Seawater Adaptation Research Articles

Changes in Plasma and Tissue Long-Chain Polyunsaturated Fatty Acid (LC-PUFA) Content in the Eel Anguilla japonica After External and Internal Osmotic Stress.

We investigated the effect of external and internal osmotic stress on the profile of long-chain polyunsaturated fatty acids (LC-PUFA) in euryhaline eels Anguilla japonica. Freshwater (FW) fish were transferred to seawater (SW) for external osmotic stress or subjected to internal stress through injection with hypertonic saline. FW eels injected with isotonic saline served as controls. Plasma osmolality, Na+ concentration, and gill Na+/K+ -ATPase activity increased, but hematocrit decreased compared with controls in eels exposed to external or internal osmotic stress. The expression of two major transporter genes for SW adaptation, the Na+ -K+ -2Cl - co-transporter 1a (NKCC1a) in the gill and NKCC2b in the intestine, was up-regulated only in SW-transferred eels, suggesting a direct impact of SW on the gill and intestine via SW ingestion. Total LC-PUFA contents and DHA (22:6 n-3) increased in the gill and liver of SW-transferred eels and in the intestine of hypertonic saline-injected eels. However, total LC-PUFA content in plasma decreased after both external and internal osmotic stimuli. In contrast, the gene expression of two key enzymes involved in the LC-PUFA biosynthesis, Δ6 fatty acid desaturase and elongase, did not change in the gill, intestine and liver of osmotically stressed eels. These results indicate that LC-PUFA is possibly involved in osmoregulation and the increased LC-PUFA contents of osmoregulatory organs might be a result of LC-PUFA transport via circulation, rather than through de novo biosynthesis.

NMR-based metabolomic study on the toxicological effects of pesticide, diazinon on adaptation to sea water by endangered Persian sturgeon, Acipenser persicus fingerlings

NMR-based metabolomics was applied to explore metabolic impacts of diazinon on sea water adaptation of Persian sturgeon fingerlings, Acipenser persicus. Fingerlings were exposed to sub-lethal concentrations of diazinon in freshwater (FW) for 96 h (short-term trial) and 12 days (long-term trial) and then exposed in brackish water (BW) (12 mg L−1 salinity) for 24 h. After 96 h and 12 days exposure in FW, identified metabolites (amino acids, osmolytes, energy metabolites) showed different change-patterns compared to control group (P < 0.05) as follow: (A) short-term trial: higher plasma levels of glucose, lactate (in all diazinon-exposed fish), acetate and acetoacetate (in 0.9 mg L−1diazinon treatment); lower levels of creatine (in all diazinon-exposed fish), trimethylamine-N-oxide, choline, taurine, betaine, N,N-dimethylglycine and almost all amino acids in fish exposed to high concentrations of diazinon (0.54 and 0.9 mg L−1 diazinon). (B) Long-term trial: higher plasma levels of lipid oxidation metabolites and almost all amino acids in fish exposed to 0.54 and 0.9 mg L−1 diazinon; lower levels of creatine, trimethylamine-N-oxide, N,N-dimethylglycine, betaine, choline (in all diazinon-exposed fish), glucose (in 0.54 and 0.9 mg L−1diazinon treatments) and taurine (in 0.9 mg L−1 diazinon treatment).When fish were exposed in BW for 24 h, the plasma levels of osmolytes decreased significantly in almost all experimental groups of short-term and long-term trial (P < 0.05). In short-term trial, the plasma levels of glucose in all groups and lactate in 0.18 and 0.54 mg L−1 diazinon treatments increased after salinity challenge (P < 0.05). However, a significant decrease was observed in lactate levels in 0.9 mg L−1 diazinon treatment (P < 0.05). Also, the plasma levels of amino acids decreased mostly in fish of control group than exposed fish (P < 0.05). The plasma glycerol concentration showed a significant decrease only in fish of 0.54 mg L−1 diazinon treatment (P < 0.05). In long term trial, the energetic metabolites (acetate, acetoacetate, glycerol) showed significant increases mostly in fish exposed to high concentrations of diazinon (P < 0.05). Phosphocreatine was detected only in groups exposed to 0.54 and 0.9 mg L−1 diazinon. Some amino acids decreased in control and diazinon-exposed groups while glycine (in control and 0.18 mg L−1 diazinon treatment), glutamine and alanine (in 0.9 mg L−1 diazinon treatment) elevated significantly after 24 h acclimation in BW (P < 0.05). Our results may help to understand the effects of pesticides on fish osmoregulation from a metabolic approach.

