Adaptation Mechanisms of Aquatic Animals to Saline–Alkaline Water Aquaculture: Physiological, Energetic and Molecular Perspectives

Abstract

Saline–alkaline water constitutes a vital strategic non-traditional fishery resource in China, characterized by high pH values, elevated carbonate alkalinity, and complex ionic compositions. These extreme environmental conditions impose significant stress on aquatic animals, mainly by inducing ionic toxicity and disrupting acid–base regulatory mechanisms. Such disruptions subsequently lead to osmotic imbalance, metabolic dysregulation, and immunosuppression, thus restricting the survival and growth of aquatic species in aquaculture systems. Consequently, the sustainable development of the saline–alkaline aquaculture is imperative for enhancing production efficiency and promoting the utilization of marginal land and water resources. This review comprehensively summarizes the current status of saline–alkaline aquaculture and highlights the stress-inducing impacts of salinity, alkalinity, and specific ionic ratios on teleost fishes and crustaceans. It further explores key adaptive mechanisms, including osmoregulatory and ionoregulatory strategies, bioenergetic trade-offs related to oxygen consumption and ammonia excretion, coordinated antioxidant and innate immune responses, as well as recent findings from multi-omics research. This review aims to offer a scientific foundation for the selection and breeding of saline–alkaline-tolerant strains, the precise regulation of aquaculture water environments, and the development of ecological aquaculture models in saline–alkaline regions, thereby facilitating the sustainable utilization of saline–alkaline land and water resources.

1. Overview of Saline–Alkaline Water

Inland saline–alkaline water, a non-traditional water resource of substantial strategic significance, generally denotes non-marine water bodies situated in inland areas with mineralization levels typically surpassing 1 g·L⁻¹. These water bodies predominantly exist in saline–alkaline lakes, low-lying seepage areas, and underground brines [1,2], and are also commonly referred to as inland saline water (ISW) under the same definition. On a global scale, such water bodies are extensively distributed in arid and semi-arid regions, including deserts, grasslands, and plateaus. China is endowed with abundant inland saline–alkaline water resources, covering a total area of approximately 46 million hectares, which accounts for 55% of the total lake area in the country and is distributed across 19 provinces, municipalities, and autonomous regions [3]. Influenced by the combined effects of geographical location and formation mechanisms (e.g., evaporative concentration and geological leaching), the physicochemical properties of these water bodies demonstrate high habitat heterogeneity.

From a hydrochemical standpoint, the attributes of saline–alkaline water are defined by the dual indicators of “salinity” and “alkalinity”. Salinity represents the total mineralization of major cations (Na⁺, Mg²⁺, Ca²⁺, K⁺) and anions (Cl⁻, SO₄²⁻, HCO₃⁻, CO₃²⁻), while alkalinity reflects the buffering capacity of the carbonate system [4]. Based on the stoichiometric relationships of major ions, saline–alkaline waters are generally classified into three major categories, namely chloride type, carbonate type, and sulfate type, which are further subdivided into subtypes I–IV according to specific ionic ratios [5]. In China, coastal regions are predominantly characterized by chloride-type waters, whereas inland regions are primarily typified by carbonate-and sulfate-type waters [6].

In comparison to natural freshwater and seawater, saline–alkaline water exhibits distinct habitat stress characteristics. In contrast to freshwater, the high osmotic pressure environment and specific ion accumulation in saline–alkaline water exert significant “physiological drought” and ion toxicity stress on aquatic organisms, severely restricting nutrient uptake and metabolic functions. Secondly, compared to seawater with a relatively stable ionic composition, inland saline–alkaline water not only displays high salinity fluctuations but often features higher carbonate/bicarbonate ratios and high pH [7]. Research by Szabó et al. [8] verified that this unique ionic composition ratio is a core factor driving changes in the structure of the aquatic microbial community, with ecological effects being potentially as significant as salinity itself. Owing to the complex water chemistry, most saline–alkaline waters are unsuitable for direct consumption or agricultural irrigation and have long remained under-exploited [6]. Under natural conditions, only a few euryhaline species such as the naked carp (Gymnocypris przewalskii) and the Amur ide (Leuciscus waleckii) can survive [9,10].

2. Current Status of Saline–Alkaline Water Aquaculture

Despite the limitations imposed on conventional agriculture by the complex physicochemical environments of saline–alkaline waters, their substantial resource abundance and unique ecological potential offer extensive opportunities for aquaculture. Currently, saline–alkaline aquaculture has gained significant global momentum as an innovative model to address freshwater scarcity and alleviate pressure on land resources. Geographically, this practice is primarily concentrated in two types of regions: arid and semi-arid areas possessing vast expanses of saline–alkaline land, and regions rich in underground brackish water resources. In China, northwestern provinces serve as key regions for this practice. By utilizing abundant saline–alkaline water resources to develop aquaculture on saline–alkaline wastelands, these regions have achieved a “win–win” outcome of ecological amelioration and economic benefits—a strategy often described as “controlling alkalinity through fisheries” [11]. Figure 1 provides an overview of saline–alkaline aquaculture regions and major cultured species in China.

In South Asia, shrimp farming based on underground brackish water is widely practiced in Indian states such as Haryana, Punjab, and Rajasthan, where it has gradually become a vital pillar of the local rural economy [12]. Similarly, the Punjab and Sindh provinces of Pakistan, parts of Egypt, and regions of the North American Great Plains have emerged as potential or active areas for saline–alkaline aquaculture due to comparable land and water resource conditions [13]. In these regions, where traditional agriculture is constrained by high soil salinity or freshwater deficits, utilizing inland saline–alkaline water for aquaculture offers novel possibilities for the development and utilization of these marginal lands.

In the selection of candidate species for aquaculture, euryhalinity and economic value are the primary considerations. Currently, the Pacific white shrimp (Litopenaeus vannamei) stands out as the most critical and economically valuable species. Owing to its superior osmoregulatory capacity and growth advantages, it has become the predominant species in inland saline–alkaline aquaculture worldwide [14,15]. In addition to crustaceans, various fish species have also demonstrated significant aquaculture potential. The Genetically Improved Farmed Tilapia (GIFT Oreochromis niloticus) has been widely promoted due to its robust environmental adaptability. Furthermore, research indicates that dietary supplementation with additives such as bile acids can further optimize the lipid utilization efficiency and growth performance of this species in saline–alkaline waters [16]. Several carnivorous and omnivorous fish species have also been successfully cultured in these environments, including the red drum (Sciaenops ocellatus) [17], channel catfish (Ictalurus punctatus) [18], and Indian major carps such as rohu (Labeo rohita) [19].

Table 1. Major cultured species and their adaptability in saline–alkaline aquaculture.

