Effect of cadmium on survival,osmoregulation and gill structure of the Sunda prawn,Macrobrachium sintangense (de Man), at different salinities

The prawn Macrobrachium sintangense is likely to be subjected to occasional exposure to combined metal and saline stressors in its natural environment. This research evaluated the acute toxicity (96?h LC₅₀) of cadmium (Cd) on the prawn M. sintangense, with respect to the osmoregulatory capacity (OC) of prawns and to document histological changes in the gills after exposure to sublethal Cd concentrations at different salinities. The 96?h LC₅₀ of Cd to M. sintangense decreased with increasing salinity. The 96?h LC₅₀ values were 89.12 (72.53–109.50), 681.26 (554.20–837.46) and 825.37 (676.99–1006.27) μg CdL^?1 at 0, 10 and 20 ppt, respectively. The OC of prawns exposed to 30?μg?CdL^?1 at 0 ppt and to 300?μg?CdL^?1 at10 ppt decreased significantly compared with that of control prawns exposed to 0 and 10 ppt respectively. Swelling, hyperplasia and necrosis of gill lamellae resulting in the loss of marginal canals were observed in the gills of prawns exposed to 30?μg?CdL^?1 at 0 ppt and to 300?μg?CdL^?1 at 10 ppt for 7?days.     

Salinity tolerance in different life history stages of an invasive false mussel Mytilopsis sallei Recluz, 1849: implications for its restricted distribution

ABSTRACT

Although the false mussel Mytilopsis sallei Recluz, 1849 is recognised as an aggressive invasive species, its populations in several estuaries in Thailand are restricted to small areas. A salinity gradient is a major characteristic of its habitat, hence the effect of various salinity levels (0–40?ppt) on the mortality of larvae, juveniles and adults of M. sallei was investigated. Condition Indices of adults reared at different salinity levels for two months were measured. Spatial and temporal variations of salinity and false mussel abundance in a canal with a salinity gradient were also monitored. After an acute (48?h) test, survival of larvae was highest at salinity levels of 12.5 and 16.25?ppt and decreased at lower and higher levels. Juveniles survived at all salinity levels, but most adults died in the first 24?h at a salinity of 40?ppt, while condition indices were lowest at salinity levels of 30 and 35?ppt. In the field survey, highest false mussel abundance was consistently found at the middle part of the canal with mid-range salinity. The results suggested that salinity is a determinant of survival in M. sallei larvae and potentially regulates the dispersal success of false mussels. However, the importance of salinity was marginal in the later stages of its life history.     

Effect of salinity on osmoregulatory patch epithelia in gills of the blue crab Callinectes sapidus

In euryhaline crabs, ion-transporting cells are clustered into osmoregulatory patches on the lamellae of the posterior gills. To examine changes in the branchial osmoregulatory patch in the blue crab Callinectes sapidus in response to change in salinity and to correlate these changes with other osmoregulatory responses, crabs were acclimated to a range of salinities between 10 and 35 ppt. When crabs that had been acclimated to 35 ppt were subsequently transferred to 10 ppt, both the size of the osmoregulatory patch on individual gill lamellae and the specific activity of Na+, K+-ATPase in whole-gill homogenates increased only after the first 24 h of exposure to dilute seawater. Enzyme activity and size of patch area increased gradually and reached their maxima (increasing by 200% and 60%, respectively) 6 days following transfer to 10 ppt seawater and then remained at these levels. Patch size at acclimation varied inversely with the salinity for seawater dilutions below 26 ppt (the isosmotic point of the crab), although it did not vary in salinities at or above 26 ppt. Thus, the size of the patch clearly is modulated with acclimation salinity, but it increases only in those salinities in which the crab hyperosmoregulates. An increase in the total RNA/DNA ratio in gill homogenates, the lack of mitotic figures in the lamellae, and the lack of incorporation of bromodeoxyuridine into nuclei of lamellar epithelial cells during acclimation to dilute seawater were interpreted as evidence that no cell proliferation had occurred and that increases in the size of the osmoregulatory patch occurred through differentiation of existing gas exchange cells or of undifferentiated epithelial cells into ion-transporting cells.     