Smoltification and seawater transfer of Atlantic salmon (Salmo salar L.) is associated with systemic repression of the immune transcriptome

Smoltification and seawater adaptation of Atlantic salmon are associated with profound alterations in the endocrine status, osmoregulation and behaviour. Little is known about immunological changes during smoltification, although increased incidences of infectious diseases after seawater transfer (SWT) may indicate weakened protection. We report microarray gene expression analyses in farmed Atlantic salmon during smoltification stimulated with constant light and early seawater adaptation (one and three weeks after SWT). Gene expression changes were large, their magnitude in the head kidney and proximal intestine was greater than in the gill. Among 360 differentially expressed immune genes, 300 genes were down-regulated, and multiple functional groups were affected such as innate antiviral immunity, chemokines, cytokines and receptors, signal transducers, effectors of humoral and cellular innate immunity, antigen presentation and lymphocytes, especially T cells. No recovery was observed after three weeks in seawater. A notable exception was a transient up-regulation of immunoglobulin transcripts in the gill after SWT. Genes involved in stress responses and xenobiotic metabolism were up-regulated in respectively intestine and gill. The duration of this observed immune suppression and the possible consequences for susceptibility to infections and diseases need further exploration.

Past seawater experience enhances seawater adaptability in medaka, Oryzias latipes.

BackgroundDuring the course of evolution, fishes have acquired adaptability to various salinity environments, and acquirement of seawater (SW) adaptability has played important roles in fish evolution and diversity. However, little is known about how saline environments influence the acquirement of SW adaptability. The Japanese medaka Oryzias latipes is a euryhaline species that usually inhabits freshwater (FW), but is also adaptable to full-strength SW when transferred through diluted SW. In the present study, we examined how past SW experience affects hyposmoregulatory ability in Japanese medaka.ResultsFor the preparation of SW-experienced fish, FW medaka were acclimated to SW after pre-acclimation to 1/2 SW, and the SW-acclimated fish were transferred back to FW. The SW-experienced fish and control FW fish (SW-inexperienced fish) were transferred directly to SW. Whereas control FW fish did not survive direct transfer to SW, 1/4 of SW-experienced fish adapted successfully to SW. Although there were no significant differences in blood osmolality and plasma Na+ and Cl− concentrations between SW-experienced and control FW medaka in FW, increments in these parameters following SW transfer were lower in SW-experienced fish than in control FW fish. The gene expression of SW-type Na+, K+-ATPase (NKA) in the gills of SW-experienced medaka increased more quickly after direct SW transfer compared with the expression in control FW fish. Prior to SW transfer, the density of NKA-immunoreactive ionocytes in the gills was higher in SW-experienced fish than in control FW fish. Ionocytes expressing CFTR Cl− channel at the apical membrane and those forming multicellular complexes, both of which were characteristic of SW-type ionocytes, were also increased in SW-experienced fish.ConclusionThese results indicate that past SW experience enhances the capacity of Na+ and Cl− secretion in ionocytes and thus hypoosmoregulatory ability of Japanese medaka, suggesting the presence of epigenetic mechanisms involved in seawater adaptation.Electronic supplementary materialThe online version of this article (doi:10.1186/s40851-016-0047-2) contains supplementary material, which is available to authorized users.

Appearance of Infectious Hematopoietic Necrosis Virus in Rainbow Trout, Oncorhynchus mykiss, During Seawater Adaptation

AbstractAbout 7% mortality occurred in rainbow trout, Oncorhynchus mykiss, during seawater adaptation at a marine farm in the South Sea of Korea during the winter of 2014. Most diseased fish showed petechial hemorrhaging of gills and internal fat with enlarged spleen. Although no parasites or bacteria were isolated from the diseased fish, all tissue filtrates produced cytopathic effects (CPEs) in fathead minnow and Chinook salmon embryo‐214 cells. The cell culture supernatant showing CPE contained specific 1527‐bp fragment for the infectious hematopoietic necrosis virus (IHNV) glycoprotein gene by polymerase chain reaction. Their nucleotide sequences shared 98.1–98.2% identities with IHNV RtUi02 isolated from rainbow trout in Korea. This isolate (RtGoH14) was closely related to Korean IHNV isolates of genogroup JRt rather than to those of North American and European genogroups. These results suggest that this IHNV isolate might have been introduced to rainbow trout farm (land‐based culture system) in Korea. This is the first report of IHNV infection in rainbow trout during seawater adaptation in Korea.

