A fish’s capacity to adapt to different levels of
environmental salinity ultimately depends on its
capacity to regulate the uptake and excretion of ions,
and to maintain its hydromineral equilibrium. The
present study shows an increase of serum osmolarity
and ion concentrations (Na+, Cl- and Ca2+) immediately
after the transference of juvenile Chinese
sturgeon from FW to BW. At 24 h after transition to
BW of 10%, serum osmolarity and ion concentrations
(Na+, Cl- and Ca2+) had begun to decrease, and
reached a new steady state after 216 h. These results
are consistent with other studies in several sturgeon
species (A. oxyrinchus, Altinok et al. 1998; A.
gu¨eldensta¨edti, Natochin et al. 1985; A. transmontanus,
McEnroe and Cech 1985; A. brevirostrum and A.
oxyrhynchus, Krayushkina 1998; A. naccarii, Cataldi
et al. 1995b, 1999; McKenzie et al. 1999; Martı´nez-
A ´
lvarez et al. 2002). These changes can be attributed
to changes in the water content in the blood, caused by
the change in environmental salinity (Plaut 1998). Like
other sturgeon, juvenile Chinese sturgeon are hyperosmotic
to fresh water and hyposmotic to sea water.
Thus, at the beginning of exposure to a hyperosmotic/
isosmotic environment, the fish would lose water
passively, and thereby undergo increases in the
concentrations of serum ions. Afterwards, the compensatory
increase in water ingestion would provide a
transitory dilution of the blood parameters. Finally,
these would return to new steady values as a result of
the rest of the osmoregulatory mechanisms (Martı´nez-
A ´
lvarez et al. 2002).
In euryhaline fish, abrupt transfer fromSWtoBWor
from BW to SW induces changes in osmotic plasma
parameters and the consequent activation of osmoregulatory
system to try to recover the original values. In
this process, two periods were described: (1) the
adaptative period, with changes in osmotic parameters,
and (2) the chronic regulatory period, where these
parameters again reach homeostasis (Holmes and
Donaldson 1969; Maetz 1974). In agreement with this,
our study shows that the acclimation of Chinese
sturgeon to BW involves two different physiological
periods, the crisis period and the stabilization period.
The crisis period, when the serum osmolarity and ion
concentrations (Na+, Cl- and Ca2+) exhibit transient
increase, occurs in the first 24 h after transference to
BW. Immediately post-transfer to BW, the critical
problem faced by sturgeon is dehydration, caused by
osmotic removal of water in the gills (Cataldi et al.
1999; McKenzie et al. 1999; Martı´nez-A´ lvarez et al.
2002). This crisis period occurs in Gulf of Mexico
sturgeon (A. oxyrinchus) at 24 h after transition to BW
(Altinok et al. 1998). The subsequent 24–216 h marks
the beginning of the stabilization period, when the
serum osmolarity and ion concentrations (Na+, Cl-
and Ca2+) start to decrease. Altinok et al. (1998) also
reported a strong increase of the osmolarity and ion
concentrations after the transference to BW in A.
oxyrinchus, peaking at 24 h, followed by a decline to
basal level.
Ion regulation maintains stable concentrations of
electrolytes in extracellular fluids. Generally, extracellular
Na+, Cl- and Ca2+ concentrations are elevated
while K+ concentrations are depressed relative to the
intracellular medium. This unequal distribution of ions
across cell membranes results in the establishment of
membrane potential. As regulatory capacity is lessened,
electrolyte concentrations are altered, disrupting
the membrane potential. As the cell is no longer
excitable, function is lost, and the cell dies (Holmes
and Donaldson 1969). Ion regulations in sturgeon are
thought to be similar to those of teleosts (Krayushkina
et al. 1995). Na+,K+-ATPase plays a critical role in
sodium and water balance in both marine and fresh
water fishes because it is involved in active sodium
transport (Borgatti et al. 1992). In the gill, Na+,K+-
ATPase drives the active ion transport processes of the
‘‘chloride’’ or ‘‘mitochondria-rich’’ cells (Marshall and
Bryson 1998). Na+,K+-ATPase similarly activates the
ion pumps in the intestine. In sea water fish, the
intestine actively absorbs Na+ and Cl- until intestine
fluid become hyposmotic to plasma, at which time
water moves passively into the plasma, thereby
maintaining overall osmotic balance (Kirsch et al.
1985; Karnaky 1998).