Variety		Aquaculture Area	Adaptability	Main Farming Models	Physical Injury
Crustaceans	Pacific white shrimp (Litopenaeus vannamei)	China coastal saline–alkali areas [20], Inland saline–alkali areas [21,22]; Southeast Asia (e.g., Indonesia, Vietnam [23,24]); South Asia (e.g., India [25]); South America (e.g., Mexico [26,27,28]); Other arid regions (e.g., Iran [29])	Salinity tolerance: Capable of surviving in a salinity range of 1–50 [30,31]. Particularly adapted to low–salinity conditions [26,32]. Exhibits stress response mechanisms to acute alkalinity stress, such as specific gene expression regulation [21,22]. Physiological constraints and optimization: Adaptability is challenged by alkalinity stress [21,22], cold stress [33], water quality variations [34], and microbial influences (e.g., antibiotic resistance gene transmission) [35].	Traditional pond farming [36,37]; Recirculating aquaculture systems [38,39]; Bioflocculation technology (BFT) systems [40]; Others: fish-shrimp co-culture [36], Rice-shrimp rotation [41], Ecological simulation system farming [42], etc.	Altering the activity of Na+/K+-ATPase and heat shock proteins (altered gene expression) [43].
	Chinese Mitten Crab (Eriocheir sinensis)	China, especially the alkaline regions in the northwest [44,45], the saline–alkali areas in the northeast [46,47], and the saline–alkali areas along the coast of Jiangsu [48].	Salinity tolerance: Optimal growth occurs at low salinity (≤5) [49,50], with a tolerance range from 0 to 25 [51]. Salinity exceeding 15 may restrict gonadal development [49,50]. Quality improvement: Crabs cultured in saline–alkali water (salinity 6–12) exhibit superior meat quality [52,53]. Osmotic regulation burden: Carbonate alkalinity (CA) affects osmotic regulation [54]. High-salt environments increase the energy expenditure required to maintain osmotic balance [55].	Common pond aquaculture [56,57], Rice-crab co-culture [44,47,58]	There was a marked increase in MDA content and a decrease in SOD, CAT, and T-AOC levels in the hemolymph. Additionally, there was an elevation of ROS levels in the gills and hepatopancreas, along with Keap1 expression, leading to tissue oxidative damage [59].
	Giant mud crab (Scylla serrata)	The Indo-Western Pacific region, including the southern coastline of China, Southeast Asian countries, and other coastal nations [60,61]	Basic adaptability: Due to its descending migration, it can adapt to a wide range of salinity, from fresh water to high-salinity water. Adaptation differences in saline–alkaline water types: The body establishes a new homeostasis to adapt to chloride-based low-salinity alkaline water [62]. High concentrations of sulfate disrupt intestinal immunity and undermine the integrity of hepatopancreatic tissues [63]. Sudden changes in salinity may damage gill tissues, impairing osmoregulation and antioxidant capacity [64].	Pond aqaculture [65,66]; Recirculating aqaculture system [67]	Abnormal fluctuations in hemolymph biochemical markers—such as acid phosphatase (ACP), alkaline phosphatase (AKP), aspartate aminotransferase (AST), and alanine aminotransferase (ALT)—further confirmed tissue damage. Moreover, the expression of key osmoregulatory enzymes in the gills, including Na+/K+-ATPase (NKA) and Ca2+/Mg2+-ATPase (CaMgA) [68].
Fishes	Mozambique tilapia (Oreochromis mossambicus), Nile tilapia (Oreochromis niloticus)	China region [69,70]; South Asia region (such as Bangladesh, India [71,72,73])	Salt tolerance variations: Significant differences in salt tolerance exist among tilapia species. For example, Oreochromis mossambicus (Mozambique tilapia) performs better in high-salinity environments, while Oreochromis niloticus (Nile tilapia) shows significantly reduced growth and feed efficiency in saline–alkaline waters [74,75]. GIFT strains of tilapia demonstrate superior performance in saline–alkaline aquaculture [76]. High concentrations of sulfate exhibit certain toxicity to tilapia [34]. Low-potassium water affects the ion homeostasis in fish [77]. High alkalinity (e.g., 23.8 µmol/L) impairs tilapia growth [78,79]. Generally, tilapia can enhance their salt tolerance through specific adaptation periods [80].	Pond culture [81,82]; Integrated aquaculture–agriculture system (IAAS) [83]; Bio-floc culture [84,85,86]	The activity of pancreatic amylase and the number of white blood cells (WBCs), red blood cells (RBCs), and the activity of alanine aminotransferase (ALT) was significantly lower; The mRNA expression of peptide transporter 1 (PEPT-1) was significantly increased [87].
	Turbot (Scophthalmus maximus)	China region [88,89,90]	Salinity tolerance: The large flounder (Paralucranus argenteus) exhibits broad salinity tolerance (5–50, with 11 being optimal), and its kidney morphology undergoes adaptive changes under salinity stress [91]. Prolactin (PRL) and its receptor (PRLR) play critical roles in osmoregulation [92]. Increasing dietary salt levels can partially alleviate osmoregulatory stress and enhance the physiological tolerance of fish [93].	Recirculating Aquaculture System [88,94]	knockdown of genes involved in myo-inositol biosynthesis and the consequent decrease in myo-inositol concentration significantly impair osmoregulatory capacity [95].
	Largemouth bass (Micropterus salmoides)	China inland saline water and coastal saline water areas [96,97,98,99]; North America [100]	Salt tolerance: Juvenile fish exhibit basic tolerance to salinity levels of 0–12 [101], while adult fish selectively breed at 9 salinity to develop tolerance [101]. However, high salinity may cause tissue damage [96]. Cultivation at 3–9 salinity for 10 weeks can enhance the flavor of fish meat [102]. Nevertheless, long-term adaptation to high-osmotic environments (e.g., saline–alkaline water) remains limited, and there is a lack of readily available salt-tolerant germplasm resources [98].	Ecological pond aquaculture [103,104]	Serum osmolality, Na+, Cl−, and cortisol levels of the high salinity group were significantly higher than of the low salinities [101].

summarizes the major global distribution regions, common cultured species, and culture models associated with inland saline–alkaline aquaculture.

The feasibility of aquaculture in saline–alkaline water is fundamentally determined by its distinctive physicochemical properties. Different from natural seawater and freshwater, the habitat characteristics of these waters are mainly defined by variations in salinity, pH, alkalinity, and specific ionic concentrations [105]. These environmental factors directly impact the physiological functions of aquatic animals, including osmoregulation, oxygen consumption and metabolism, digestive and absorptive capabilities, and immune defense mechanisms [106,107]. Although studies in the past few decades have offered preliminary understandings of the molecular mechanisms underlying species adaptation to saline–alkaline environments, a systematic assessment of the overall adaptability of cultured animals is still lacking. In particular, there is a significant dearth of comprehensive summaries regarding specific physiological response processes.