Adaptive branchial mechanisms in the sturgeon Acipenser naccarii during acclimation to saltwater

Variations of Na(+)/K(+)-ATPase activity and fatty-acid composition in the gills of the sturgeon Acipenser naccarii subjected to progressive acclimation to full seawater (35 ppt) were determined in relation to the hypo-osmoregulatory capacity of this species in the hyperosmotic medium. Blood samples were taken and gills arches were removed at intermediate salinity levels between 0 and 35 ppt and after 20 days at constant salinity (35 ppt). Plasma osmolality and Na(+)/K(+)-ATPase activity increased significantly with growing environmental salinity. Total saturated fatty acids (SFAs) decreased, while total polyunsaturated fatty acids (PUFAs) increased significantly with increasing salinity due mainly to changes in n-3 PUFAs (20:5n-3 and 22:6n-3). The n-3/n-6 ratio increased significantly during the acclimation process. The results show a direct relationship between salinity, increased gill Na(+)/K(+)-ATPase activity and ultrastructural changes of the gill chloride cells. Changes in the fatty-acid composition in gills of A. naccarii during progressive acclimation to full seawater suggest that variations of gill fatty acids may also have a role in osmoregulatory mechanisms.     

Salinity tolerance and osmoregulation of the arctic marine amphipods Onisimus litoralis (Kroyer) and Anonyx nugax (Phipps)

Summary The gammarid amphipod Onisimus litoralis, which inhabits arctic and subarctic intertidal and under-ice habitats, is a euryhaline hyperosmotic regulator. It survives 10 d exposures to salinities from 5 to 55 ppt. It hyperregulates its hemolymph osmolality during 3 h exposures to dilutions of 33 ppt seawater and remains hyperosmotic for at least 2 w. The hemolymph is isosmotic to the medium after 12 h exposures to salinities higher than 33 ppt. The gammarid amphipod Anonyx nugax, which inhabits arctic and subarctic subtidal areas, tolerates salinities from 23 to 45 ppt with little mortality. Unlike Onisimus, however, it is an osmoconformer and its hemolymph becomes isosmotic to all dilute salinities within its tolerance range after 12 h and to concentrated media after 3 h. The salinity tolerances and osmoregulatory abilities of both species are reflected in their distributions in the field.     

Survival,haemolymph osmolality and tissue water of Penaeus chinensis juveniles acclimated to different salinity and temperature levels

Survival, growth, haemolymph osmolality and tissue water of Penaeus chinensis (Osbeck) juveniles (0.11 ± 0.04 g) were investigated, after they were acclimated to 10, 20, 30 and 40 ppt from 33 ppt for 14 days at 24°C, and then acclimated to 12, 18, 24 and 30°C at each salinity for 14 days. The survival of shrimp was the lowest at 10 ppt and 12°C. Growth of shrimp increased with increased temperature in the range 12–24°C, with no significant difference among four salinity levels at 18, 24 and 30°C. Haemolymph osmolality increased with increased salinity, and decreased with increased temperature. The isosmotic point computed from the linear relationship between haemolymph osmolality and medium osmolality was 664, 632, 629 and 602 mOsm/kg which is equivalent to 25.2, 24.1, 24.0 and 23.1 ppt at 12, 18, 24 and 30°C, respectively. Tissue water decreased with increased medium osmolality and haemolymph osmolality. The slope obtained from the relationship between haemolymph osmolality and medium osmolality indicated that there is an impairment of osmoregulatory ability for the P. chinensis juveniles at 12°C.     

Cloning and Expression of Four Aquaporin Homologs from the Chinese Black Sleeper (Bostrychus sinensis): The Effects of Salinity Acclimation

Several fish species are known to possess mechanisms that allow them to adapt to environments with different salinities. The aim of this study was to investigate the effects of salinity on the expression of aquaporins (aqp1a, aqp3a, aqp8a, and aqp9a) in the gills and intestines of Chinese black sleeper. After 30 days of acclimation, the expression of aqp1a, aqp3a, and aqp9a in the gills was significantly higher in fish transferred to 5 ppt than in those transferred to 40 ppt seawater, whereas aqp8 expression was lower. In contrast, aqp1a, aqp3a, and aqp8a expression in the intestines was higher in fish acclimated in 40 ppt than in those acclimated in 5 ppt. During abrupt salinity acclimation, the levels of aqp1a and aqp9a in the gills varied over time in fish acclimated in 5 ppt, but not in 40 ppt. The aqp3a levels in gills were higher in the 5 ppt group after 24 h than in the 40 ppt. The expression level of aqp8a in gills was higher in 40 ppt than in 5 ppt, except for that at 12 h. In the intestines, expression level of aqp1a and aqp8a were significantly upregulated from 12 to 48 h following acclimation in 40 ppt and aqp3a was higher in 40 ppt group than in 5 ppt, while aqp9a expression exhibited an opposite trend. These findings suggest that aqp1a, aqp3a, aqp8a and aqp9a may play a major osmoregulatory role in water transport in the gills and intestines during acclimation to different salinity environment.