In our experiment, we detected a progressive
increase in gill Na+,K+-ATPase activity of fish
inhabiting brackish water, peaking at 24 h after
transference and remaining at a new steady state which
was significantly higher than the levels of fresh water
control fish (Fig. 3). The increase in plasma osmolality
reflects the challenge of the hyperosmotic medium
against the body fluids of the fish, whereupon greater
Na+,K+-ATPase activity occurs—that is, the branchial
hypo-osmoregulatory mechanisms participating
in the re-establishment of the normal values of blood
concentration were activated. Greater activity of
Na+,K+-ATPase in the gills of teleosts during acclimation
to sea water has been reported (Kirschner 1980;
Johnston and Cheverie 1985; Boeuf et al. 1985;
Fuentes et al. 1997). According to McCormick
(1995), in almost all the teleosts studied (several dozen
species), stronger salinity provoked high activity levels
of this transporter enzyme. In sturgeons, increased
activity of this enzyme has also been demonstrated
during the transition from fresh water to salt water. In
the Siberian sturgeon A. baerii, this enzyme increased
when exposed to an iso- and hyperosmotic medium
(Rodrı´guez et al. 2002, 2003). McKenzie et al. (1999),
also working with A. naccarii, found a significant
increase in branchial Na+,K+-ATPase in fish acclimated
to brackish water (23%).
When fish were transferred fromFWtoBWof 10%,
there was a marked, short-term (3 h after transfer to
brackish water) drop in the gill Na+,K+-ATPase
activity compared with that of the fish kept in fresh
water. In contrast, the spiral valve Na+,K+-ATPase
activity increased rapidly. These changes in the spiral
valve and the gill Na+,K+-ATPase activities might be
attributed to the hyperosmoregulatory mechanism
used to adapt to hyposmotic environment, i.e., active
uptake of Na+ and Cl- ions, and producing a large
amount of dilute urine (Jobling 1995). Na+-K+-
ATPase activity in the spiral valve fell rapidly at 3 h,
reaching the lowest at 24 h after transference. From
24 h to 480 h, a significant increase in the spiral valve
Na+-K+-ATPase activity was recorded in fish exposed
to 10% water salinity, and at the end of the period no
statistically significant differences were recorded
between fish kept in 0% and 10% media. This
decrease in the spiral valve Na+,K+-ATPase activity
might be the mechanism used to adapt to isosmotic/
hyperosmotic environments, i.e., reducing the ionic
Na+ and Cl- inflow. These changes of the spiral valve
Na+-K+-ATPase activity indicate that Chinese sturgeon
exposed to isoosmotic media may ingest water in
order to compensate for the osmotic water losses, as
Kirsch et al. (1984) reported for euryhaline teleost
species. This reduction in the spiral valve Na+,K+-
ATPase activity, however, also compromised intestinal
nutrient absorption, since the spiral valve is the
main site of intestinal absorption in sturgeons (Gawlicka
et al. 1995), directly affecting fish nutrition and
growth performance (Morgan and Iwana 1991).
Rodriguez et al. (2002) showed that A. baerii exposed
to isosmotic and hyperosmotic media were unable to
grow normally and had actually lost weight in
hyperosmotic media. Allen and Cech (2007) also
reported an age/body size of fish effect in hyperosmotic
adaptability. Our data do not coincide with the
results reported by Rodriguez et al. (2002). The final
body weight and SGR values in our study revealed that
fish exposed to isosmotic media grew at a faster rate
than those kept in hyposmotic media. These differences
might be due to species-specific morphophysiological
mechanisms for salinity adaptation and
tolerance, which would be directly related to the
natural history of this species. The Siberian sturgeon is
semi-anadromous, spending a significant portion of its
life in brackish water habitats (Rodriguez et al. 2002),
and Chinese sturgeon is truly anadromous, spending a
significant portion of its life in full-strength oceanic
sea water (Zhuang et al. 2002).
Na+,K+-ATPase activities in the gills and spiral
valve, SGR values and new steady states of plasma
osmolality and electrolyte concentrations of fish kept
in 273 mOsmol kg-1 all seem to indicate that
Chinese sturgeon juveniles (7 months old) were able
to regulate their hydromineral content satisfactorily
during the 20-day period. However, further research
is needed to evaluate the long-term growth of
juvenile Chinese sturgeon exposed to a plasmatic
isosmotic/hyperosmotic medium. The results of our
study indicate the existence of hyposmoregulatory
adaptive mechanisms in 7-month-old juvenile Chinese
sturgeon which enable this fish to acclimate
itself successfully to brackish water.
