Tolerance of seahorse Hippocampus kuda (Bleeker) juveniles to various salinities

In line with current conservation efforts, some success in the captive breeding of the seahorse Hippocampus kuda (Teleostei: Syngnathidae) has been achieved. To evaluate the salinity tolerance of these hatchery‐bred juveniles, 9‐week‐old H. kuda were transferred without prior acclimatization from ambient full strength seawater (32–33 ppt) to salinities ranging from freshwater to 85 ppt. Survival, growth, and total body water content were determined after 4 and 18 days of exposure. Juvenile H. kuda are able to survive in dilute seawater (15 ppt) for at least 18 days without any compromise in growth (both wet and dry body weight), survival, and total body water. Fish abruptly transferred to freshwater succumbed within 4–24 h, while survival of 5 ppt‐reared fish decreased to ca. 65% in 18 days. Although 10 ppt‐reared seahorses had growth and survival comparable with the control (30 ppt seawater), total body water was significantly elevated indicating reduced adaptability. The upper limit of H. kuda salinity tolerance was 50 ppt. Fish reared at salinities ≥55 ppt succumbed within 24 h. Like several other marine teleosts, growth and survival of juvenile H. kuda tended to peak in diluted seawater salinities of 15 and 20 ppt. These results indicate the possibility of growing hatchery‐bred H. kuda in brackishwater environments.     

Cold resistance and metabolic responses to salinity variations in the amphipod Eusirus antarcticus and the krill Euphausia superba

Summary The krill Euphausia superba, unlike the amphipod, Eusirus antarcticus, tolerates being frozen into solid sea-ice at temperatures down to about-4°C. Cooled in air, the amphipod and the krill freeze and will die at temperatures of-11° and-9°C respectively, representing the supercooling points of the animals. The krill is an osmoconformer in the salinity range of 25 to 45 ppt, while the amphipod conforms in the salinity range of 26 to 40 ppt. The animals thereby lower the melting point of their body fluids in the vicinity of the freezing sea ice, preventing internal ice formation at low temperatures. The mean oxygen consumption rates, at raised and lowered salinities, were not significantly different from rates obtained in normal (35 ppt.) seawater, indicating that salinity has little effect on the metabolism of either species.     

Multiple stress tolerance in an antarctic terrestrial arthropod: Belgica antarctica

Terrestrial arthropods of the maritime Antarctic experience a diverse range of environmental Stressors including extended periods of ice and snow cover, anoxia, immersion in water of variable pH and salinity, and extensive habitat drying. The collembolan Cryptopygus antarcticus and the mite Alaskozetes antarcticus seasonally depress whole body supercooling points to avoid the lethal effects of freezing. Alternatively, the wingless chironomid Belgica antarctica has a relatively limited supercooling capacity (between −6 and −8 °C) and tolerates extracellular freezing. The lower limit of freeze tolerance remains unchanged near −13 °C throughout the year in B. antarctica. Summer larvae tolerate dehydration to a limit of 35% of initial body weight as well as extended periods of anoxia and immersion in freshwater and saltwater. Two weeks of exposure to variable pH (3–12) induced no mortality.     

Temperature preferences of the mite, Alaskozetes antarcticus, and the collembolan, Cryptopygus antarcticus from the maritime Antarctic