Consequently, this review endeavors to clarify the influence of core environmental factors, namely salinity, pH, ionic concentrations, and ionic ratios, on the growth and physiological state of cultured animals. It commences with an analysis of the crucial chemical differences between saline–alkaline water and seawater. Through the systematic synthesis of extant research findings to interpret the adaptive patterns of aquatic animals to saline–alkaline habitats, this paper aims to offer a scientific reference for the healthy and sustainable development of the inland saline–alkaline aquaculture industry.

3. Impacts of the Saline–Alkaline Environment on Cultured Organisms

3.1. Effects of Salinity on Cultured Aquatic Animals

Salinity represents a crucial abiotic environmental factor within aquatic ecosystems and serves as one of the pivotal determinants regulating the physiological functions of aquatic organisms [108,109]. In natural settings, fluctuations in salinity that surpass a species’ potential tolerance threshold not only present direct perils to survival but may also lead to the decline in aquatic biodiversity [110,111]. In aquaculture systems, salinity stress exerts a significant influence on the physiological processes of cultured species, encompassing growth performance, survival rates, metabolism, and nutrient absorption [112,113,114,115]. Moreover, sub-optimal salinity environments can inhibit non-specific immune functions, consequently enhancing the susceptibility to pathogens and restricting the sustainable productivity of the aquaculture industry [116].

Salinity tolerance demonstrates substantial inter-specific variation. For most commercial freshwater species, mortality rates usually increase steeply when the ambient salinity surpasses 10 ppt [117,118]. Conversely, estuarine or coastal euryhaline commercial fish, like Takifugu flavidus, generally display an optimal growth salinity range of 10–20 ppt [119]. This strong adaptability to low-to-moderate salinity environments makes these species suitable candidates for aquaculture development in inland saline waters. Globally, successful applications of saline–alkaline water resources in aquaculture have been established for a long time in countries such as Thailand [120], the United States [121], Brazil [122], Ecuador [123], and Australia [124]. The research results on the salinity adaptability of several commercial species in saline–alkaline waters are summarized in

Table 2. Study on Optimal Salinity of Major Economic Species in Saline Water Culture.

Economic Species (Latin)	Area	Salinity (g/L)	Survival Rate	Note	References
Black Sea Bream (Pagrus auratus)	southwestern New South Wales, Australia	12–48	Same as the control group (100%)	Add seawater equivalent of 50–100% K+ concentration	Partridge et al. [125]; Fielder et al. [126]
Silver Perch (Bidyanus bidyanus)	southwestern New South Wales, Australia	10	96.1 ± 3.9%	After adding the K+ concentration equivalent to seawater	Doroudi et al. [127]
Nibea albiflora (Argyrosomus japonicus)	southwestern New South Wales, Australia	15–35	96%	Add K+ concentration equivalent to more than 40% of seawater	Doroudi et al. [128]
King prawn (Penaeus latisulcatus)	Curtin Aquatic Research Laboratory, Australia	25–32	≈64%	Addition of 80% to 100% K+ concentration	Prangnell et al. [129,130]
Black tiger shrimp (Penaeus monodon)	Udaipur, Rajasthan, India.	10~12.5	88~100%	Supplementation of K+ concentration exceeding 100%, with Mg2+ concentration not exceeding 48% of the equivalent seawater level	Shakeeb et al. [131]; Raizada et al. [132]
Blue mussel (Mytilus edulis)	Aquatic Research Laboratory, Curtin University, Australia	27	62%	100% K+ enhancement	Huy et al. [133]
Indian prawn (Penaeus indicus)	Tamil Nadu, India	5	91.1%	Adjust the Na+/K+ ratio to 44:1	Antony et al. [134]
Red tilapia (Oreochromis sp.)	Kumari, India.	20	95.56%		Kumari et al. [135].
Carp (Cyprinus carpio)	Rohtak, Haryana, India	5	≈74.28%		Iffat et al. [136]
Roho labeo (Labeo rohita)	Rohtak, Haryana, India	4–12	72~100%	Optimal at salinity of 4	Patel et al. [137]
Pacific white shrimp (Litopenaeus vannamei)	Rohtak Center, Rohtak, Haryana, India	15	>92%		Kumar et al. [138]

.

Studies in fish indicate that salinity exerts a dual effect on flesh quality and physiological health. On the one hand, moderate salinity can improve filet quality. Evidence shows that red drum (Sciaenops ocellatus) and other euryhaline fishes cultured in inland low-salinity saline–alkaline waters exhibit significantly higher contents of umami- and sweet-tasting amino acids in muscle, resulting in superior flavor compared with conspecifics reared in seawater [139,140]. However, inappropriate salinity stress often causes more harm than benefit. Under high-salinity conditions, fish may face pronounced pathological challenges. For example, turbot (Scophthalmus maximus) exposed to salinities exceeding its tolerance can develop histopathological lesions such as glomerular atrophy and renal tubular dilation [141]. In addition, some species show splenic congestion or enlargement, hepatocellular degeneration, and thickening of the intestinal mucosa under salinity stress [141,142,143]. Moreover, Sai-Ut et al. [144] reported that the combined effects of high-salt diets and high-salinity water can decrease muscle protein content and increase lipid peroxidation (TBARS), thereby promoting excessive salt accumulation in muscle and ultimately compromising nutritional value.

Notably, fish are more sensitive to salinity during early life stages. The hatching rate of tilapia embryos decreases markedly at 15–20, indicating that salinity tolerance in adults does not necessarily reflect developmental-stage adaptability [145]. In practical farming, salinity frequently interacts with other environmental factors. For instance, when salinity stress is combined with ammonia–nitrogen exposure, the antioxidant status and immune function of tilapia are more severely suppressed [146]. Therefore, in low-salinity saline–alkaline aquaculture, in addition to prioritizing low-salinity-tolerant species such as common carp (Cyprinus carpio) [136], nutritional modulation is also required to alleviate oxidative stress and enhance immune responses [147].

For crustaceans, salinity responses are mainly manifested in immune defense and the integrity of organ structure. In typical euryhaline species such as the Pacific white shrimp (Litopenaeus vannamei) and black tiger shrimp (Penaeus monodon), although survival salinity ranges are broad, total hemocyte count (THC) still declines significantly over time under low-salinity conditions (e.g., <10 or even 1), accompanied by elevated glucose and protein levels, suggesting an immunosuppressed and stress-associated state [148,149]. In addition, low-salinity stress can cause edema of gill filaments with separation of the cuticle from the epithelium; such gill lesions markedly impair osmoregulatory capacity [150,151]. By contrast, the giant river prawn (Macrobrachium rosenbergii) exhibits better adaptation to freshwater and low-salinity environments [152,153].