A fish’s capacity to adapt to different levels ofenvironmental salinity ultimately depends on itscapacity to regulate the uptake and excretion of ions,and to maintain its hydromineral equilibrium. Thepresent study shows an increase of serum osmolarityand ion concentrations (Na+, Cl- and Ca2+) immediatelyafter the transference of juvenile Chinesesturgeon from FW to BW. At 24 h after transition toBW of 10%, serum osmolarity and ion concentrations(Na+, Cl- and Ca2+) had begun to decrease, andreached a new steady state after 216 h. These resultsare consistent with other studies in several sturgeonspecies (A. oxyrinchus, Altinok et al. 1998; A.gu¨eldensta¨edti, Natochin et al. 1985; A. transmontanus,McEnroe and Cech 1985; A. brevirostrum and A.oxyrhynchus, Krayushkina 1998; A. naccarii, Cataldiet al. 1995b, 1999; McKenzie et al. 1999; Martı´nez-A ´lvarez et al. 2002). These changes can be attributedto changes in the water content in the blood, caused bythe change in environmental salinity (Plaut 1998). Likeother sturgeon, juvenile Chinese sturgeon are hyperosmoticto fresh water and hyposmotic to sea water.Thus, at the beginning of exposure to a hyperosmotic/isosmotic environment, the fish would lose waterpassively, and thereby undergo increases in theconcentrations of serum ions. Afterwards, the compensatoryincrease in water ingestion would provide atransitory dilution of the blood parameters. Finally,these would return to new steady values as a result of
the rest of the osmoregulatory mechanisms (Martı´nez-
A ´
lvarez et al. 2002).
In euryhaline fish, abrupt transfer fromSWtoBWor
from BW to SW induces changes in osmotic plasma
parameters and the consequent activation of osmoregulatory
system to try to recover the original values. In
this process, two periods were described: (1) the
adaptative period, with changes in osmotic parameters,
and (2) the chronic regulatory period, where these
parameters again reach homeostasis (Holmes and
Donaldson 1969; Maetz 1974). In agreement with this,
our study shows that the acclimation of Chinese
sturgeon to BW involves two different physiological
periods, the crisis period and the stabilization period.
The crisis period, when the serum osmolarity and ion
concentrations (Na+, Cl- and Ca2+) exhibit transient
increase, occurs in the first 24 h after transference to
BW. Immediately post-transfer to BW, the critical
problem faced by sturgeon is dehydration, caused by
osmotic removal of water in the gills (Cataldi et al.
1999; McKenzie et al. 1999; Martı´nez-A´ lvarez et al.
2002). This crisis period occurs in Gulf of Mexico
sturgeon (A. oxyrinchus) at 24 h after transition to BW
(Altinok et al. 1998). The subsequent 24–216 h marks
the beginning of the stabilization period, when the
serum osmolarity and ion concentrations (Na+, Cl-
and Ca2+) start to decrease. Altinok et al. (1998) also
reported a strong increase of the osmolarity and ion
concentrations after the transference to BW in A.
oxyrinchus, peaking at 24 h, followed by a decline to
basal level.
Ion regulation maintains stable concentrations of
electrolytes in extracellular fluids. Generally, extracellular
Na+, Cl- and Ca2+ concentrations are elevated
while K+ concentrations are depressed relative to the
intracellular medium. This unequal distribution of ions
across cell membranes results in the establishment of
membrane potential. As regulatory capacity is lessened,
electrolyte concentrations are altered, disrupting
the membrane potential. As the cell is no longer
excitable, function is lost, and the cell dies (Holmes
and Donaldson 1969). Ion regulations in sturgeon are
thought to be similar to those of teleosts (Krayushkina
et al. 1995). Na+,K+-ATPase plays a critical role in
sodium and water balance in both marine and fresh
water fishes because it is involved in active sodium
transport (Borgatti et al. 1992). In the gill, Na+,K+-
ATPase drives the active ion transport processes of the
‘‘chloride’’ or ‘‘mitochondria-rich’’ cells (Marshall and
Bryson 1998). Na+,K+-ATPase similarly activates the
ion pumps in the intestine. In sea water fish, the
intestine actively absorbs Na+ and Cl- until intestine
fluid become hyposmotic to plasma, at which time
water moves passively into the plasma, thereby
maintaining overall osmotic balance (Kirsch et al.
1985; Karnaky 1998).
