Hypoosmoregulatory Ability Research Articles

Effects of freshwater hyperoxia and hypercapnia and their influences on subsequent seawater transfer in Atlantic salmon (<i>Salmo salar</i>) smolts

Atlantic salmon (Salmo salar) presmolts, smolts, and postsmolts compensate for a respiratory acidosis associated with 96 h of exposure to hyperoxia (100% O2; hO2), hypercapnia (2% CO2 and 98% air; hCO2), and combined hO2/hCO2 in freshwater (FW) by increasing strong ion difference, predominantly through a reduction in plasma [Cl-] (presumably via branchial Cl-/HCO3- exchange). In smolts, compensation during hO2 or hCO2 occurred within 24 h, whereas that in combined hO2/hCO2 was much slower, resulting in 33% mortality by 96 h. FW hO2 and combined hO2/hCO2 appeared to impair gill function, likely through oxidative cell damage. This resulted in reduced hypoosmoregulatory ability following subsequent transfer to seawater (SW), as indicated by changes in plasma ion levels, osmolality, and muscle water content, resulting in considerable mortalities. Interestingly, FW hCO2 appeared to enhance hypoosmoregulatory ability during subsequent SW transfer. Smolts are often transported from FW to a subsequent SW release ...

Metamorphosis in the summer flounder, Paralichthys dentatus: thyroidal status influences salinity tolerance.

Metamorphosis in the summer flounder (Paralichthys dentatus) is controlled by thyroid hormones (TH) and takes place as the larvae move from a salinity of about 35 parts per thousand (ppt) in the ocean to salinity ranging from 0-35 ppt in estuaries. Historically, the role of TH in juvenile and adult teleost osmoregulation has been ambiguous, and it is not known if TH influences larval teleost osmoregulatory development. This study addresses the influence of thyroxine (T4) on the development of tolerance to low (5 ppt) and high salinity (45 and 50 ppt) as determined by salinity tolerance tests. In untreated larvae, tolerance to both low and high salinity was high during early premetamorphosis (early pre-M) and decreased or was very low from late prometamorphosis (late pro-M) through mid-metamorphic climax (mid-MC). Salinity tolerance increased 2-3-fold during late MC when whole-animal T4 levels are highest, and reached maximum tolerance at the juvenile stage. The early induction of metamorphosis by exposing larvae in pre-M to exogenous T4 reduced tolerance to low salinity during early and mid-MC, though tolerance of fish that had developed into juveniles was not impaired. In contrast, T4 increased high salinity tolerance during early and mid-MC, and the juvenile stage. This T4-induced heterochrony in salinity tolerance with regards to developmental stage suggests that the effects of T4 on salinity tolerance may be uncoupled from accelerated metamorphosis. Treatment of larvae with thiourea (TU, an inhibitor of T4 synthesis) inhibited metamorphosis and reduced tolerance to high salinity, but did not affect tolerance to low salinity. Reduced tolerance to high salinity by TU was only partially counteracted by T4 treatment, suggesting that TU also affects hypoosmoregulatory activity by an extrathyroidal mechanism. Our findings suggest that in the summer flounder T4 plays a more important role in the development of hypoosmoregulatory ability than hyperosmoregulatory ability. J. Exp. Zool. 284:414-424, 1999.

The effect of diet and short duration hyperoxia exposure on seawater transfer in coho salmon smolts ( Oncorhynchus kisutch)

Coho salmon smolts ( Oncorhynchus kisutch) were fed one of three casein-based diets containing different levels of vitamin E (Vit. E) and Selenium (Se). Diet 1 contained no supplemental Vit. E or Se, diet 2 contained moderate levels (Vit. E=30 IU kg −1 and Se=0.2 mg kg −1) and diet 3 contained high levels (Vit. E=900 IU kg −1 and Se=1.0 mg kg −1). Smolts were reared for 6 weeks on their respective diets and then exposed to hyperoxia (364±10% atmospheric levels) for 0 h, 6 h, 24 h or 1 week (wk) and then subject to a 24-h normoxic seawater (27 ppt) challenge (0 h-SWC, 6 h-SWC, 24 h-SWC and 1 wk-SWC respectively). In diet 1, no significant increase in plasma [Cl −] was observed in the 0 h-SWC treatment relative to freshwater values, however, plasma [Cl −] was significantly elevated in seawater following 6 h, 24 h and 1 wk exposure to hyperoxia. In diets 2 and 3, plasma [Cl −] levels were significantly elevated over freshwater levels except for the 0 h-SWC at the end of the experiment. In diets 2 and 3, 2 of 12 fish died following seawater transfer in each group after being exposed to hyperoxia for 1 wk (1 wk-SWC). No other mortalities were observed during this study. Thus, exposure to hyperoxia for as little as 6 h impaired hypoosmoregulatory ability in coho salmon smolts and elevated levels of Vit. E and Se were not protective. Furthermore, plasma [Cl −] following seawater transfer was significantly greater in diet 3 than diets 1 and 2 throughout the data set, indicating that high levels of Vit. E and Se may impair hypoosmoregulatory ability during seawater transfer.