Abstract. The thermal preferences of Alaskozetes antarcticus (Acari, Cryptostigmata) and Cryptopygus antarcticus (Collembola, Isotomidae) were investigated over 6 h within a temperature gradient (?3 to +13 °C), under 100% relative humidity (RH) conditions. After 10 days of acclimation at ?2 or +11 °C, individual supercooling points (SCP) and thermopreferences were assessed, and compared with animals maintained for 10 days under fluctuating field conditions (?6 to +7 °C). Acclimation at ?2 °C lowered the mean SCP of both A. antarcticus (?24.2 ± 9.1) and C. antarcticus (?14.7 ± 7.7) compared to field samples (?19.0 ± 9.0 and ?10.7 ± 5.2, respectively). Acclimation at +11 °C increased A. antarcticus mean SCP values (?13.0 ± 8.5) relative to field samples, whereas those of C. antarcticus again decreased (?16.7 ± 9.1). Mites acclimated under field conditions or at +11 °C selected temperatures between ?3 and +1 °C. After acclimation at ?2 °C, both species preferred +1 to +5 °C. Cryptopygus antarcticus maintained under field conditions preferred +5 to +9 °C, whereas individuals acclimated at +11 °C selected +9 to +13 °C. For A. antarcticus, thermopreference was not influenced by its cold hardened state. The distribution of field specimens was further assessed within two combined temperature and humidity gradient systems: (i) 0–3 °C/12% RH, 3–6 °C/33% RH, 6–9 °C/75% RH and 9–12 °C/100% RH and (ii) 0–3 °C/100% RH, 3–6 °C/75% RH, 6–9 °C/33% RH and 9–12 °C/12% RH. In gradient (i), C. antarcticus distributed homogeneously, but, in gradient (ii), C. antarcticus preferred 0–3 °C/100% RH. Alaskozetes antarcticus selected temperatures between 0 and +6 °C regardless of RH conditions. Cryptopygus antarcticus appears better able than A. antarcticus to opportunistically utilize developmentally favourable thermal microclimates, when moisture availability is not restricted. The distribution of A. antarcticus appears more influenced by temperature, especially during regular freeze‐thaw transitions, when this species may select low temperature microhabitats to maintain a cold‐hardened state.     

Salinity Tolerance in Some Selected Aquatic Oligochaetes

Salt tolerance of the freshwater tubificid Limnodrilus hoffmeisteri varied from an LD 50 of 10 ppt to 10.5 ppt for mature and immature worms without sediment, 9.4 to 10.5 in sand, and 14.3 for immatures acclimated to 5 ppt. Ilyodrilus templetoni was similarly intolerant to higher salinities. The estuarine Tubificoides gabriellae tolerated salt water, but was susceptible to freshwater, with LD 50's of 2 and 3.5 when acclimated to 5 and 10 ppt sea water. It survived immersion in freshwater for 3 hours. The estuarine naidids Paranais litoralis and frici experienced mortalities at both high and low salinities but showed a wide tolerance range. The coastal tubificid Monopylephorus irroratus showed almost complete tolerance to the salinity range employed. The findings reflected field distributions especially when interstitial salinities were measured, but worms did not occupy the full range of habitats possible in terms of salinity alone.     

The salinity tolerance of the invasive golden apple snail (Pomacea canaliculata)

We exposed snails of an invasive species of golden apple snail (Pomacea canaliculata) to five artificial sea water treatments at salinity levels of 0, 5, 10, 15 or 20 parts per thousand (ppt) to assess their salinity tolerance. We observed the behaviour, heart rate, total haemocyte counts, haemolymph ionic concentration and Na⁺/K⁺-ATPase activity in the mantle at 0, 12, 24, 48, 72 and 96 h post salinity exposures. The heart rate declined with increasing salinity, while Na⁺/K⁺-ATPase activity in the mantle presented a reverse trend, possibly to maintain normal osmolality. A trend of rising total haemocyte count was observed from 0 ppt and 5 ppt to 10 ppt salinities, while a sudden increase in the count was observed at 15 ppt and 20 ppt salinity groups. Furthermore, haemolymph Cl^?, Na⁺ and K⁺ concentrations increased directly with elevated salinity. An additional trial was performed to assess the growth performance of the snails under exposure to low salinities. During a 1 month trial, snails grew better at 5 ppt salinity treatment. Taken together, our results demonstrate that P. canaliculata can tolerate salt stress to some extent. The finding also obviously implies a possible invasive risk to estuaries.     

Heat tolerance and physiological plasticity in the Antarctic collembolan, Cryptopygus antarcticus, and mite, Alaskozetes antarcticus