Latent Toxicity of Endothall to Anadromous Salmonids During Seawater Challenge

Limited evidence exists on the latent effects of toxicant exposure on the seawater adaptability of anadromous salmon and steelhead. It is unclear whether such an effect exists for the widely used and relatively non-toxic herbicide endothall. Coho salmon, Oncorhynchus kisutch (coho), Chinook salmon, O. tshawytscha (Chinook), and anadromous rainbow trout, O. mykiss (steelhead) were subjected to a 10-day seawater challenge following freshwater treatments [0-12 mg acid equivalent (a.e)./L at 96 h]. Mean survival resulted in 82 % (n = 225), 84 % (n = 133), 90 % (n = 73) and 59 % (n = 147) survival for 0, 3-5, 6-8, and 9-12 mg a.e./L, respectively. Our results indicate a lower toxicity threshold compared with previously reported acute toxicity results, but higher compared with previous seawater challenge studies. We demonstrate the utility of the seawater challenge assay to accurately define toxic effects of pesticides on salmonids with complex life-histories.

Observations of initial water ingestion and ion absorption in the digestive tract of Japanese eel larvae

The onset of osmoregulation for seawater adaptation was examined during the early life stages of the Japanese eel. Ingested seawater was detected in the digestive tract by using fluorescent dextran as an inert marker. Ingested seawater remained in the forepart of the digestive tract at 0 and 1 days post hatching (dph), but reached the anus at 2 dph. Scanning electron microscope observations showed that the mouth appeared as a slit at 0 dph and developed into a hole-like shape at 2 dph. Expressions of Na+, K+, and 2Cl− cotransporter 2β (NKCC2β), and Na+ and Cl− cotransporter β (NCCβ) mRNAs were detected mainly in the intestine and rectum, respectively. These results are consistent with those of the adult eel, suggesting that the intestine and rectum are the sites of active ion absorption in larvae also. Expression levels of NKCC2β steeply increased from 4 to 6 dph, while NCCβ levels were highest on the day of hatching, presumably due to a maternal factor. The expression levels of NCCβ decreased by 3–4 dph and then increased to a constant status at 7 dph. Our findings confirmed that osmoregulation started as early as the day of hatching and stabilized within a week.

Profiles of hypothalamus–pituitary–interrenal axis gene expression in the parr and smolt stages of rainbow trout, Oncorhynchus mykiss: Effects of recombinant aquaporin 3 and seawater acclimation

The objective of this investigation was to quantify how the hypothalamus–pituitary–interrenal (HPI) axis in the rainbow trout, Oncorhynchus mykiss (parr/smolt), responds to salinity changes during transfer from freshwater (FW) to seawater (SW) and recombinant aquaporin 3 (rAQP3) injection. mRNA expression levels of HPI axis genes [corticotropic-releasing hormone (CRH) and adrenocorticotropic hormone (ACTHα and ACTHβ)] significantly increased when the fish were transferred from FW to SW (parr: 16.4-, 13.2-, 21.4-, and 11.9-fold higher than FW; smolt: 2.3-, 2.7-, 13.6-, and 6.2-fold higher than FW, respectively). Furthermore, and the plasma ACTH, Na+, Cl−, and K+ levels were the highest at 50% SW. Moreover, these parameters were significantly lower in the rAQP3-treated group than those in the control (parr: 2.0-, 2.4-, 2.1-, and 2.0-fold lower than SW; smolt: 4.2-, 1.9-, 2.4-, and 2.3-fold lower than SW, respectively). Hence, HPI axis genes may play a role in SW adaptation during migration from FW to SW environments. We showed that there was a negative correlation between rAQP3, HPI axis genes, and ion levels when the fish were transferred to SW, with levels being significantly lower in the rAQP3-injected group. Hence, cortisol appears to be a stress hormone and plasma Na+ and Cl− levels significantly increased when the fish were transferred to SW, with levels being significantly lower in the rAQP3-treated group. These results indicate that rAQP3 modulates the HPI axis and ion transportation in rainbow trout.