Post-smolt growth and maturation of out-of-season 0+ Atlantic salmon ( Salmo salar) reared under different photoperiods

Out-of-season smolts are currently transferred to the sea in the autumn under a decreasing photoperiod. The following experiment was undertaken to determine the effect of photoperiod on the seawater performance of out-of-season smolts. The growth and maturation of out-of-season 0+ post-smolts were monitored under different photoperiod regimes following seawater transfer. Prior to seawater transfer all fish were subjected to the same photoperiod which advanced the development of hypoosmoregulatory ability and smolt coloration. 600 smolts were transferred to 6 seawater tanks on the 11th December to form 6 groups. From the 11th December to the 11th July, three duplicate groups were subjected to a simulated natural photoperiod (SNP), a 9 h daylength (LD 9:15) or constant light (LL, i.e., a continuation of the freshwater photoperiod). From the 11th July until the termination of the experiment on the 4th December one group from each of treatments LD 9:15 and LL was subjected to SNP (groups LD 9:15/SNP and LL/SNP). The three treatments exhibited significantly ( P<0.01) different growth; fish in treatment LL and LD 9:15 exhibited the highest and lowest weights respectively. Percentage maturation was highest in group LL (24.6%) compared to the other groups (range 0–9.3%). Mortality was similar in all groups. Although these experiments were conducted in a pumped-ashore seawater system the results demonstrate the potential for photoperiod manipulation of fish in cages for improving the seawater performance of out-of-season smolts.

Changes in branchial and intestinal osmoregulatory mechanisms and growth hormone levels during smolting in hatchery-reared and wild brown trout

Evidence of smolting was studied in Danish hatchery-reared brown trout Salmo trutta L. Twenty-four hour seawater (SW) challenge tests (28‰, 10°C) at regular intervals showed that maximal hypo-osmoregulatory ability developed within a 3–4-week period in March and April. The improved ability to regulate plasma osmolality, muscle water content and plasma total [Mg] developed asynchronously, indicating that developmental changes in the gill, the gastrointestinal system and the kidney may not necessarily concur during smolting. Gill Na+, K+-ATPase activity peaked in April at the time of optimal hypo-osmoregulatory ability. Na+, K+-ATPase a -subunit mRNA level in gills was unchanged from January until April, but decreased in May in parallel with a decrease in the activity of the enzyme. In the middle region of the intestine, Na+, K+-ATPase activity increased in February and remained high until April. In the posterior region of the intestine, the activity was stable from January until April after which it decreased. In vitro fluid transport capacitity, Jv, in the middle intestine fluctuated throughout the spring. In the posterior intestine, Jv was low until late March, when it increased fivefold until early May. Drinking rate in fish transferred to SW for 24 h surged during spring. Na+, K+-ATPase activity in the pyloric caeca was elevated from March until May, and increased in response to SW transfer in June, suggesting a hypo-osmoregulatory function of the pyloric caeca. Plasma GH levels surged in FW trout during spring, concurring with the increase in gill Na+, K+-ATPase activity and SW tolerance, but peaked in May when gill Na+, K+-ATPase activity and SW tolerance were regressing. GH levels were generally low in SW-challenged fish, and there was no consistent effect of 24-h SW exposure on GH levels. In wild anadromous trout, gill Na+, K+-ATPase activity varied seasonally as in hatchery-reared fish, but peaked at higher levels suggesting a more intense smolting in fish living in their natural environment.

