chloride but not potassium, magnesium, and calcium. As was established earlier, urinary bladders of seawater flounder show considerable mucosa-to-serosa water movement whereas prolactin treatment or freshwater acclimation markedly decreases bladder water permeability. The flounder kidney was studied by cannulating the urinary bladder in vivo. Prolactin treatment and freshwater acclimation caused increased urine production, increased sodium excretion, and decreased urine osmolarity. These changes suggest diminished fluid reabsorption resulting from decreased water permeability of the kidney tubule. Furthermore, prolactin treatment lowered urine magnesium content without affecting net renal magnesium excretion. Kidneys of freshwater flounder excreted considerably less magnesium and produced urine of lower magnesium concentration than kidneys of seawater flounders. Maximal effects were manifested 48 hrs after a single prolactin injection; thereafter, values slowly returned toward initial levels. A primary and inital action of prolactin on renal excretion in the starry flounder appears to be a decrease in water permeability of the kidney tubules and urinary bladder.