해수순치 무지개송어 (Oncorhynchus mykiss)의 Vibrio anguillarum 감염증

Department of Aqualife Medicine, Pukyong National University, Busan 608-737, KoreaWe examined the cause of a disease outbreak in rainbow trout Oncorhynchus mykiss, which were adapting to a seawater in an aquaculture farm in Jeju on April, 2013. Most of the diseased fish showed a severe ulcer on the skin, enlarged spleen, expanded stomach and hemorrhage of abdominal and pyloric region. Although no parasites, fungi or viruses were isolated from diseased fish, over 200 same bacterial colonies were isolated from liver, spleen and kidney. Nucleotide sequences of the 16S rDNA gene of the bacterium in this study showed 100% identity with Vibrio anguillarum. This study is the first report of V. anguillarum infection in rainbow trout during sea adaption in Korea.Key words: Rainbow trout, Seawater adaptation, Vibrio anguillarum, Infection

Guanylin peptides regulate electrolyte and fluid transport in the Gulf toadfish (Opsanus beta) posterior intestine.

The physiological effects of guanylin (GN) and uroguanylin (UGN) on fluid and electrolyte transport in the teleost fish intestine have yet to be thoroughly investigated. In the present study, the effects of GN, UGN, and renoguanylin (RGN; a GN and UGN homolog) on short-circuit current (Isc) and the transport of Cl-, Na+, bicarbonate (HCO3-), and fluid in the Gulf toadfish (Opsanus beta) intestine were determined using Ussing chambers, pH-stat titration, and intestinal sac experiments. GN, UGN, and RGN reversed the Isc of the posterior intestine (absorptive-to-secretory), but not of the anterior intestine. RGN decreased baseline HCO3- secretion, but increased Cl- and fluid secretion in the posterior intestine. The secretory response of the posterior intestine coincides with the presence of basolateral NKCC1 and apical cystic fibrosis transmembrane conductance regulator (CFTR), the latter of which is lacking in the anterior intestine and is not permeable to HCO3- in the posterior intestine. However, the response to RGN by the posterior intestine is counterintuitive given the known role of the marine teleost intestine as a salt- and water-absorbing organ. These data demonstrate that marine teleosts possess a tissue-specific secretory response, apparently associated with seawater adaptation, the exact role of which remains to be determined.

Enteric neuroplasticity in seawater‐adapted European eel (Anguilla anguilla)

European eels live most of their lives in freshwater until spawning migration to the Sargasso Sea. During seawater adaptation, eels modify their physiology, and their digestive system adapts to the new environment, drinking salt water to compensate for the continuous water loss. In that period, eels stop feeding until spawning. Thus, the eel represents a unique model to understand the adaptive changes of the enteric nervous system (ENS) to modified salinity and starvation. To this purpose, we assessed and compared the enteric neuronal density in the cranial portion of the intestine of freshwater eels (control), lagoon eels captured in brackish water before their migration to the Sargasso Sea (T0), and starved seawater eels hormonally induced to sexual maturity (T18; 18weeks of starvation and treatment with standardized carp pituitary extract). Furthermore, we analyzed the modification of intestinal neuronal density of hormonally untreated eels during prolonged starvation (10weeks) in seawater and freshwater. The density of myenteric (MP) and submucosal plexus (SMP) HuC/D-immunoreactive (Hu-IR) neurons was assessed in wholemount preparations and cryosections. The number of MP and SMP HuC/D-IR neurons progressively increased from the freshwater to the salty water habitat (control>T0>T18; P<0.05). Compared with freshwater eels, the number of MP and SMP HuC/D-IR neurons significantly increased (P<0.05) in the intestine of starved untreated salt water eels. In conclusion, high salinity evokes enteric neuroplasticity as indicated by the increasing number of HuC/D-IR MP and SMP neurons, a mechanism likely contributing to maintaining the body homeostasis of this fish in extreme conditions.