In our experiment, we detected a progressive
increase in gill Na+,K+-ATPase activity of fish
inhabiting brackish water, peaking at 24 h after
transference and remaining at a new steady state which
was significantly higher than the levels of fresh water
control fish (Fig. 3). The increase in plasma osmolality
reflects the challenge of the hyperosmotic medium
against the body fluids of the fish, whereupon greater
Na+,K+-ATPase activity occurs—that is, the branchial
hypo-osmoregulatory mechanisms participating
in the re-establishment of the normal values of blood
concentration were activated. Greater activity of
Na+,K+-ATPase in the gills of teleosts during acclimation
to sea water has been reported (Kirschner 1980;
Johnston and Cheverie 1985; Boeuf et al. 1985;
Fuentes et al. 1997). According to McCormick
(1995), in almost all the teleosts studied (several dozen
species), stronger salinity provoked high activity levels
of this transporter enzyme. In sturgeons, increased
activity of this enzyme has also been demonstrated
during the transition from fresh water to salt water. In
the Siberian sturgeon A. baerii, this enzyme increased
when exposed to an iso- and hyperosmotic medium
(Rodrı´guez et al. 2002, 2003). McKenzie et al. (1999),
also working with A. naccarii, found a significant
increase in branchial Na+,K+-ATPase in fish acclimated
to brackish water (23%).
When fish were transferred fromFWtoBWof 10%,
there was a marked, short-term (3 h after transfer to
brackish water) drop in the gill Na+,K+-ATPase
activity compared with that of the fish kept in fresh
water. In contrast, the spiral valve Na+,K+-ATPase
activity increased rapidly. These changes in the spiral
valve and the gill Na+,K+-ATPase activities might be
attributed to the hyperosmoregulatory mechanism
used to adapt to hyposmotic environment, i.e., active
uptake of Na+ and Cl- ions, and producing a large
amount of dilute urine (Jobling 1995). Na+-K+-
ATPase activity in the spiral valve fell rapidly at 3 h,
reaching the lowest at 24 h after transference. From
24 h to 480 h, a significant increase in the spiral valve
Na+-K+-ATPase activity was recorded in fish exposed
to 10% water salinity, and at the end of the period no
statistically significant differences were recorded
between fish kept in 0% and 10% media. This
decrease in the spiral valve Na+,K+-ATPase activity
might be the mechanism used to adapt to isosmotic/
hyperosmotic environments, i.e., reducing the ionic
Na+ and Cl- inflow. These changes of the spiral valve
Na+-K+-ATPase activity indicate that Chinese sturgeon
exposed to isoosmotic media may ingest water in
order to compensate for the osmotic water losses, as
Kirsch et al. (1984) reported for euryhaline teleost
species. This reduction in the spiral valve Na+,K+-
ATPase activity, however, also compromised intestinal
nutrient absorption, since the spiral valve is the
main site of intestinal absorption in sturgeons (Gawlicka
et al. 1995), directly affecting fish nutrition and
growth performance (Morgan and Iwana 1991).
Rodriguez et al. (2002) showed that A. baerii exposed
to isosmotic and hyperosmotic media were unable to
grow normally and had actually lost weight in
hyperosmotic media. Allen and Cech (2007) also
reported an age/body size of fish effect in hyperosmotic
adaptability. Our data do not coincide with the
results reported by Rodriguez et al. (2002). The final
body weight and SGR values in our study revealed that
fish exposed to isosmotic media grew at a faster rate
than those kept in hyposmotic media. These differences
might be due to species-specific morphophysiological
mechanisms for salinity adaptation and
tolerance, which would be directly related to the
natural history of this species. The Siberian sturgeon is
semi-anadromous, spending a significant portion of its
life in brackish water habitats (Rodriguez et al. 2002),
and Chinese sturgeon is truly anadromous, spending a
significant portion of its life in full-strength oceanic
sea water (Zhuang et al. 2002).
Na+,K+-ATPase activities in the gills and spiral
valve, SGR values and new steady states of plasma
osmolality and electrolyte concentrations of fish kept
in 273 mOsmol kg-1 all seem to indicate that
Chinese sturgeon juveniles (7 months old) were able
to regulate their hydromineral content satisfactorily
during the 20-day period. However, further research
is needed to evaluate the long-term growth of
juvenile Chinese sturgeon exposed to a plasmatic
isosmotic/hyperosmotic medium. The results of our
study indicate the existence of hyposmoregulatory
adaptive mechanisms in 7-month-old juvenile Chinese
sturgeon which enable this fish to acclimate
itself successfully to brackish water.
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