Under high-salinity or combined-stressor conditions, crustacean growth is also constrained. It has been reported that high salinity and high stocking density show a significant negative synergistic effect; in a stress trial on Pacific white shrimp (Litopenaeus vannamei), growth performance was poorest under 44 g/L salinity and a density of 600 individuals m⁻³ [154]. The interaction between salinity and alkalinity should also be considered. For example, under combined salinity–alkalinity stress, the Chinese mitten crab (Eriocheir sinensis) exhibits evident pathological alterations in the gills, hepatopancreas, and intestine, potentially involving apoptosis mediated via the ROS/MAPK signaling pathway [155]. To mitigate such stress, dietary supplementation with feed additives such as Chlorella vulgaris and non-lethal heat shock (NLHS) preconditioning have been demonstrated to be effective interventions, and both strategies can significantly enhance cross-tolerance to environmental fluctuations in crustaceans [156,157,158].

Notably, aquaculture in saline–alkaline waters is challenged not only by total salinity but also by pronounced ionic imbalances, particularly potassium (K⁺) deficiency, which impose additional physiological burdens on cultured organisms. As a result, aquatic animals must rely on complex regulatory mechanisms involving ion transport, osmoregulation, and energy reallocation to maintain internal homeostasis.

3.2. Effects of Water pH on Cultured Animals

As a core indicator of the aquatic chemical environment, pH functions as a crucial ecological factor that determines the success of aquaculture activities. Generally, the optimal pH range for the growth of crustaceans is 7.0–8.5 [159]; cyprinids also flourish within a similar range (7.0–8.5), while salmonids typically favor near-neutral environments (pH ≈ 7.0). Deviations from these safe thresholds (e.g., pH > 9.0 or <4.0) can directly lead to the mortality of cultured organisms [160].

The dynamic equilibrium of water pH is mainly regulated by the carbon dioxide–carbonate system. In conventional aquatic environments, pH variations are predominantly driven by fluctuations in free CO₂ resulting from biological respiration, organic matter oxidation, and photosynthesis [161]. However, in saline–alkaline waters, which cover approximately 46 million hectares in China, high carbonate alkalinity serves as the dominant factor maintaining consistently elevated pH levels. This unique habitat characteristic makes high-pH stress the primary environmental bottleneck restricting the spatial expansion and productivity improvement in saline–alkaline aquaculture.

The influence of high-pH environments on aquatic animals is initially presented as direct histopathological impairment. Since organs like the gills and skin are directly exposed to the water, high pH can induce epithelial cell swelling, fusion, hyperplasia, and even necrosis and exfoliation [162,163]. A research on the mud crab (Scylla paramamosain) indicated severe morphological destruction of gill filaments in high-pH saline–alkaline water, which directly undermined respiratory and ion-exchange functions [68]. Similarly, other investigations have reported renal tubular epithelial degeneration in crucian carp (Carassius auratus) and loss of hepatopancreatic structural integrity in red swamp crayfish (Procambarus clarkii) [164,165]. Such structural damage to physical barriers and metabolic organs forms the morphological foundation for the decline in physiological function.

Beyond direct physical harm, elevated pH modifies water chemical equilibria, thereby instigating more profound physiological and metabolic disruptions. Notably, pH plays a crucial role in regulating the speciation of ammonia nitrogen: high pH promotes the transformation of ionized ammonium (NH₄⁺) into the more toxic unionized ammonia (NH₃). Considering that NH₃ is highly lipophilic and can freely penetrate biological membranes, high-pH environments not only aggravate exogenous ammonia toxicity but also impede the excretion of endogenous ammonia wastes [46]. Studies have verified that in walking catfish (Clarias batrachus) exposed to pH 10, ammonia excretion is hindered, resulting in a large-scale accumulation of ammonia in plasma and brain tissue, which subsequently triggers severe metabolic dysfunction [164]. Moreover, high pH alters the solubility and bioavailability of heavy metals and hydrogen sulfide [166]; the synergistic effects between these environmental factors and toxic substances further enhance the intensity of combined stress.

The physiological impacts of pH on aquatic animals are depicted in Figure 2.

3.3. Effects of Water Hardness on Cultured Animals

Water hardness, predominantly determined by the concentrations of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, represents another crucial hydrochemical factor in maintaining physiological homeostasis in aquatic animals, along with salinity and pH [167]. Appropriate concentrations of Ca²⁺ and Mg²⁺ are indispensable for osmoregulation, skeletal development, and neuromuscular transmission. Nevertheless, saline–alkaline waters are characterized by intricate hydrochemistry, typically presenting as high salinity, high carbonate alkalinity, and high pH [168]. In such distinctive habitats, abnormal fluctuations in hardness, whether extreme deficiency or excessive load, disrupt physiological equilibrium, resulting in growth retardation or mortality [169].

For commercial fish species, the impacts of water hardness on growth performance, metabolic rate, and stress resistance display complex dose-dependent and species-specific patterns. Research suggests that most fish have an optimal hardness range within which growth and feed utilization efficiency are maximized. For example, water hardness significantly regulates the physiological state of juvenile largemouth bass (Micropterus salmoides). Varying hardness levels (100–600 mg/L CaCO₃) not only directly modify growth rates but also notably influence oxygen consumption, ammonia excretion rates, and the activities of ion-regulatory enzymes [170]. This implies that optimizing hardness within an appropriate range can improve the energy budget of fish, enabling more energy to be allocated to growth rather than basal metabolic maintenance. Moreover, in saline–alkaline environments, higher water hardness often exerts a “protective effect,” enhancing tolerance to other environmental stressors. Sinha et al. [171] demonstrated that juvenile channel catfish (Ictalurus punctatus) acclimated to high-hardness environments exhibited significantly enhanced resilience to subsequent high ammonia exposure and salinity shock. This increased tolerance may be ascribed to the stabilizing effect of hardness on gill epithelial membranes. However, this protection has physiological limitations. Once hardness exceeds the tolerance threshold, it transforms from a protective factor to a lethal agent. For instance, a 96 h LC₅₀ was recorded for channel catfish (Ictalurus punctatus) at extremely high hardness (~4939 mg/L CaCO₃), indicating direct acute toxicity [172].

Compared to fish, crustaceans exhibit higher sensitivity to hardness variations in saline–alkaline waters, and these impacts extend beyond survival to physiological growth and final product quality. As key organs for osmoregulation and excretion, crustacean gills are directly exposed to the water and are highly susceptible to combined damage from high hardness and alkalinity [173]. Studies on Pacific white shrimp (Litopenaeus vannamei) and ridgetail white prawn (Exopalaemon carinicauda) reveal that suboptimal hardness alters gill filament histology, thereby affecting ion transport enzyme activity and respiratory function, ultimately impairing adaptation to saline–alkaline conditions [174,175]. Crucially, chronic exposure to inland waters characterized by intersecting high hardness and carbonate alkalinity significantly suppresses reproductive development and muscle quality. For example, in the Chinese mitten crab (Eriocheir sinensis), combined stress from high hardness and alkalinity not only induced apoptosis mediated by metabolic disorders of ammonia but also significantly reduced the gonadosomatic index (GSI), meat yield, and total edible yield [176]. This implies that hardness management is vital for securing the economic traits of high-value crustaceans in saline–alkaline aquaculture.