Effects of exogenous cortisol and seawater adaptation on thyroid hormone receptors in the smolt stage of the sockeye salmon, Oncorhynchus nerka

The objective of this investigation was to quantify how thyroid hormone receptors of the sockeye salmon, Oncorhynchus nerka, respond to salinity changes from freshwater (FW) to seawater (SW) conditions. Thy- roid hormone receptors (TRs) mRNA and protein expres- sions levels significantly increased when the fish were transferred from FW to SW, and the plasma T3 and T4 levels were significantly highest at 50 % SW and then maintained as control. Moreover, these parameters were significantly lower in the cortisol-injected groups than in the control. Hence, TRs, T3, and T4 may play a role in SW adaptation, when the fish migrate from FW to SW envi- ronments. We showed a negative correlation between cortisol and thyroid hormone levels, and a significant increase in plasma K ? levels in the kidney when the fish were transferred to SW, with levels being significantly lower in the cortisol-injected group. Hence, cortisol appears to be a stress hormone, and the plasma Na ? and Cl - levels significantly increased when the fish were transferred to SW, with levels being significantly lower in the cortisol- injected group. These results indicate that cortisol modu- lates ion transportation in the plasma.

Environmental induced methylation changes associated with seawater adaptation in brown trout

Migratory trout (sea trout) and sedentary trout (freshwater trout) coexist and interbreed. However, the mechanisms underlying the development of the migrant morphotype are still unclear. At one point parr–smolt transformation is initiated and, as a result, juvenile trout are ready to leave the river and migrate to the sea. The smoltification process has been linked to various factors such as body size, growth rate and the physiological state of the fish. In addition, the process is also strongly influenced by environmental factors such as year, seasonality, water temperature and flow rate. For example, hatchery environment can depress the natural parr–smolt transformation and consequently, the success of the seawater migration of reared trout from these hatchery programmes might be adversely affected. We have investigated whether changes in DNA methylation, by means of MSAP (methylation-sensitive amplified polymorphism), could be involved in anadromy. We identified dramatic differences in genome-wide methylation patterns between hatchery reared and seawater brown trout. Furthermore, we demonstrated that salt enriched diets can trigger short-term genome-wide methylation changes in hatchery reared trout. However, these changes only lasted for a short period of time. Determining the duration of this effect could result in increased survival of hatchery-reared trout in seawater when fed on salt-enriched diets. Altogether, these results suggest that salt-induced alterations in DNA methylation patterns could play an important role in enabling fish acclimation to seawater conditions, potentially with important economic consequences for fish farming.

Role of cardiac natriuretic peptides in seawater adaptation of medaka embryos as revealed by loss-of-function analysis

Cardiac natriuretic peptides (atrial natriuretic peptide, ANP; b-type natriuretic peptide, BNP; ventricular natriuretic peptide, VNP) and their direct ancestor C-type natriuretic peptide 3 (CNP3) exert potent osmoregulatory actions in fish. However, very little is known about their roles in embryonic osmoregulation. In this study, we performed loss-of-function analysis using euryhaline medaka (Oryzias latipes), which has lost ANP and VNP during evolution and thus possesses only BNP and CNP3. We found that the maintenance of whole-body osmolality in seawater embryos was impaired by the knockdown of BNP+OLGC7 (BNP receptor) or CNP3 alone from 1 day postfertilization, and the CNP3 knockdown was accompanied by greater water loss. The impaired osmoregulation in the knockdown embryos was not due to the suppressed expression of major transporters for NaCl excretion via ionocytes or of key enzyme genes for metabolic water production, but to the impaired blood circulation to the yolk-sac membrane caused by abnormal heart development. We detected a strong positive correlation between impaired blood circulation and increased body fluid osmolality and pharmacological blockade of blood flow increased body fluid osmolality in seawater embryos. We also found that the exaggerated water loss in CNP3 knockdown embryos is related to the failure to suppress aquaporin (AQP3, AQP4, and AQP9) gene expression. These results show that CNP3 decrease water permeability of body surfaces and that both BNP and CNP3 ensure sufficient blood flow to the yolk-sac membrane for efficient salt excretion by ionocytes and sufficient water production by yolk metabolism to promote seawater adaptation during early development in medaka.