     

Effects of salinity on growth and survival of African sharptooth catfish (clarias gariepinus) larvae

Growth and survival of replicate batches of African sharptooth catfish (Clarias gariepinus) larvae were monitored in 0, 2.5, 5.0, 7.5 and 10 ppt salinity. No significant differences in mortality or growth rate were evident between 0 and 5 ppt salinity. At 7.5 ppt mortality rate was higher and larval growth rate declined in comparison to the lower salinities. At 10 ppt all larvae died within 48 hours. The condition factor of the larvae similar between 0–2.5 ppt and displayed a declining trend between 2.5–7.5 ppt. Osmoconcentratkm of blood plasma of C. gariepinus in fresh water was 280 ± 20 mOsm/kg which is equivalent to 9.5 ppt salinity. It was concluded that 0–2.5 ppt is the optimal sclinity range for larval rearing and that short-term exposure to higher salinities (2.5–7.5 ppt) could be effective in the treatment of ectoparasitic diseases.     

Multiple Euryhaline Lineages of Centrohelids (Haptista: Centroplasthelida) in Inland Saline Waters Revealed with Metabarcoding

The diversity of centrohelids in inland saline waters was studied with metabarcoding for the first time. The fragment of V6–V7 regions of 18S rDNA was sequenced with newly designed primers. Obtained OTUs were identified with molecular phylogenetic analysis and comparison of the signatures in 39es9 hairpin of V7. The obtained data included some OTUs, which could be attributed to four described species, but the majority belonged to previously established or novel environmental clades. Along with some presumably marine/brackish clades and freshwater/low salinity (0–2 ppt) clades, seven presumable species demonstrating broad (from 1–2 up to 78 ppt) salinity tolerance were detected. A number of OTUs belonged to Raphidocystis contractilis, which is known from three independent findings in brackish habitats only. Thus, it was assumed that this species is stenohaline and specifically adapted to salinity 5–15 ppt. The high level of salinity tolerance was suggested for centrohelids before based on morphology, which was used to justify their cosmopolitan distribution. Later these views were criticized based on environmental sequencing, but the results of the current survey indicate, that at least some species are present at salinities from almost freshwater (1–2 ppt) to twice oceanic (78 ppt) and are presumably capable of overcoming oceanic salinity barriers for their distribution.     

Growth and physiological parameters of whiting (Sillaginodes punctata) in relation to salinity

This study assessed the impact of salinity on whiting (Sillaginodes punctata) in an attempt to understand the mechanisms by which salinity could potentially influence habitat selection and growth of King George whiting in southern Australia. The experiment included whiting of two age classes, young of the year (YOY) and 2⁺ year, at three salinities (30, 40, 50 ppt). YOY whiting showed no significant difference in length or weight gain, specific growth rate, feed intake, food conversion ratio or condition factor when exposed to the three salinities for 72 day. Plasma osmolality of YOY whiting was not significantly different at any salinity, although it was significantly lower than that of 2⁺ year whiting. The 2⁺ year whiting showed significantly higher plasma osmolality than the YOY. Blood plasma potassium and chloride levels of 2⁺ year fish at 50 ppt were significantly higher than those at 30 ppt and 40 ppt. Blood sodium levels at 50 ppt were significantly higher than at 30 ppt but the sodium level at 40 ppt was not different from 30 ppt or 50 ppt. Haematocrit of 2⁺ whiting was significantly higher at 30 than at 50 ppt while haematocrit at 40 ppt was not different from 30 or 50 ppt. The 2⁺ year‐old whiting had a more pronounced increase in plasma osmolality and plasma ions at high salinities, indicating poorer osmoregulatory capacity in older fish. This study provides physiological evidence to partially explain habitat occupancy and growth in relation to salinity of different age groups of whiting in southern Australia.     