Therefore, when evaluating the potential of saline–alkaline aquaculture, water hardness should not be viewed as an isolated parameter; it interacts closely with salinity, carbonate alkalinity, and nutrient levels [177]. High-hardness environments may alleviate the toxicity of certain ions through competitive antagonism mechanisms, yet they may also form synergistic stressors with high alkalinity, exacerbating the energetic burden on cultured animals. Such complex environmental interactions necessitate precise, species-specific water quality management strategies.

3.4. Effects of the Four Major Cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) and Their Ratios on Cultured Animals

Major waterborne ions, including sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺), play critical roles in physiological and metabolic processes such as osmoregulation and energy transformation, serving as essential nutrients for aquatic organisms. However, the ionic concentrations and ratios of inland saline–alkaline waters differ significantly from those of seawater. Consequently, ionic deficiencies or imbalances are detrimental to the growth and survival of cultured aquatic animals.

3.4.1. Na⁺ and K⁺

Sodium (Na⁺) and potassium (K⁺) constitute the fundamental physicochemical basis for maintaining cellular osmolarity, acid–base balance, and neuromuscular excitability; moreover, they are critical elements for energy metabolism and osmoregulation in aquatic animals coping with environmental salinity fluctuations [178,179]. Physiologically, ion transport relies heavily on the activity of membrane-bound Na⁺/K⁺-ATPase (NKA), which maintains intracellular homeostasis and membrane potential equilibrium through ATP hydrolysis [180]. However, NKA activity is highly sensitive to environmental ionic composition. In contrast to natural seawater, which maintains a relatively stable Na⁺/K⁺ ratio, inland saline–alkaline waters are frequently characterized by K⁺ deficiency and abnormally elevated Na⁺/K⁺ ratios. Research indicates that when environmental Na⁺/K⁺ ratios deviate significantly from the normal range found in seawater, NKA activity in the gill tissues of aquatic animals is markedly inhibited [181,182], thereby impairing their osmoregulatory capacity.

This ionic imbalance inflicts multidimensional physiological and pathological impairments on cultured animals. Histomorphologically, environments characterized by K⁺ deficiency or elevated Na⁺/K⁺ ratios predispose animals to gill pathologies; typical symptoms include cellular apoptosis, gill filament edema, and atrophy [183], which directly compromise respiratory and excretory functions. At the biochemical and metabolic levels, ionic stress triggers oxidative stress responses and metabolic disturbances. For instance, crucian carp (Carassius auratus) exhibits significant hepatic oxidative damage under saline–alkaline stress [184], whereas black tiger shrimp (P. monodon) exposed to potassium-deficient environments display failure in serum ion regulation and shifts in the isosmotic point [185]. Furthermore, the deficiency of specific ions extends its impact to reproductive performance; in salmonids, sperm motility is severely inhibited or even totally inactivated in purely potassium-deficient environments [186,187,188,189].

In response to these challenges, numerous empirical studies have confirmed that optimizing the aquatic Na⁺/K⁺ ratio significantly improves the survival and growth performance of cultured organisms. A study on the razor clam (Sinonovacula constricta) demonstrated that larvae maintained at a Na⁺/K⁺ ratio of 31.9–47.3 exhibited significantly superior growth rates compared to those in high-ratio groups [178]. Similarly, in Pacific white shrimp (Litopenaeus vannamei) cultured in low-salinity or saline–alkaline water, K⁺ supplementation—such as increasing concentrations to 110–221 mg/L or adding 10–40 mg/L K⁺ to low-salinity water—effectively alleviated ionic stress, restored growth performance, and reduced mortality [190,191]. Likewise, studies on blue mussels (Mytilus edulis) and cobia (Rachycentron canadum) have identified K⁺ fortification as a requisite measure for ensuring larval development and survival [133,192]. Given the ubiquitous K⁺ deficiency in saline–alkaline waters, correcting the Na⁺/K⁺ ratio through aqueous potassium addition or dietary supplementation has emerged as a critical technical strategy to mitigate osmotic stress and enhance aquaculture productivity [129,193].

3.4.2. Ca²⁺ and Mg²⁺

In inland saline–alkaline aquaculture ecosystems, calcium (Ca²⁺) and magnesium (Mg²⁺) serve not only as macro-minerals constituting the skeletons, scales, and exoskeletons of aquatic animals but also as critical signaling molecules and cofactors. They are intricately involved in core physiological processes such as neural transmission, muscle contraction, enzyme activation, and the regulation of cell membrane permeability [194,195]. Consequently, the concentrations of Ca²⁺ and Mg²⁺, along with their ratio (Ca²⁺/Mg²⁺), directly govern the growth, metabolic homeostasis, and environmental adaptability of cultured organisms.

At the microscopic level of physiological mechanisms, Ca²⁺ acts as an absolute limiting factor in the molting cycle of crustaceans. Because crustaceans lack effective calcium storage mechanisms, the calcium required for post-molt exoskeleton mineralization must be absorbed directly from the aquatic environment [196]. Fluctuations in environmental Ca²⁺ concentrations not only directly affect the exoskeleton hardening process but also regulate the molting regulatory network through molecular pathways; for instance, they influence the gene expression of molt-inhibiting hormone (MIH) and the ecdysone receptor (EcR), thereby determining molting frequency and energy budgets [197,198]. When water is deficient in Ca²⁺, juvenile Pacific white shrimp (Litopenaeus vannamei) exhibit symptoms such as difficulty in cuticular calcification, reduced feeding rates, and prolonged molting cycles [199,200]. For fish, the physiological significance of Ca²⁺ extends to reproductive biology. Research confirms that fish sperm motility is highly dependent on environmental Ca²⁺; calcium deficiency prevents the successful activation and maturation of sperm in various species [187,201,202]. For euryhaline fish such as tilapia, adequate calcium concentration is also a prerequisite for maintaining sperm motility in hypertonic environments [203]. Mg²⁺ functions as an activator for numerous enzymes in metabolic pathways such as glycolysis and protein synthesis. Following entry into the organism via passive absorption, Mg²⁺ not only participates in hemolymph osmoregulation but is also crucial for maintaining elemental balance. While cultured animals may sustain growth when aquatic Mg²⁺ is insufficient, this can induce “magnesium deficiency,” a metabolic disorder characterized by decreased magnesium content in scales and bones accompanied by a compensatory increase in sodium [204].