Osmolality and ionic status of hemolymph and branchial Na+/K+-ATPase in adult mitten crab during seawater adaptation

Introduction Mitten crabs, genus Eriocheir, are notorious invasive animals and are considered to be the first (1912) documented case of a ballast water-transported species [1]. In addition, they are of considerable value for aquaculture in both local and international markets [2]. They are strongly euryhaline brachyurans and provide a unique model

Vigorous SO42– influxviathe gills is balanced by enhanced SO42– excretion by the kidney in eels after seawater adaptation

Sulfate (SO(4)(2-)) is maintained at ∼1 mmol(-1) l(-1) in teleost fishes that are exposed to media of varying SO(4)(2-) concentrations. We first measured plasma SO(4)(2-) concentration in euryhaline fishes that adapt to both SO(4)(2-)-poor freshwater (<0.5 mmol l) and SO(4)(2-)-enriched seawater (30 mmol l(-1)). Unlike Mozambique tilapia and chum salmon, Japanese eels maintained higher plasma SO(4)(2-) concentration in freshwater (6.2±2.3 mmol l(-1)) than in seawater (0.7±0.1 mmol l(-1)). We then analyzed the whole-body SO(4)(2-) budget using (35)SO(4)(2-). (35)SO(4)(2-) influx in seawater-adapted eels occurred by 84.5% via body surfaces and 15.5% via digestive tracts. The SO(4)(2-) influx was higher in seawater eels (1.55 μmol kg(-1) h(-1)) than in freshwater eels (0.09 μmol kg(-1) min(-1)), but it was facilitated in freshwater eels when the difference in SO(4)(2-) concentrations between plasma and environment was taken into account (freshwater eels, 6.2 vs 0.3 mmol l(-1); seawater eels, 0.7 vs 30 mmol l(-1)). One hour after injection of (35)SO(4)(2-) into the blood of seawater eels, the kidney excreted ∼97% of the ionized form, whereas the radioactivity increased gradually in the medium and the rectal fluid more than 3 h after injection. As the radioactivity was poorly adsorbed by anion-exchange resin, (35)SO(4)(2-) in the blood may be incorporated into cells and excreted by the intestine, gills and skin, probably as mucus. These results show that freshwater eels take up SO(4)(2-) actively from the environment, but seawater eels cope with the obligatory influx of SO(4)(2-) through the gills by excreting excess SO(4)(2-) via the kidney and in mucus.

Recruitment and degeneration of mitochondrion-rich cells in the gills of Mozambique tilapia Oreochromis mossambicus during adaptation to a hyperosmotic environment

Cellular recruitment and degeneration of branchial mitochondrion-rich (MR) cells were examined in Mozambique tilapia transferred from hypoosmotic to hyperosmotic water. To examine apoptosis in the gills associated with salinity change, tilapia were directly transferred from freshwater to 70% seawater. The TUNEL assay showed that apoptotic cells in the gills were significantly increased at 1day after transfer, which was supported by an electron-microscopic observation that gill MR cells underwent morphological changes characteristic of apoptosis such as an irregularly shaped electron-dense nucleus and distension of the tubular system. To further examine MR–cell recruitment, freshwater-acclimated tilapia were transferred either to freshwater or to 70% seawater after BrdU injection. Immunohistochemical detection of BrdU-labeled nuclei and Na+/K+-ATPase-rich MR cells allowed us to classify BrdU-labeled MR cells into two subtypes: a single MR cell and an MR–cell complex. Although newly generated single MR cells were observed similarly in both freshwater and 70% seawater-transferred fish, the density of MR–cell complexes was much higher in 70% seawater than in freshwater. Our findings indicated that transfer from hypoosmotic to hyperosmotic water enhanced apoptosis of freshwater-type MR cells, resulting in reduction in hyperosmoregulatory ability for freshwater adaptation, and stimulated the recruitment of MR–cell complexes to develop hypoosmoregulatory ability for seawater adaptation.