Influence of salinity on dimethyl sulfide and methanethiol formation in estuarine sediments and its side effect on nitrous oxide emissions

We investigated the regulatory effect of salinity on the production of dimethylsulfide (DMS) and methanethiol (MeSH) in estuarine sediments and the potential interactions with the nitrous oxide (N₂O) reductase step of the denitrification pathway. This was achieved by monitoring DMS, MeSH and N₂O accumulation in sediment slurries retrieved from a temperate estuary (Ave, NW Portugal). Treatments were performed with and without amendments of potential sulfur gas precursors, DMSP (0–50?μM) or methionine (0–500?μM) at different salinities (0, 15 and 30?ppt). Experimental increases of salinity inhibited DMS accumulation under both oxic and anoxic incubation conditions, and the pattern was observed whether DMSP or methionine was added or not, i.e. lower salinities stimulated DMS net production. In contrast, MeSH tended to accumulate to higher concentrations in higher salinity treatments (15 and 30?ppt). Our results also suggest that while salinity had a direct influence on N₂O accumulation, it also may modulated N₂O production through its regulatory effect on the formation of MeSH, a compound previously shown to inhibit N₂O reduction activity. Overall, our results suggest that changes in salinity may have an important regulatory role in net production of DMS, MeSH and N₂O and their potential emissions to the atmosphere.     

Acute changes in gill Na+-K+-ATPase and creatine kinase in response to salinity changes in the euryhaline teleost, tilapia (Oreochromis mossambicus)

Some freshwater (FW) teleosts are capable of acclimating to seawater (SW) when challenged; however, the related energetic and physiological consequences are still unclear. This study was conducted to examine the changes in expression of gill Na(+)-K(+)-ATPase and creatine kinase (CK) in tilapia (Oreochromis mossambicus) as the acute responses to transfer from FW to SW. After 24 h in 25 ppt SW, gill Na(+)-K(+)-ATPase activities were higher than those of fish in FW. Fish in 35 ppt SW did not increase gill Na(+)-K(+)-ATPase activities until 1.5 h after transfer, and then the activities were not significantly different from those of fish in 25 ppt SW. Compared to FW, the gill CK activities in 35 ppt SW declined within 1.5 h and afterward dramatically elevated at 2 h, as in 25 ppt SW, but the levels in 35 ppt SW were lower than those in 25 ppt SW. The Western blot of muscle-type CK (MM form) was in high association with the salinity change, showing a pattern of changes similar to that in CK activity; however, levels in 35 ppt SW were higher than those in 25 ppt SW. The activity of Na(+)-K(+)-ATPase highly correlated with that of CK in fish gill after transfer from FW to SW, suggesting that phosphocreatine acts as an energy source to meet the osmoregulatory demand during acute transfer.     

Alteration of environmental salinity modulates atrial natriuretic peptide concentrations in the gills of the oyster,Crassostrea virginica

Because gills are frequently important in ion transport and osmoregulation, the present investigation was designed to determine if atrial natriuretic peptides (putative osmoregulatory peptides) are present within the gills of a representative euryhaline osmoconforming invertebrate. Utilizing three radioimmunoassays devised to amino acids 1–30, 31–67, and 99–126 of the 126 amino acid prohormone, the gills of 72 eastern oysters, Crassostrea virginica were examined and each found to contain atrial natriuretic peptides, whose content was approximately one-seventh of that within the heart of the oyster. In high salinity (32 parts per thousand, ppt) the content of atrial natriuretic peptides recognized by each of the three radioimmunoassays was lower at 3 days (P < 0.005) and then became higher (P < 0.001) compared with their content in medium salinity (21 and/or 27 ppt). The content of atrial natriuretic peptides within gills in a low salinity environment (8 ppt) was significantly lower (P < 0.05) at 3, 7, and 14 days compared with their content in either medium or high salinity environments. We conclude that atrial natriuretic peptides are present within the gill tissues of Crassostrea virginica and that their content changes with alterations in environmental salinity. Their lower content at 3 days in high salinity implies a release of stored natriuretic-like peptides that are re-synthesized by 1 week. The decreased content of atrial natriuretic peptides at 3 days, 1 and 2 weeks in low salinity versus medium or high salinity environments suggests that their synthesis is decreased when exposed to a lower salinity in the environment.     

Effect of sudden salinity change on Penaeus latisulcatus Kishinouye osmoregulation, ionoregulation and condition in inland saline water and potassium-fortified inland saline water