Effects of Insulin-like Growth Factor-I and Cortisol on Na+,K+-ATPase Expression in Osmoregulatory Tissues of Brown Trout (Salmo trutta)

The effect of recombinant bovine IGF-I (rbIGF-I) on hypo-osmoregulatory ability and the effect of rbIGF-I and cortisol (F) alone and in combination on Na+,K+-ATPase expression in fresh water (FW) acclimated brown trout (Salmo trutta) were examined in two experiments. In Experiment 1, fish were given three injections of saline or 0.01 or 0.1 μg rbIGF-I/g, respectively, and subjected to a 24-h 25 ppt seawater (SW) challenge test 24 h after the last injection. Fish treated with 0.01 and 0.1 μg rbIGF-I/g had better hypo-osmoregulatory ability than control fish as judged by their higher level of muscle water content and lower plasma osmolality after 24 h exposure to 25 ppt SW. Compared with control fish, gill Na+,K+-ATPase activity was unchanged 24 h after the first injection at either dose but significantly stimulated after three injections of either dose of rbIGF-I. In Experiment 2, fish were given three injections of saline, 0.1 μg rbIGF-I/g, 4 μg F/g, or 0.1 μg rbIGF-I + 4 μg F/g and sampled in FW 24 h after the last injection. IGF-I and F had additive stimulatory effects on Na+,K+-ATPase activity and α-subunit Na+,K+-ATPase mRNA levels in the gill. Injections of IGF-I and F alone and in combination increased Na+,K+-ATPase-immunoreactive (NKIR) cell number in the primary gill filament but had no effect on secondary lamellar NKIR cell number. NKIR cells were abundant in kidney tubules, pyloric ceca, and posterior intestine, but Na+,K+-ATPase enzyme activity was unaffected by treatment with F and/or IGF-I in these tissues. F but not rbIGF-I increasedin vitrofluid transport capacity in the posterior intestine. In addition to confirming an overall SW-adaptive effect of rbIGF-I and F in FW-acclimatedS. trutta,the study suggests the effect to be associated with stimulation of chloride cell development and Na+,K+-ATPase expression in the gill. The study indicates that the stimulatory effects of the two hormones on Na+,K+-ATPase expression are additive, highly organ specific, and restricted to the primary filament epithelium of the gill.

Influence of cortisol, growth hormone, insulin-like growth factor I and 3,3′,5-triiodo-l-thyronine on hypoosmoregulatory ability in the euryhaline teleost Fundulus heteroclitus

The capacity of cortisol, ovine growth hormone (oGH), recombinant bovine insulin-like growth factor I (rbIGF-I) and 3,3′,5-triiodo-l-thyronine (T3) to increase hypoosmoregulatory capacity in the euryhaline teleost Fundulus heteroclitus was examined. Fish acclimated to brackish water (BW, 10 ppt salinity) were injected with a single dose of hormone suspended in oil and transferred to seawater (SW, 35 ppt salinity) 10 days post-injection. Fish were sampled 24 h after transfer and plasma osmolality and gill Na+, K+-ATPase activity were examined. Transfer from BW to SW induced significantly increased plasma osmolality but not gill Na+, K+-ATPase activity. Cortisol (50 μg g−1 body weight) improved the ability to maintain plasma osmolality and to increase gill Na+, K+-ATPase activity. oGH (5 μg g−1 body weight) also increased hypoosmoregulatory ability and gill Na+, K+-ATPase activity. A cooperation between oGH and cortisol was observed in increasing hypoosmoregulatory ability but not in increasing gill Na+, K+-ATPase activity. rbIGF-I (0.5 μg g−1 body weight) alone was without effect in increasing salinity tolerance or gill Na+, K+-ATPase activity. rbIGF-I and oGH showed a positive interaction in increasing salinity tolerance, but not gill Na+, K+-ATPase activity. Treatment with T3 (5 μg g−1 body weight) alone did not increase salinity tolerance or gill Na+, K+-ATPase activity, and there was no consistent significant interaction between cortisol and T3 or between GH and T3. The results confirm the classical role of cortisol as a seawater-adapting hormone and indicate an interaction between cortisol and the GH/IGF-I axis during seawater acclimation of Fundulus heteroclitus.