Polar amplification of global warming has led to an average 2 °C rise in air temperatures in parts of the polar regions in the last 50 years. Poikilothermic ectotherms that are found in these regions, such as Collembola and mites, may therefore be put under pressure by changing environmental conditions. However, it has also been suggested that the thermal sensitivity of invertebrates declines with higher latitudes and, therefore, that polar ectotherms may not be at risk. In the current study, the heat tolerance and physiological plasticity to heat stress of two well-studied Antarctic invertebrates, the collembolan, Cryptopygus antarcticus, and the mite, Alaskozetes antarcticus, were investigated. Both species showed considerable heat tolerance, with each having an Upper Lethal Temperature (ULT) above 35 °C (1 h exposure). These species were also able to survive for over 43 d at 10 °C and for periods of 5–20 min at 40 °C. Across all experimental procedures, A. antarcticus possessed a somewhat greater level of heat tolerance than C. antarcticus. Water loss during short duration exposures did not differ between the two species at 30, 35 and 40 °C, suggesting that the greater tolerance of A. antarcticus over this timescale was not due to higher desiccation resistance. Physiological plasticity was investigated by testing for Rapid Heat Hardening (RHH) and long-term acclimation. RHH was observed to a small degree in both species at a warming rate of 0.5 °C min⁻¹, and also 0.2 °C min⁻¹ in A. antarcticus alone. Longer-term acclimation (1 week at 10 °C) did not enhance the heat tolerance of either species. Even with this limited physiological plasticity, the results of this study indicate that C. antarcticus and A. antarcticus have capacity in their heat tolerance to cope with current and future environmental extremes of high temperature.     

Variations in renal morphology during adaptation to salinities in tilapias

Synopsis Fifteen days oldOreochromis niloticus andO. mossambicus juveniles were gradually adapted to 10, 20, 30 and 35 ppt sea water. Kidney morphology was observed after transfer to higher salinities and compared with freshwater controls. Despite differences in salinity tolerance, both species have similar transformations when transferred at high salinity, particularly reduction of the glomerular area after 2 months adaptation to sea water, and change in tubule morphology.     

Extremely large aggregations of collembolan eggs on Humble Island,Antarctica: a response to early seasonal warming?

In January 2007 we discovered numerous large aggregations of collembolan eggs on Humble Island near Palmer Station (64°46′S, 64°03′W). These aggregations were found in damp areas under rocks, moss, mats of the terrestrial alga Prasiola crispa, and at the interface between vegetation and the rocky substrate. Aggregations ranged in size from hundreds of eggs to the largest, whose estimated size exceeded 2 million eggs. Such aggregations were not observed in previous years. Associated with these aggregations were two collembolan species, Cryptopygus antarcticus (Willem) and Friesea grisea (Schaffer). Spring warming occurred approximately 7 weeks earlier in 2006 compared to the previous year. This early warming and consequent extended period of relatively high temperatures may have modified thermal and hydric conditions favoring collembolan growth and development and the formation of these large aggregations.     

Behavioral aspects of cold tolerance in blackchin tilapia,Sarotherodon melanotheron,at different salinities

Synopsis Cold tolerance and behavioral responses of blackchin tilapia, Sarotherodon melanotheron, to rapidly decreasing temperatures were investigated at salinities of 5, 15, and 35 parts per thousand (ppt). Cold tolerance did not significantly differ with salinity or social rank. Mean temperatures were 10.7° C for beginning loss of equilibrium, 9.6° C for complete loss of equilibrium, and 6.9° C for death at all salinities. Behavioral activity declined with decreasing temperature and ceased between 10–12° C. Certain behavioral actions were significantly more frequent at 15 or 35 ppt salinity than at 5 ppt. The northward range expansion by introduced populations of the blackchin tilapia in the United States probably will be limited by its lower lethal temperature limits, but may also be affected by temperatures at which social behavior becomes disrupted.     

Response of the freshwater marsh species, Panicum hemitomon Schult., to increased salinity

SUMMARY. 1. Panicum hemitomon Schult. plants were colleeted from Louisiana's Mississippi River deltaic plain freshwater marsh and subjected to salinities of 5–12 ppt under controlled environment conditiotis.
2. The condition was designed to simulate salt stress resulting from storm surges, brine spills associated with oil recovery operations and salt water intrusion due to rapid subsidence and parallel increases in apparent sea level in Louisiana's coastal areas.
3. Plant stomatal conductance was reduced between 55 % and 80% and net photosynthesis declined between 20% and 67% in response to the different salinities within 1 day of salt application. Both responses lasted throughout the entire experiment.
4. Salinities ranging between 10 and 12 ppt resulted in tissue death in the plants 4 days after salt exposure.
5. In addition to the short-term impact of salt water influx on gas exchange of P, hemitomon reported here, the potential long-term effects on habitat change are discussed.     