However, the ionic composition of inland saline–alkaline waters is highly heterogeneous, often manifesting as abnormal fluctuations in Ca²⁺ and Mg²⁺ concentrations. Although some euryhaline species, such as the naked carp (Gymnocypris przewalskii), possess a wide tolerance range for Ca²⁺, their growth is significantly inhibited when concentrations exceed 10 times or fall below 1% of that in seawater of equivalent salinity [205]. More critically, satisfying single-ion requirements is often insufficient to support homeostasis; a significant synergistic effect exists between Ca²⁺ and Mg²⁺. For example, studies on the larval rearing of Chinese mitten crab (Eriocheir sinensis) found that raising freshwater Ca²⁺ concentration alone had a limited effect on survival. In contrast, simultaneously increasing Mg²⁺ and maintaining a Ca²⁺/Mg²⁺ ratio between 1:1.25 and 1:3 significantly improved juvenile survival [206]. Similarly, under natural saline–alkaline conditions, Litopenaeus vannamei requires specific Ca²⁺/Mg²⁺ ratios (e.g., 1:3) and total concentrations to achieve optimal growth performance [191,207,208].

3.5. Effects of Major Anions in Water on Cultured Animals

3.5.1. Alkalinity

In saline–alkaline aquaculture systems, Total Alkalinity (TA) serves as the chemical buffering foundation for maintaining relative pH stability, primarily composed of anions such as bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻). High concentrations of carbonate and bicarbonate not only directly elevate water pH but also disrupt acid–base balance and ionic homeostasis in aquatic animals. Numerous studies have previously demonstrated the impact of alkalinity levels on cultured aquatic organisms [203,204]. By interfering with physiological metabolism, inducing oxidative damage, and modulating key gene expression, they exert significant stress effects on the growth, survival, and tissue structure of cultured organisms [155,173,209,210,211]

Crustaceans are particularly sensitive to high-alkalinity environments due to their lack of comprehensive body fluid regulatory mechanisms. High-carbonate alkalinity primarily impairs ion transport functions in the gills, leading to osmoregulatory dysfunction and acid–base imbalance. Research indicates that under high carbonate alkalinity stress, the Chinese mitten crab (Eriocheir sinensis) exhibits significant alterations in gene expression and DNA methylation levels of carbonic anhydrase (CAs) in the gills. Although CAs play a pivotal role in maintaining the CO₂/HCO₃⁻ balance, their catalytic efficiency is compromised under extreme alkalinity, often exacerbating acid–base imbalance and triggering metabolic pathway disorders [155,212]. At the molecular regulation level, the ridgetail white prawn (Exopalaemon carinicauda) displays significant transcriptomic changes under acute alkalinity stress, increasing transcriptional diversity through alternative splicing (AS) to cope with environmental pressure; however, aberrant splicing may lead to adaptive impairment [174,213]. Furthermore, high-alkalinity environments predispose crustaceans to oxidative stress, resulting in lipid peroxidation of cell membranes and the accumulation of reactive oxygen species (ROS) in critical tissues such as the hepatopancreas, which subsequently activates the ROS/MAPK signaling pathway and induces apoptosis [155,214]. These physiological impairments are directly reflected in production performance, manifested as retarded ovarian development in Exopalaemon carinicauda and significantly reduced gonadosomatic index (GSI) and meat yield in Eriocheir sinensis [175,176].

For fish, high alkalinity stress similarly causes severe histopathological damage and alterations in energy metabolism. The gills, liver, and kidneys, serving as core organs for ion regulation and metabolic excretion, frequently exhibit structural degeneration in high-carbonate environments. Studies show that in staple fish species such as crucian carp (Carassius auratus) and grass carp (Ctenopharyngodon idella), alkalinity stress induces cell swelling and necrosis in the gill filament epithelium. Additionally, it disrupts antioxidant defense mechanisms in hepatic and renal tissues and suppresses digestive enzyme activity, thereby reducing feed utilization efficiency and inhibiting growth [215,216]. Regarding energy metabolism, fish under alkalinity stress must expend additional energy to maintain internal homeostasis, leading to a reduction in energy allocated for growth. Research has indicated that while long-term alkalinity stress reduces energy metabolic efficiency in Nile tilapia (Oreochromis niloticus), this issue can be effectively ameliorated by adjusting the dietary protein-to-carbohydrate ratio [80].

Although high alkalinity is generally regarded as an environmental stressor, its impact on the quality of aquaculture products is complex. While chronic high alkalinity stress leads to reduced edible yield in species like Eriocheir sinensis, low-salinity saline–alkaline environments within a moderate range have been proven to enhance the quality of certain aquatic animals. This view is supported by the increased accumulation of flavor-imparting amino acids and non-volatile flavor substances in the muscle of black tiger shrimp (Penaeus monodon) and red drum (Sciaenops ocellatus) reared in saline–alkaline water [17,140]. Wang’s research indicates that during the fattening period of the Eriocheir sinensis, its color, nutritional value and flavor quality perform better in saline–alkali water [56].

3.5.2. Sulfate

In inland saline–alkaline aquaculture ecosystems, the physiological and ecological impacts of sulfate (SO₄²⁻) on aquatic animals demonstrate notable environmental dependence and ionic synergistic properties. Distinct from chlorides or carbonates, the direct toxicity of the sulfate ion itself is generally relatively low. Nevertheless, its bioavailability and toxicity thresholds undergo substantial alterations under specific hydrochemical conditions, especially with respect to variations in ionic composition. Toxicological investigations have verified that in K⁺-deficient environments, the acute toxicity of aqueous sulfate to GIFT tilapia (Oreochromis niloticus) increases remarkably, with the 96 h median lethal concentration (96 h LC₅₀) decreasing to 2.56 g/L, which is far lower than the levels observed in the freshwater and artificial seawater control groups [34]. The fundamental physiological mechanism of this heightened toxicity resides in the competitive inhibition of ion transport channels and the depletion of osmoregulatory functions. Specifically, high-sulfate environments not only impede NKA activity on the basolateral membrane of gill epithelial cells, disrupting transmembrane ion gradients, but also induce antagonistic effects in the absorption kinetics of key divalent cations such as Ca²⁺, Mg²⁺, and Mn²⁺ [217]. Particularly in low-hardness waters, excessive sulfate ions competitively bind to transport proteins or form insoluble complexes and down-regulate the expression of sulfate transporter genes, thereby obstructing the assimilation of essential minerals in fish and triggering systemic electrolyte imbalances and nutritional metabolic disorders [218].

Beyond direct osmoregulatory interference, exposure to high sulfate concentrations serves as a crucial inducer for triggering oxidative stress cascades and histopathological lesions in aquatic animals. When environmental sulfate surpasses the organism’s regulatory threshold, it results in electron leakage within the mitochondrial respiratory chain and an excessive accumulation of reactive oxygen species (ROS), thereby activating compensatory responses in the antioxidant defense system. Research on the silver carp (Hypophthalmichthys molitrix) demonstrates that chronic exposure to high-salinity and high-sulfate environments significantly upregulates the activities of superoxide dismutase (SOD) and catalase (CAT) while inhibiting the activity of glutathione peroxidase (GSH-Px) [217]. Such abnormal fluctuations in enzyme activity signify the disruption of redox homeostasis.