Two trials were conducted to determine the effect of sudden decrease in salinity of raw and potassium-fortified inland saline water on western king prawn Penaeus latisulcatus osmoregulation, ionoregulation and condition. Prawns were subjected to salinity decrease over 1 h from 32 to 25 ppt in the first trial and from 27 to 20 ppt in the second trial in three water types: inland saline water with potassium fortified to 100% and 80% of the marine water concentration (IS100, IS80), and raw inland saline water (ISW). In the first trial condition and ingestion rate were monitored over 19 days following salinity change. In the second trial condition, haemolymph osmo- and iono-regulation were recorded over 48 h following salinity change. In the first trial, 100% mortality was observed in ISW by day 13, with final survival 94% in IS80 and 100% in IS100. Tail muscle moisture content increased significantly (P < 0.05) over time in both trials and in all water types, suggesting loss of energy reserves. In the second trial, serum osmolality, sodium concentration and osmoregulatory capacity decreased following salinity change, stabilising by 24 h in IS100 and IS80 but continuing to decrease till 48 h in ISW, suggesting partial breakdown of osmoregulatory function in the potassium-deficient medium. Prawns were stronger regulators of divalent than monovalent cations. These trials demonstrate that potassium-deficient inland saline water requires fortification with potassium to allow prawn survival and efficient osmoregulation.     

Effect of temperature and salinity on the energetics of juvenile gray snapper (Lutjanus griseus): implications for nursery habitat value

Gray snapper (Lutjanus griseus) encounter a wide range of temperatures and salinities in nearshore and estuarine juvenile habitats. The energetic response of juvenile gray snapper to temperature and salinity was measured in laboratory experiments to determine the influence of these physicochemical factors on the potential value of different juvenile nurseries. Maximum consumption and growth rates of juvenile (25-50 mm SL) gray snapper were determined in 12-day trials at 20 temperature/salinity combinations representing conditions in juvenile habitats. Ad libitum feeding level of individual fish was measured daily. Maximum weight specific feeding rate increased significantly with temperature and salinity; however, the effect of salinity was much less than that of temperature. Linear growth rate and specific growth rate both increased with temperature, and salinity did not have a significant effect on either. Gross growth efficiency (K₁, growth×consumption⁻¹*100) increased with temperature and was significantly lower at high salinities, indicating increased energetic costs. The higher K₁ at lower salinities has several implications for juvenile gray snapper in low salinity habitats: (1) they would need less food to achieve the same somatic growth as juveniles in high salinity habitats; (2) they would have higher growth at limited ration levels as compared to high salinity habitats; and (3) they would have less impact on prey populations than higher salinity habitats assuming similar gray snapper densities.     

Osmo and ionic regulation of black tiger prawn (Penaeus monodon Fabricius 1798) juveniles exposed to K(+) deficient inland saline water at different salinities

An 11-day trial was conducted to investigate the osmoregulatory capacity (OC) and regulation of K(+), Na(+), Ca(2+) and Mg(2+) of Penaeus monodon juveniles when exposed to K(+) deficient inland saline water (ISW) of four different salinities (5, 15, 25 and 35 ppt). The survival of juveniles showed a positive linear relationship (R(2) ranging from 0.72 to 0.98) with salinity. At the end of the trial, juveniles were able to survive only in 5 ppt of ISW and showed no changes in OC when transferred from ocean water (OW) to ISW. Further, the OC of juveniles in 5 ppt of ISW was significantly different (P<0.05) from the OC of juveniles exposed to 15, 25 and 35 ppt and exhibited strong serum K(+), Na(+), Ca(2+) and Mg(2+) regulation monitored over 16 h. In contrast, at 35 ppt, significant decrease (P<0.05) in serum K(+) and Mg(2+) concentrations and accumulation of serum Na(+) concentration occurred after 16 h of exposure to ISW. At higher salinity, an increase in serum Na(+) concentration leads to an increase in the serum osmolality of the juveniles, which in turn causes decrease in the OC of the juveniles. The results of this study suggest that K(+) deficiency in ISW has a negative effect on survival, OC and the ability of P. monodon juveniles to regulate serum Na(+), K(+), Ca(2+) and Mg(2+) concentrations. These effects are compounded as salinity increases.     

Effects of the multiple stressors high temperature and reduced salinity on the photosynthesis of the hermatypic coral Galaxea fascicularis

This study investigates the physiological responses in the hermatypic coral Galaxea fascicularis exposed to salinity stress (from 37 ppt to 15 ppt) for 12 h, combined effects of reduced salinity (from 37 ppt to 20 ppt) and two temperatures (26 °C and 32 °C) for 12 h and combined effects of reduced salinity (from 37 ppt to 25 ppt) and two temperatures (26 °C and 29.5 °C) for 10 d. The results demonstrate that the coral is tolerant to 12 h exposure to extremely low salinity (15 ppt). The study also shows that combined effects of temperature and low salinity aggravate the damage on the photosynthesis of the symbiotic dinoflagellates in 12 h exposure to 20 ppt sea water. This study suggests that high temperature (29.5 °C) aggravates the damage of trivially low salinity (30 ppt) on the holobiont (the coral and its symbiotic dinoflagellates) in 10 d exposure. However, high temperature (29.5 °C) may have an antagonistic effect between temperature and low salinity (25 ppt) on metabolism of the holobiont. Based on the above results, we suggest that (1) the true mechanism of corals exposed to combined effects of low salinity and high temperature is complicated. This calls for more studies on different corals. Future studies should aim at investigating long-term low-level stress in order to simulate in situ conditions more accurately; (2) when corals exposed to extremely severe combined stressors for short-term or trivially severe stressors for relative long-term, the combined effects of two stressors (such as low salinity and high temperature) may be negative, otherwise, the effects may be additive.     