Immunolocalization of Na+/K+-ATPase in branchial epithelium of chum salmon fry during seawater and freshwater acclimation

Immunolocalization of the -subunit of Na+/K+-ATPase was examined in the gill epithelium of chum salmon (Oncorhynchus keta) fry during acclimation to brackish water (25 salinity) and reintroduction to fresh water. In freshwater fish, strong immunoreactivity was associated with the large spherical cells located on the free surface of the primary lamellae, especially in those found at the base of the secondary lamellae, and with the large spherical cells located on the secondary lamellae.The large spherical cells located near the central venous sinus at the base of the secondary lamellae and in the interlamellar regions, however, showed little or no immunoreactivity. When freshwater fish were acclimated to brackish water, immunoreactivity developed in the large spherical cells near the central venous sinus concomitant with an increase in the hypo-osmoregulatory ability of the fish. In contrast, reintroduction from brackish water to fresh water caused the disappearance of the immunoreactivity in the large spherical cells near the central venous sinus and a reduction in hypo-osmoregulatory ability. During acclimation to brackish water and reintroduction to fresh water, the hypo-osmoregulatory ability of the fish did not correlate with the total number of large spherical cells located on the primary lamellae but was closely correlated with the number of large spherical cells showing strong immunoreactivity for Na+/K+-ATPase. We conjecture that these immunopositive large spherical cells are mature differentiated chloride cells, whereas the immunonegative large spherical cells are young developing chloride cells. The development of immunoreactivity for Na+/K+-ATPase in young chloride cells may be one of the most important factors in the development of hypo-osmoregulatory ability by chum salmon fry.

The effect of different periods of constant short days on smoltification in juvenile Atlantic salmon ( Salmo salar).

The present study investigated the differential effects of a range of constant photoperiods on the development of biomodality and hypoosmoregulatory ability and the changes in coloration and morphology which occur during the process of smoltification in the Atlantic salmon. The aim being to determine the photoperiod conditions required for the successful production of out-of-season (0+) smolts. In 2 separate experiments, groups of parr reared from hatch under a long day (LL or LD 23:1) and in heated water (10°C or 13°C) were subjected to a short day photoperiod (LD 8:16 or LD 10:14) for periods of 4, 5, 6, 7, 8 or 12 weeks starting at varying times from July through to September after which the groups were then returned to a long day (LD 19.5:4.5 or LD 20:4). Control groups of fish were held under a natural photoperiod and also constant long days (LD 19.5:4.5). The timing of the changes from long to short and short to long daylengths in different photoperiod regimes induced differences in seasonal profile of the various parr–smolt changes in morphology and hypo-osmoregulatory ability. The development of bimodality was related to the timing of the decrease in photoperiod, whereas hypoosmoregulatory ability and smolt coloration appeared related to the increase. The regimes which included periods of 6 or more weeks of short days advanced smoltification compared to the group under ambient photoperiod. By contrast, smoltification was not completed in the groups exposed to 4 or 5 week periods of short days or constant long days. Under all the artificial photoperiod regimes there was some degree of desynchronisation of the various constituent processes of smoltification. Collectively, these data provide information on the specific components of photoperiod regimes which are necessary for the successful production of out-of-season smolts; they also suggest that smoltification is controlled by a series of interrelated endogenous rhythms or clocks.

Circulating growth hormone levels in Atlantic salmon smolts following seawater transfer: effects of photoperiod regime, salinity, duration of exposure and season

Atlantic salmon juveniles in fresh water, both on simulated natural photoperiod (LDN) and continuous light (LL) regime, were transferred to fresh water (controls) and different salinities (28, 35 and 40‰) at five different dates over the period of parr-smolt transformation (February to June). Survival was noted and plasma samples obtained after 12, 24, 48, 96 and 192 h of salinity exposure. Survival and plasma sodium levels were used to assess hypoosmoregulatory ability, and plasma growth hormone (GH) levels measured to elucidate the possible role of this hormone in the salinity tolerance of smolting salmon. The fish on LDN photoperiod had markedly elevated plasma GH levels as well as improved hypoosmoregulatory ability in June, whereas these changes were inhibited by the LL regime. Salinity exposure caused a transient increase in circulating GH levels. The response was complex, both in regard to the date of exposure, the photoperiod regime, and the level of salinity. The data indicate that elevated plasma GH levels in smolting Atlantic salmon in freshwater are of importance for the subsequent hypoosmoregulatory ability in seawater. However, it is questioned whether the transient increase in GH following salinity exposure represents a specific short-term hypoosmoregulatory role.