The persistence of oxidative stress further gives rise to the lipid peroxidation of cell membranes and irreversible damage to tissue structure, which is especially prominent in metabolically active organs. Histopathological observations of the mud crab (Scylla paramamosain) reveal that chronic sulfate stress causes thickening and shrinkage of gill filament epithelial cells, leading to a reduction in the efficiency of gas exchange and ion excretion. Simultaneously, the hepatopancreas, which functions as the center for detoxification and metabolism, exhibits severe cellular vacuolization, nuclear pyknosis, and a disordered cellular arrangement [63,219].

Moreover, impairment of intestinal barrier function induces immune inflammatory responses, characterized by the downregulation of relevant immune gene expression and the release of inflammatory factors. Ultimately, this leads to a reduction in growth performance and a compromise in the immune defense capabilities of cultured animals [220].

Notably, within intensive aquaculture systems, particularly recirculating aquaculture systems (RASs) or pond bottoms replete with organic sediments, the perils of sulfate are also evident in the secondary risks stemming from its biochemical transformation processes. In hypoxic or anaerobic microenvironments, sulfate-reducing bacteria (SRB) employ sulfate as an electron acceptor for dissimilatory reduction metabolism, converting relatively low-toxicity sulfate into highly neurotoxic hydrogen sulfide (H₂S). H₂S binds to cytochrome c oxidase, obstructing the mitochondrial electron transport chain and inducing cellular asphyxiation [221]. For species with high sensitivity to substrate conditions, such as Atlantic salmon (Salmo salar), the accumulation of even trace quantities of H₂S can instigate mass mortality [222]. Nevertheless, the tolerance mechanisms for sulfate display significant inter-specific specificity. For example, largemouth bass (Micropterus salmoides) and large-scale loach (Paramisgurnus dabryanus) show robust adaptability to high sulfate levels, while tilapia exhibit extreme sensitivity in environments characterized by low potassium and high sulfate coupling [101,223,224].

Consequently, evaluating the ecological risks posed by sulfate in saline–alkaline waters requires comprehensive consideration of aqueous ionic composition (particularly K⁺ and Ca²⁺ concentrations), redox potential, and the physiological tolerance of cultured species.

4. Adaptation Mechanisms of Aquatic Animals Under Saline–Alkaline Conditions

4.1. Osmoregulation

In both teleosts and crustaceans, the gill serves as the principal organ for osmoregulation and ion transport [225,226]. Functioning through a typical process of extracellular anisosmotic regulation, aquatic animals rely on mitochondria-rich ionocytes (also known as chloride cells) within the gill epithelium to drive transmembrane ion transport via ATP energy supply [227,228]. Specifically, NKA is primarily responsible for establishing transmembrane electrochemical gradients, which drive the synergistic action of the Na⁺-K⁺-2Cl⁻ cotransporter (NKCC) and Cl⁻/HCO₃⁻ exchangers to facilitate active ion excretion or absorption [43,229].

In fish, seawater-adapted species counteract dehydration through a coordinated mechanism involving the excretion of excess salts via the gills, reduced urine production by the kidneys, and active water absorption in the intestine. Conversely, freshwater-adapted species maintain their hyperosmotic state (relative to the environment) by minimizing salt excretion, increasing the production of dilute urine, and accelerating intestinal metabolism; cortisol and prolactin play pivotal roles in these regulatory processes [230]. In crustaceans, in addition to the gills, the antennal glands and hepatopancreas also participate in the regulation of water–salt balance [231,232,233]. Research indicates that under salinity fluctuations, crustaceans significantly modulate the expression abundance of genes such as the NKA αα-subunit, V-type H⁺-ATPase (VHA), and aquaporins (AQPs) to maintain relatively constant hemolymph osmolality [226,234].

In addition to maintaining extracellular fluid homeostasis (e.g., hemolymph) through ion transport, intracellular isosmotic regulation represents a critical mechanism for protecting cell volume and function. This process primarily relies on the accumulation or degradation of intracellular organic osmolytes to achieve regulation [235]. Common osmolytes include amino acids and their derivatives, polyols and sugars, and methylamines [236]. Previous studies have shown that the free amino acid content in Pacific white shrimp (Litopenaeus vannamei) exhibits a significant linear positive correlation with environmental salinity [237]. However, this regulatory mechanism often displays pronounced non-singularity. For example, the mud crab (Scylla paramamosain) employs a combined strategy involving both extracellular and intracellular regulation to cope with environmental stress [151]. Metabolomic analyses have further revealed that under short-term low-salinity stress, organisms primarily rely on energetically costly ion transport (anisotonic regulation) to counteract osmotic shock, whereas during long-term acclimation, amino acid metabolic pathways are activated, and organic osmolytes (isosmotic regulation) gradually become dominant [238]. Figure 3 illustrates the osmotic regulation mechanisms of aquatic animals under salinity fluctuations.

4.2. Oxygen Consumption, Ammonia Excretion, and Metabolism

In inland saline–alkaline aquaculture environments, aquatic animals face severe challenges regarding osmotic gradient imbalances and ion regulation. These challenges compel them to initiate energy-intensive adaptive physiological mechanisms, leading to significant remodeling of oxygen consumption rates, ammonia excretion, and bioenergetic profiles. As the primary cultured species, teleosts and crustaceans have evolved distinct physiological regulatory strategies during their long-term adaptation.

For teleosts, maintaining internal homeostasis in high-salinity and high-alkalinity environments entails significant bioenergetic costs. This trade-off in energy allocation is directly reflected in the physiological compensation of respiratory oxygen consumption and nitrogen metabolism. Fish rely primarily on the synergistic action of the gills, kidneys, and intestines for osmoregulation. Since osmoregulation constitutes a substantial portion of the metabolic budget, oxygen consumption rates typically rise with fluctuations in environmental salinity. For instance, when Nile tilapia (Oreochromis niloticus) are transferred from freshwater to high-salinity environments, their oxygen consumption rate increases significantly during the initial phase, indicating active energy metabolism to cope with osmotic shock [239]. However, the oxygen consumption response is not influenced solely by environmental salinity; temperature and feeding status also exert superimposed effects. A study on channel catfish (Ictalurus punctatus) and blue catfish (Ictalurus furcatus) found that at high temperatures (25 °C and 32 °C), post-prandial oxygen consumption increased significantly, peaking at 1.8–2.0 times pre-feeding levels [240]. Notably, the high pH characteristic of saline–alkaline waters significantly inhibits the transmembrane diffusion of un-ionized ammonia (NH₃), leading to internal ammonia nitrogen accumulation [241,242,243]. To mitigate ammonia toxicity, fish have evolved specific adaptive mechanisms for excretion. On one hand, they responsively upregulate the expression of Rh glycoproteins (Rhbg, Rhcgb) via the HIF1A signaling pathway and enhance active ammonia transport in coordination with H⁺ pumps [244,245]. On the other hand, they activate metabolic detoxification pathways to convert endogenous ammonia into glutamine or urea [243,244,246]. To meet the energy demands of these processes, fish undergo metabolic reprogramming, characterized by prioritizing carbohydrate utilization via glycolysis, mobilizing lipid metabolism to maintain cell membrane fluidity, and activating amino acid catabolism to assist in osmoregulation [215,240,247,248,249].