Tissue Distribution, Effects of Salinity Acclimation, and Ontogeny of Aquaporin 3 in the Marine Teleost, Silver Sea Bream (Sparus sarba)

The purpose of the present study was to ascertain the tissue-specific expression of the water channel protein, aquaporin 3 (AQP3), during salinity acclimation and larval development of silver sea bream (Sparus sarba). A cDNA fragment encoding aquaporin 3 (aqp3) from silver sea bream gill was cloned and from the deduced amino acid sequence a polyclonal antibody was prepared. AQP3 was found to be present in gill, kidney, liver, brain, heart, and spleen but not in whole blood. The abundance of AQP3 was significantly highest in gills of hypoosmotic (6 ppt) and isoosmotic (12 ppt) acclimated sea bream when compared to seawater (33 ppt) and hypersaline (50 ppt)- acclimated sea bream. Spleen tissue also displayed significantly high levels of AQP3 protein in hypoosmotic and isoosmotic salinities whereas the AQP3 abundance in brain, liver, heart, and kidney remained unchanged across the range of salinities tested. The ontogenetic profile of AQP3 was also investigated from developing sea bream larvae and AQP3 was first detected at 14 days posthatch (dph) and increased steadily up to 28–46 dph. In conclusion, this study has demonstrated that AQP3 expression is modulated in gill and spleen tissue of salinity acclimated sea bream and that it can be detected relatively early during larval development.     

High temperature Chlorellaceae (Chlorophyta) strains from the Syrian-African Rift Valley: the effect of salinity and temperature on growth,morphology and sporulation mode

Two algal cultures, TvB and SH, were isolated from extreme habitats along the Syrian-African rift Valley (Israel). These cultures were initially identified as Chlorella spp. according to their morphology and lack of bristles, but following molecular phylogenetic analyses, re-identified as Micractinium spp. closely related to Chlorella. The strains were subjected to a bi-factorial study in the search for algae that grow well at elevated temperatures and salinities for future biotechnological uses. Cell density (CD) and optical density (OD) were measured for each strain at three temperatures: 35, 40 and 45ºC, and five salinities of seawater (SW): 34.8 ppt (100% SW), 26.5 ppt (75% SW), 18.3 ppt (50% SW) 10 ppt (25% SW) and 1.8 ppt (0% SW). Both strains grew best at 35–40ºC and at 0–50% SW. Increased salinity enhanced temperature tolerance to 45ºC, particularly for strain TvB. At 45ºC, following a short initial growth spurt, cultures underwent a lag period of c. 7 days, followed by a significant growth phase. During the lag period, algae underwent a substantial increase in average cell diameter (ACD). These enlarged or gigantic cells with diameters of up to ~20 μm, produced and eventually released multiple autospores. By day 13, original size distribution was almost restored. The observed morphological alterations appear to enable these strains to survive and grow autotrophically at supra-optimal temperatures (SOT). These natural adjustments may be exploited for reducing costs associated with both cooling and desalination in future cultivation.     

Acclimation trials of wild and hatchery sea bass (Dicentrarchus labrax) fry at different salinities

Hatche and wild sea bass (Dicentrarchus labrax) fry were transferred from sea water to low salinities and freshwater (FW) usiny three different operational protocols: 1) direct exposure to ow salimues and FW (LC 50 test, ST 50 test); 2) quick acclimation to FW (48h); 3) slow acclimation to FW (17d). Direct exposure to freshwater led to e death of all fry. Comete survival was observed after direct transfer to salinities ≥ 9 ppt, both in wild and hatchery fry. Eatchery fry tolerate direct transfer to low salinities (LC 50=4.393 ppt) better than wild fry (LC 50 = 3.215 ppt). Slow acclimation rotocol leads to higher survival rates (nearly 90 %) than does the quick procedure. In 48h and 17d accimation trials wild fry tolerate FW better tlan hatchery fry. Hypotheses to explain observed differences in wild and hatchery tolerances to low salinities and FW are discussed.     

Growth and survival of the invasive alga, <Emphasis Type="Italic">Caulerpa taxifolia</Emphasis>, in different salinities and temperatures: implications for coastal lake management

The alga Caulerpa taxifolia is an invasive pest species in many parts of the world and has recently become established in several estuaries in south eastern Australia. A major infestation has occurred in Lake Conjola, an intermittently open and closed coastal lagoon in southern NSW. Short term (1 week) laboratory experiments were carried out to investigate growth and survival of fragments of C. taxifolia collected from this outbreak, under a range of salinities (15–30 ppt) and water temperatures (15–30°C). Fronds, stolons and thalli of the alga all displayed similar responses. Many of the algal fragments doubled in size over the week and a maximum growth rate of 174 mm/week was recorded. Fragments showed good growth (> 20 mm/week) at salinities > 20 ppt and temperatures > 20°C. Almost total mortality occurred at salinities lower than 20 ppt and temperatures less than 20°C. Historical records of water quality demonstrate that prior to entrance manipulation in 2001, salinities in Lake Conjola had often dropped to below 17 ppt for extended periods (up to 2 years). This suggests that management of the alga may be improved if the lake was allowed to undergo its normal cycles of opening and closing to the ocean, and that entrance manipulation may be one factor that has influenced the success of this invasive species.     