Growth, maturation and survival of out-of-season 0+ and 1+ Atlantic salmon ( Salmo salar) smolts

Out-of-season 0+ and 1+ smolts were reared at a Scottish hatchery using two photoperiod regimes. Potential S2 parr (group 1+) were reared under a 7-month compressed photoperiod regime beginning on the 22nd of February. Potential S1 parr (group 0+) reared in heated water under LL were exposed to a photoperiod of LD 8:16 from the 9th of August to the 4th of October when the photoperiod was increased to LD 18:6. Changes in condition factor and hypo-osmoregulatory ability were used to assess the completion of smoltification in the two groups. 27 000 1+ smolts (group 1+) were transferred to sea water on the 15th of November (mean weight 109±3.1 g) and 54 000 0+ smolts (group 0+) were transferred on the 21st of December (mean weight 59.8±1.4 g). The group 1+ fish were harvested after 12 months in sea water at harvest weights ranging from 2.5–2.7 kg. The group 0+ fish were harvested over a 5-month period, which began 15 months after seawater transfer, harvest weights ranged from 2.3 to 4.2 kg. The growth rates over the entire cycle were similar (range 0.9–0.99% day −1) in the two out-of-season groups and a group of natural smolts transferred during April. However, the pattern of growth during the production cycle differed between out-of-season and natural smolts. Cumulative mortality did not exceed 8% and maturation did not exceed 6.5% for the out-of-season smolts during the seawater period. It is concluded that the overall performance of out-of-season smolts parallels that of natural smolts and that their production provides a commercially important strategy for spreading the availability of market-sized (3–4 kg) salmon.

Gill Na +–K +-ATPase activity and hypoosmoregulatory ability of seaward migrating smolts of anadromous Atlantic salmon ( Salmo salar), sea trout ( Salmo trutta) and Arctic char ( Salvelinus alpinus) in the Hals river, northern Norway

Gill Na +–K +-ATPase activity and hypoosmoregulatory ability of seaward migrating smolts of anadromous Atlantic salmon, sea trout and Arctic char were compared in a river system where all three species co-exist. Migrating Atlantic salmon and Arctic char were of similar size (mean lengths 13.3 and 14.6 cm, respectively) whereas the trout smolts were significantly larger (mean length 19.1 cm). The char smolts had the lowest condition factor and the trout smolts the highest. Migrating smolts of all three species had similar high gill Na +–K +-ATPase activity. The mean plasma Cl − concentrations after 24 h in seawater were not significantly different between Atlantic salmon and sea trout (about 150 and 160 mM, respectively), whereas these of Arctic char had significantly higher mean concentrations (about 180 mM). Smolts of both Atlantic salmon and sea trout in river Halselva in northern Norway were pre-adapted to seawater, perhaps the salmon smolts somewhat better despite their smaller body size. The small-size, first-time migrants of the Arctic char were not pre-adapted to life in full-strength seawater.

Changes in seawater tolerance and gill Na +,K +-ATPase activity during desmoltification in Atlantic salmon kept in freshwater at different temperatures

Atlantic salmon ( Salmo salar L.) smolts were maintained in freshwater past their normal time of seawater transfer (early June) and reared at temperatures of 6, 10, 12 and 14°C until late July. Reduction in gill Na +,K +-ATPase activity was temperature dependent, with a significant decrease at 12 and 14°C towards the end of the study. The duration of the period of good smolt status, as determined by gill Na +,K +-ATPase activity, was approximately 280–350°C at 12 and 10°C, with an extended period at 14°C (450°C). All groups maintained good hypo-osmoregulatory ability despite a reduction in gill Na +,K +-ATPase activity in freshwater. Gill Na +,K +-ATPase activity was higher in seawater challenged smolts at 12 and 14°C towards the end of the study. Desmoltification in Atlantic salmon is not a synchronised process as normally observed during smolting, nor is it a parr-reversion as seen among some Pacific salmonids ( Oncorhynchus spp.).

Wild and hatchery-reared brown trout, Salmo trutta, differ in smolt related characteristics during parr–smolt transformation

Morphological changes during parr–smolt transformation are generally less apparent in hatchery-reared than wild salmonids. The aim of the present study was to investigate possible differences between wild and hatchery-reared brown trout in regard to physiological characteristics during smoltification. Plasma growth hormone levels, hypoosmoregulatory ability, gill Na +, K +-ATPase activity and condition factor were compared between wild and hatchery-reared fish from the same river stock, in two different streams on the Swedish coast. Plasma growth hormone levels were consistently higher in wild compared with hatchery-reared trout, and the growth hormone levels increased in wild fish from one of the streams after a 24 h seawater challenge test. At the time of parr–smolt transformation, there was a peak in gill Na +, K +-ATPase activity, which coincided with the lowest plasma sodium levels. Wild fish possessed consistently higher gill Na +, K +-ATPase activity and lower plasma sodium levels compared with hatchery-reared fish. The condition factor of wild fish decreased throughout the smoltification period, in both river strains, whereas the hatchery-reared fish had consistently high condition factor. It is concluded that the artificial environment of hatchery-reared anadromous brown trout can depress the natural parr–smolt transformation, and that this may adversely affect the success of seawater migration and long-term survival of the fish.