Salinity selection and preference of the grey snapper Lutjanus griseus: field and laboratory observations

Field observations were supplemented with laboratory experiments to reveal patterns of salinity selection and preference for grey snapper Lutjanus griseus (c. 21 cm total length, L_T), an ecologically and economically important species in the south‐eastern U.S.A. Fish abundance data were examined from a long‐term field survey conducted in the mangrove habitats of Biscayne Bay, Florida, where salinities ranged from <1 to 40. First, regression analyses indicated significant, positive linear relationships with salinity for both L. griseus frequency of occurrence and concentration (density when present). These patterns are inconsistent with physiological expectations of minimizing energetic osmoregulatory costs. Next, the salinity preference and swimming activity of 11 L. griseus (ranging from 18 to 23 cm L_T) were investigated using a newly developed electronic shuttlebox system. In the laboratory, fish preferred intermediate salinities in the range of 9–23. Swimming activity (measured in terms of spontaneous swimming speed) followed a parabolic relationship with salinity, with reduced activity at salinity extremes perhaps reflecting compensation for higher osmoregulatory costs. It is suspected that the basis of the discrepancy between laboratory and field observations for size classes at or near maturity ultimately relates to the reproductive imperative to move towards offshore (high‐salinity) coral‐reef habitats, a necessity that probably overrides the strategy of minimizing osmoregulatory energetic costs.     

Changes in plasma osmolality and Na+/K+ ATPase activity of juvenile obscure puffer Takifugu obscurus following salinity challenge

The osmoregulatory capabilities of 6-month-old juvenile obscure puffer Takifugu obscurus, transferred directly from fresh water to different salinities (0‰, freshwater control; 10‰; 20‰ and 30‰), were studied over an 8-day period. After transfer, plasma osmolality of the fish at 30‰ was significantly higher than those at all other salinities throughout the experiment. The Na⁺/K⁺ ATPase activity in the gills of the fish treated with various salinities increased significantly, peaking at 48 h, then decreased gradually to the control level at 192 h. Similar fluctuation trends of the Na⁺/K⁺ ATPase activity were observed in the kidneys. Modified Gaussian model provided accurate fits for the time-course changes in the Na⁺/K⁺ ATPase activities after abrupt salinity challenge. The results demonstrated that obscure puffer has strong capacity to tolerate abrupt salinity changes and can osmoregulate well over a wide range of salinities even in juvenile stage.     

The role of the antennal glands in ion and body volume regulation of cannulated Penaeus monodon reared in various salinity conditions

Urinary production rate and the osmotic and ionic concentrations in both urine and hemolymph were measured in cannulated intermolt Penaeus monodon which were either abruptly transferred from 45 ppt seawater to 15 ppt seawater (Experiment 1) or acclimated to 5, 25 and 45 ppt seawater (Experiment 2). In Experiment 1, urinary magnesium concentration fell dramatically from 228 to 30 mEq/l within 4 h post-transfer, but 8 h after transfer, U/H (urine/hemolymph) ratios stabilized at between 1.0 and 2.5. Sodium was higher in urine than in hemolymph during the first 24 h after transfer, while potassium was lower in urine than in hemolymph until 72 h after transfer, which suggests that sodium and potassium concentrations are regulated by the antennal gland after an abrupt change in media. In Experiment 2, the urinary production rate of P. monodon decreased as salinity increased, suggesting that the antennal glands also regulate body volume. In the acclimated shrimps of Experiment 2, the antennal glands did not appear to regulate osmolarity or the concentration of chloride, sodium, potassium, and calcium ions, but as salinity increased, U/H ratios of magnesium increased from 2.3 to 13.5, and active secretion by the antennal gland accounted for 57 approximately 93% of the total magnesium excretion through urine. These results suggest that active secretion of magnesium by the antennal gland enable this shrimp to maintain hypoionic levels of magnesium in the hemolymph.