Salinity‐induced changes to the survival,growth and glycogen distribution in the early fry stages of silver barb,Barbodes gonionotus (Bleeker, 1850)

Five‐week‐old silver barb, Barbodes gonionotus, fry (initial length = 10 mm) were subjected to different salinities of 0, 3, 6 or 9 ppt for 17 days, to then assess their survival and growth. Whole body histological sections were stained with Periodic‐acid Schiff (PAS). Groups of 15 fish were triplicated in each treatment with an ambient temperature (26–28°C). Results showed that growth and condition factor significantly decreased and increased (p < .05), respectively, with the increased salinities after 17 days. While survival was similar (p > .05), between 0 and 6 ppt (at 98% and 87%, respectively), the decrease was significant at 9 ppt (22%). In addition, the fry at 9 ppt had fewer gill mucous cells as well as reduced PAS positive staining intensity within the liver and intestine. This suggests energy was becoming exhausted, leading to mortalities and lower growth. Silver barb early fry were relatively sensitive to elevated salinity, which was likely due to their young age, but short‐term exposure to 3–6 ppt can be used to decrease potential freshwater diseases in the early nursery culture.     

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.     

COLD TOLERANCE OF MICROARTHROPODS

1. Microarthropods (Acari and Collembola) are dominant components of the terrestrial fauna in the Antarctic. Their cold tolerance, which forms the mainspring of their adaptational strategy, is reviewed against a background of their structure and function, and by comparison with other arthropods. 2. Two species, the isotomid collembolan Cryptopygus antarcticus Willem and the oribatid mite Alaskozetes antarcticus (Michael), are examined in detail, and afford a comparative approach to the mechanisms underlying cold tolerance in insect and arachnid types. 3. All microarthropods appear to be freezing-susceptible (unable to tolerate tissue ice), and they utilize varying levels of supercooling to avoid freezing. Gut contents are considered to be the prime nucleation site in most arthropods when supercooled, particularly for Antarctic species. Moulting also increases individual supercooling ability especially in Collembola, and the activity of ice-nucleating bacteria in cold-hardy arthropods may be important. 4. Sources of ice nucleators are many and varied, originating externally (motes) or internally (ice-nucleating agents). They act either extracellularly (mainly in the haemolymph) to promote freezing in ice-tolerant life stages, or intracellularly in freezing-susceptible forms. Thermal hysteresis proteins, acting colligatively, occur in many arthropods including Collembola; they depress both the freezing point of body fluids and the whole-body supercooling point of freezing- susceptible and freezing-tolerant species. 5. Bimodal supercooling point distributions are a feature of microarthropods and water droplets. Samples of field populations of Antarctic mites and springtails show significant seasonal changes in these distributions, which in some respects are analogous to purely physical systems of water droplets. Supercooling points are confirmed as accurate measures of cold-hardiness and survival for Antarctic species, but not necessarily for other arthropods. The effects of constant sub-zero temperatures approaching the limit of the supercooling ability of arthropods require study. 6. Desiccation and dehydration influence microarthropod physiology in several ways; in Alaskozetes it triggers glycerol synthesis. Glycerol may aid binding of water in severely dehydrated insects, but the relationship of such ‘bound’ water to cold-hardiness is unclear. 7. Sugar alcohols (polyols) and sugars are accumulated as potential cryoprotectants in many arthropods at low temperatures, and antifreeze systems may be single or multi-component in structure. Cryoprotectant synthesis and regulation have been studied principally in insects, and fresh weight concentrations of 0–3-5 M of polyols have been found. Trehalose accumulation may also influence cold-hardiness. 8. Microarthropods fall within the spectrum of cold tolerance observed for arthropods and other invertebrates. No special adaptations are found in Antarctic species, and similar strategies and mechanisms are present in both insects and arachnids. The colonization and maintenance of microarthropod populations of polar land habitats seem not to have required the evolution of any novel features with respect to cold tolerance.     