Evidence for Growth Hormone/Insulin-like Growth Factor I Axis Regulation of Seawater Acclimation in the Euryhaline TeleostFundulus heteroclitus

The ability of ovine growth hormone (oGH), recombinant bovine insulin-like growth factor I (rbIGF-I), recombinant human insulin-like growth factor II (rhIGF-II), and bovine insulin to increase hypoosmoregulatory capacity in the euryhaline teleostFundulus heteroclituswas examined. Fish acclimated to brackish water (BW, 10 ppt salinity, 320 mOsm/kg H2O) were injected with a single dose of hormone and transferred to seawater (SW, 35 ppt salinity, 1120 mOsm/kg H2O) 2 days later. Fish were sampled 24 h after transfer and plasma osmolality, plasma glucose, and gill Na+,K+-ATPase activity were examined. Transfer from BW to SW increased plasma osmolality and gill Na+,K+-ATPase activity. Transfer from BW to BW had no effect on these parameters. rbIGF-I (0.05, 0.1, and 0.2 μg/g) improved the ability to maintain plasma osmolality and to increase gill Na+, K+-ATPase activity in a dose-dependent manner. oGH (0.5, 1, and 2 μg/g) also increased hypoosmoregulatory ability but only the higher doses (2 μg/g) significantly increased gill Na+,K+-ATPase activity. oGH (1 μg/g) and rbIGF-I (0.1 μg/g) had a significantly greater effect on plasma osmolality and gill Na+,K+-ATPase activity than either hormone alone. rhIGF-II (0.05, 0.1, and 0.2 μg/g) and bovine insulin (0.01 and 0.05 μg/g) were without effect. The results suggest a role of GH and insulin-like growth factor I (IGF-I) in seawater acclimation ofF. heteroclitus.Based on these findings and previous studies, it is concluded that the capacity of the GH/IGF-I axis to increase hypoosmoregulatory ability may be a common feature of euryhalinity in teleosts.

Acute silver toxicity to seawater‐acclimated rainbow trout: Influence of salinity on toxicity and silver speciation

AbstractWhile there is considerable information on freshwater toxicity of Ag to fish, little is known about the toxicity of Ag in marine systems. In the present study, standard 96‐h acute toxicity tests were performed on seawater‐acclimated rainbow trout in water at 25‰salinity. Toxicity tests were repeated in various salinities (15, 20, 25, and 30‰) at the 50% lethal concentration (LC50) for Ag in 25‰ to investigate the influence of salinity on toxicity. Computer speciation modeling by MINEQL+ was performed in order to gain insight into the variation of Ag species over a range of salinities from freshwater to oceanic seawater at S2–‐free conditions. At 25‰salinity the LC50 was determined to be 3.75 μM (401 μg/L) Ag. No mortality was observed at this Ag concentration in 15 or 20‰ salinities. Mortality increased to 50% in 25‰ seawater and again to 67% at 30‰. Based on the results of speciation modeling, an increase in a more toxic AgCln species is unlikely. The increased toxicity with salinity may instead be linked to an incomplete hypoosmoregulatory ability of the rainbow trout.

Calcium metabolism and osmoregulation during sexual maturation of river running Atlantic salmon