순치 기간에 따른 육봉형 산천어(Oncorhynchus masou masou)의 해수 적응능력

The seawater adaptability of land-locked masu salmon Oncorhynchus masou masou via acclimation was examined for aquaculture purposes. The survival, blood chemistry, and histological changes of masu salmon (150 g) were measured after 7-, 15-, and 30-day acclimation periods. After a total of 60 days in seawater cultures that incorporated the various acclimation periods, survival was 83.5, 87.2, and 91.0% for the 7-, 15-, and 30-day periods, respectively; thus, survival increased with longer periods of acclimation. Feeding efficiencies were 32.1, 52.0, and 40.6% for the 7-, 15-, and 30-day periods, and specific growth rates were 0.14, 0.26, and 0.23%, respectively. Generally, masu salmon appeared to exhibit better growth performance after an acclimation period of 15 days. Cortisol concentrations [mean <TEX>${\pm}$</TEX> SD] for 7, 15, and 30 days of acclimation were <TEX>$21.0{\pm}6.5$</TEX>, <TEX>$17.8{\pm}4.8$</TEX>, and <TEX>$21.2{\pm}5.4\;{\mu}g/dl$</TEX>, with the lowest values occurring with 15 days of acclimation. Osmolarities were <TEX>$359.2{\pm}26.1$</TEX>, <TEX>$350.4{\pm}29.2$</TEX>, and <TEX>$354.6{\pm}29.3$</TEX> mOsm/kg, and glucose concentrations were <TEX>$60.7{\pm}20.7$</TEX>, <TEX>$72.9{\pm}17.3$</TEX>, and <TEX>$76.6{\pm}14.1$</TEX> mg/dl for the 7-, 15-, and 30-day acclimation periods, respectively (P < 0.05). The histological study revealed that both gills and both kidneys of the masu salmon exhibited middle- to end-stage and middle-stage lesions in the 7- and 15-day groups, respectively, whereas these organs only had early-stage lesions in the 30-day group in the final experiment. Therefore, the seawater acclimation of masu salmon should involve more than 30 days in seawater.

Ionic Regulation Ability in Rutilus frisii kutum Fingerlings During Sea Water Adaptation

Experiments were conducted to determine the effects of weight on the ionic regulation ability of reared Rutilus frisii kutum fingerlings during adaptation to the seawater and downstream migration. Accordingly, the ionic regulation ability of Cl-, K+, Na+ and Mg2+ in kutum fingerlings with weights of 1, 3, 5 and 7 g in three different salinities, that is 13‰ (the Caspian Sea salinity), 7‰ (estuarine area) and fresh water (as control, 0.3-0.5‰), were assessed. The blood samples were provided before being transferred as control (fresh water) and during adaptation to the sea and estuary water in a period of up to 336 h by a pooling method. The measurements of ions were carried out for blood serum Na+ and K+ and alsoplasma Cl-1 and Mg2+ by photometric methods. This investigation showed that ionic regulatory ability of kutum fingerlings depends on their weights. Results of ionic changes during the duration of 336 h (14 days) proved that unlike kutum fingerling with weights of 3, 5 and 7 g, the ionic regulation system in 1 g fingerlings were not able to expel excess ions. Further 1 g kutum were not physiologically ready (smolt) for downstream migration.

Relative Antidipsogenic Potencies of Six Homologous Natriuretic Peptides in Eels

Atrial natriuretic peptide (ANP) exhibits a potent antidipsogenic effect in seawater (SW) eels to limit excess Na(+) uptake, thereby effectively promoting SW adaptation. Recently, cardiac ANP, BNP and VNP and brain CNP1, 3 and 4, have been identified in eels. We examined the antidipsogenic effect of all homologous NPs using conscious, cannulated eels in both FW and SW together with parameters that affect drinking. A dose-response study (0.01-1 nmol/kg) in SW eels showed the relative potency of the antidipsogenic effect was in the order ANP ≥ VNP > BNP = CNP3 > CNP1 ≥ CNP4, while the order was ANP = VNP = BNP > CNP3 = CNP1 = CNP4 for the vasodepressor effect. The minimum effective dose of ANP for the antidipsogenic effect is much lower than that in mammals. ANP, BNP and VNP at 0.3 nmol/kg decreased drinking, plasma Na(+) concentration and aortic pressure and increased hematocrit in SW eels. The cardiac NPs induced similar changes in drinking, aortic pressure and hematocrit in FW eels, but aside from BNP no change in plasma Na(+) concentration. CNPs had no effect on drinking, plasma Na(+) concentration and hematocrit but induced mild hypotension in both FW and SW eels, except for CNP3 that inhibited drinking in SW eels. These results show that ANP, BNP and VNP are potent antidipsogenic hormones in eels in spite of other regulatory factors working to induce drinking, and that CNPs are without effects on drinking except for the ancestor of the cardiac NPs, CNP3.