Can differences in salinity tolerance explain the distribution of four genetically distinct lineages of Phragmites australis in the Mississippi River Delta?

In the Mississippi River Delta, the common wetland grass, Phragmites australis, displays high genetic diversity, as several genetically distinct populations are co-occurring. Differences in salinity tolerance may be an important factor determining these populations’ distribution in the delta. Our study investigated the salt tolerance of four genotypes exposed to 0, 10, 20, 30, and 40 ppt salinity. The growth rate, biomass, and the light-saturated photosynthetic rate were stimulated at 10 ppt salinity and inhibited at salinities higher than 20 ppt, compared to controls. Increased concentrations of Cl^? and Na⁺ were found in the roots and older leaves of plants exposed to high salinities. Salt tolerance levels differed between genotypes. High salinity tolerance was mainly achieved by reduced water uptake and vacuole compartmentalization of toxic ions. The most tolerant genotype sustained biomass and photosynthesis even at 40 ppt, whereas the most sensitive genotype did not survive salinities higher than 20 ppt. Our findings show that the observed occurrence of different genotypes in the Mississippi River Delta is correlated to genetically determined differences in salinity tolerance. Further investigations are needed to better understand the role that salinity tolerance plays in the invasion of certain introduced P. australis genotypes.     

Nutritional Status as the Key Modulator of Antioxidant Responses Induced by High Environmental Ammonia and Salinity Stress in European Sea Bass (Dicentrarchus labrax)

Salinity fluctuation is one of the main factors affecting the overall fitness of marine fish. In addition, water borne ammonia may occur simultaneously with salinity stress. Additionally, under such stressful circumstances, fish may encounter food deprivation. The physiological and ion-osmo regulatory adaptive capacities to cope with all these stressors alone or in combination are extensively addressed in fish. To date, studies revealing the modulation of antioxidant potential as compensatory response to multiple stressors are rather lacking. Therefore, the present work evaluated the individual and combined effects of salinity challenge, ammonia toxicity and nutritional status on oxidative stress and antioxidant status in a marine teleost, European sea bass (Dicentrarchus labrax). Fish were acclimated to normal seawater (32 ppt), to brackish water (20 ppt and 10 ppt) and to hypo-saline water (2.5 ppt). Following acclimation to different salinities for two weeks, fish were exposed to high environmental ammonia (HEA, 20 mg/L representing 50% of 96h LC₅₀ value for ammonia) for 12 h, 48 h, 84 h and 180 h, and were either fed (2% body weight) or fasted (unfed for 7 days prior to HEA exposure). Results show that in response to decreasing salinities, oxidative stress indices such as xanthine oxidase activity, levels of hydrogen peroxide (H₂O₂) and lipid peroxidation (malondialdehyde, MDA) increased in the hepatic tissue of fasted fish but remained unaffected in fed fish. HEA exposure at normal salinity (32 ppt) and at reduced salinities (20 ppt and 10 ppt) increased ammonia accumulation significantly (84 h–180 h) in both feeding regimes which was associated with an increment of H₂O₂ and MDA contents. Unlike in fasted fish, H₂O₂ and MDA levels in fed fish were restored to control levels (84 h–180 h); with a concomitant increase in superoxide dismutase (SOD), catalase (CAT), components of the glutathione redox cycle (reduced glutathione, glutathione peroxidase and glutathione reductase), ascorbate peroxidase (APX) activity and reduced ascorbate (ASC) content. On the contrary, fasted fish could not activate many of these protective systems and rely mainly on CAT and ASC dependent pathways as antioxidative sentinels. The present findings exemplify that in fed fish single factors and a combination of HEA exposure and reduced seawater salinities (upto 10 ppt) were insufficient to cause oxidative damage due to the highly competent antioxidant system compared to fasted fish. However, the impact of HEA exposure at a hypo-saline environment (2.5 ppt) also defied antioxidant defence system in fed fish, suggesting this combined factor is beyond the tolerance range for both feeding groups. Overall, our results indicate that the oxidative stress mediated by the experimental conditions were exacerbated during starvation, and also suggest that feed deprivation particularly at reduced seawater salinities can instigate fish more susceptible to ammonia toxicity.