As Atlantic salmon return from the ocean to undertake the anadromous spawning migration up the river of origin, profound changes in calcium metabolism and osmoregulation take place. Using tartrate resistant acid phosphatase as a marker, scale osteoclast activity was found to increase throughout sexual maturation and spawning migration. Thus, the participation of osteoclasts in the elevated scale resorption observed during this phase is established. As calcium was simultaneously accumulated in the female gonads, it is proposed that the scales are resorbed in order to provide calcium for the growing ovaries. Plasma oestradiol‐17β levels were elevated in females during sexual maturation, and had decreased at the time of spawning. Plasma testosterone levels were similar in males and females during the first part of the upriver migration, but had increased in males and decreased in females at spawning. In addition to the role of these sex steroids in the gonadal growth, their possible involvement in the increased scale resorption during this phase is discussed. Plasma growth hormone and thyroxine levels were elevated in both sexes at spawning, with the triiodothyronine/thyroxine (T3/T4) ratio declining sharply, indicating possible roles for these hormones in the maturational process. The relatively low gill Na+, K+‐ATPase activity of salmon caught in the estuary implies that the fish had already adapted to a hypoosmotic environment. During the upriver migration, the gill Na+, K+‐ATPase activity decreased further, indicating that the hypoosmoregulatory ability was suppressed further during sexual maturation and spawning migration.

Calcium metabolism and osmoregulation during sexual maturation of river running Atlantic salmon

As Atlantic salmon return from the ocean to undertake the anadromous spawning migration up the river of origin, profound changes in calcium metabolism and osmoregulation take place. Using tartrate resistant acid phosphatase as a marker, scale osteoclast activity was found to increase throughout sexual maturation and spawning migration. Thus, the participation of osteoclasts in the elevated scale resorption observed during this phase is established. As calcium was simultaneously accumulated in the female gonads, it is proposed that the scales are resorbed in order to provide calcium for the growing ovaries. Plasma oestradiol-17β levels were elevated in females during sexual maturation, and had decreased at the time of spawning. Plasma testosterone levels were similar in males and females during the first part of the upriver migration, but had increased in males and decreased in females at spawning. In addition to the role of these sex steroids in the gonadal growth, their possible involvement in the increased scale resorption during this phase is discussed. Plasma growth hormone and thyroxine levels were elevated in both sexes at spawning, with the triiodothyronine/thyroxine (T3/T4) ratio declining sharply, indicating possible roles for these hormones in the maturational process. The relatively low gill Na+, K+-ATPase activity of salmon caught in the estuary implies that the fish had already adapted to a hypoosmotic environment. During the upriver migration, the gill Na+, K+-ATPase activity decreased further, indicating that the hypoosmoregulatory ability was suppressed further during sexual maturation and spawning migration.

Relationships of salinity tolerance to immunolocalization of Na+,K(+)-ATPase in the gill epithelium during seawater and freshwater adaptation of the guppy, Poecilia reticulata.

The relationships of salinity tolerance to immunolocalization of Na+,K(+)-ATPase in the gill epithelium were examined during seawater and freshwater adaptation of the guppy. In fresh water, immunoreactivity for Na+,K(+)-ATPase appeared in two types of chloride cells, which are located on the primary lamellae of the gills. Immunoreactivity was strong in the chloride cells located at the base of the secondary lamellae and weak in the chloride cells located at the interlamellar region. During seawater adaptation, the strongly-immunoreactive chloride-cells increased in number and size while the weakly-immunoreactive chloride-cells decreased in number with an increase in salinity tolerance. In the fish of the seawater-adapted strain, on the other hand, most of the chloride cells were located at the base of the secondary lamellae and showed strong immunoreactivity. During freshwater adaptation, the strongly-immunoreactive chloride-cells decreased in number and size while the weakly-immunoreactive chloride-cells increased in number with a decrease in salinity tolerance. A positive correlation was observed between the salinity tolerance and the occupying area of the strongly-immunoreactive chloride-cells while a negative correlation was observed between the salinity tolerance and the occupying area of the weakly-immunoreactive chloride-cells during the seawater and freshwater adaptation. These results directly suggested that not only the occupying area of chloride cells but also the expression of Na+,K(+)-ATPase protein in the cells is important with respect to the osmoregulatory function in the gills and hypoosmoregulatory ability at the individual level.

Responses of Two Semiterrestrial Isopods, Ligia exotica and Ligia taiwanensis (Crustacea) to Osmotic Stress

When immersed in fresh water, Ligia taiwanensis is a poorer osmoregulator than Ligia exotica ,a s judged by a lower LD50 at 96 hr and by the osmolalities of haemolymph. Animals appear to osmoregulate more efficiently in air. On immersion, both species displayed hyper- and hypo-osmoregulatory ability. Both species subjected less osmotic selection pressure during their inland colonization. The results suggest that a route of terrestrial colonization not involving transitional freshwater stresses had been taken by L. exotica and L. taiwa- nensis. comp biochem physiol 118A;1:141-146, 1997. © 1997 Elsevier Science Inc.