Although the adaptive mechanisms of crustaceans (especially euryhaline species such as mud crabs and Pacific white shrimp) in saline–alkaline environments also follow the “energy for homeostasis” principle, they exhibit significant differences in organ coordination and metabolic strategies. Similarly to fish, the gills of crustaceans act as the core organ for ion transport, relying on NKA and V-type H⁺-ATPase to drive transmembrane transport [250]. However, under hyposaline or high-alkalinity stress, the antennal glands and hepatopancreas play critical compensatory roles by activating ion reabsorption pathways [231]. Fluctuations in environmental salinity directly induce adjustments in the respiratory metabolic rates of crustaceans, with oxygen consumption and ammonia excretion rates exhibiting marked fluctuations along salinity gradients [251,252]. Particularly under combined high-alkalinity stress, when ammonia excretion is inhibited, crustaceans often cope with ammonia toxicity by regulating genes related to ammonia metabolism and ion balance [253]. Furthermore, a distinguishing feature of crustacean adaptation to saline–alkaline environments is the adoption of intracellular isosmotic regulation strategies (as introduced in Section 4.1), which is a relatively energy-efficient strategy. During this process, the hepatopancreas serves as the metabolic hub, where pathways including glycolysis, the TCA cycle, and lipid metabolism are significantly regulated to ensure energy supply and maintain metabolite balance [254,255].

4.3. Immunity and Antioxidant Defense

In saline–alkaline aquaculture settings, the health condition of cultured organisms is highly contingent upon the synergistic regulation of their antioxidant capacity and immune defense systems. These two systems represent the crucial mechanisms for maintaining homeostasis and adapting to environmental stress [256]. The primary influence of saline–alkaline stress on aquatic organisms originates from the energetic compensation necessary for osmoregulation. To counteract ionic gradients in high-salinity and high-alkalinity environments, chloride cells in fish gills and epithelial cells in crustaceans must significantly upregulate Na⁺/K⁺-ATPase (NKA) activity, an active transport process that is highly ATP-consuming [231,250]. As energy production centers, mitochondria operating under such high loads are prone to electron leakage in the electron transport chain, subsequently leading to the excessive generation of reactive oxygen species (ROS), such as superoxide anions (O₂⁻) and hydrogen peroxide (H₂O₂) [257,258,259].

Oxidative stress is initiated when the rate of ROS generation induced by environmental pressure surpasses the scavenging capacity of the organism’s endogenous antioxidant system. At the molecular level, this stress is manifested as lipid peroxidation (LPO), where excess free radicals attack polyunsaturated fatty acids in cell membrane phospholipids, generating toxic byproducts such as malondialdehyde (MDA) and resulting in a loss of membrane fluidity and altered permeability [259]. At the tissue level, it is characterized by nuclear pyknosis and mitochondrial swelling in the hepatopancreas or liver, as well as the physical disruption of the intestinal mucosal barrier [256,258]. Notably, high-pH environments specifically inhibit ammonia excretion; the accumulation of intracellular ammonia and ROS produces synergistic toxic effects, further aggravating the risks of protein denaturation and DNA damage [146,259,260].

In response to oxidative damage, aquatic organisms have evolved a sophisticated antioxidant defense strategy that demonstrates significant time-dependence and tissue-specificity. During the initial stage of stress, teleosts (e.g., carp, tilapia) and crustaceans (e.g., Chinese mitten crab, Pacific white shrimp) typically initiate a “compensatory activation” mechanism. Through signaling pathways such as nuclear factor erythroid 2-related factor 2 (Nrf2), they rapidly upregulate the gene transcription and enzymatic activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) to maintain intracellular redox potential balance [261,262,263,264,265,266,267,268].

However, this compensatory mechanism has distinct physiological limitations. If the duration of stress is prolonged or the intensity exceeds tolerance thresholds, the antioxidant enzyme system enters a “decompensation phase.” This is characterized by a significant decline in enzymatic activity due to excessive consumption or structural impairment, resulting in the hyperaccumulation of ROS [269]. Furthermore, response strategies to oxidative stress vary across species and tissues. The liver of teleosts, serving as a center for metabolic detoxification, primarily relies on the synergistic action of GSH-Px and non-enzymatic antioxidants (e.g., glutathione, selenium, vitamins) [147]. In contrast, crustaceans tend to utilize multifunctional proteins and organic osmolytes within the hemolymph to assist in antioxidant defense [270].

A complex bidirectional regulatory mechanism exists between the antioxidant system and the innate immune system. ROS not only serve as metabolic byproducts but also act as crucial immune signaling molecules capable of activating nuclear factor-kappa B (NF-κB) and Toll-like receptor (TLR) signaling pathways, thereby inducing the expression of inflammatory cytokines and antimicrobial peptides [267,268,271]. At mucosal interfaces such as the gills and intestines, moderate ROS levels induced by saline–alkaline stress can upregulate the expression of immune effector molecules, including heat shock proteins (HSP70/90), crustin, and anti-lipopolysaccharide factors (ALFs), resulting in a state of immune activation [272,273,274]. However, this mechanism is notably more complex in crustaceans. Their unique prophenoloxidase (proPO) activation system is highly sensitive to redox status: while moderate stress can activate the proPO system to enhance the melanization reaction and pathogen clearance capacity, excessive oxidative stress leads to the downregulation of proPO gene expression, triggering immunosuppression [275]. Additionally, the activities of acid phosphatase (ACP) and alkaline phosphatase (AKP), key enzymes in non-specific immunity, are significantly enhanced in saline–alkaline environments. This enhancement not only aids in pathogen defense but also participates in metabolic compensatory mechanisms involving phosphorylation [276,277,278].

It is further noteworthy that the negative impacts of saline–alkaline environments on immunity are frequently mediated by the disruption of intestinal microecology. High osmotic pressure and alkalinity induce intestinal dysbiosis and compromise the physical barrier of the intestinal epithelium, thereby facilitating the invasion of opportunistic pathogens and subsequently inducing systemic inflammation [279]. Therefore, the survival of aquatic organisms in saline–alkaline environments is inherently a comprehensive outcome of dynamically adjusting resource allocation between antioxidant and immune systems. This occurs against the backdrop of high energy expenditure required for maintaining osmotic balance, aiming to tolerate oxidative damage while sustaining pathogen defense capabilities.

5. Summary and Perspectives

Saline–alkaline aquaculture serves not only as a crucial avenue for expanding the production space of fisheries but also as a vital approach for promoting the ecological restoration of saline–alkaline lands and the efficient utilization of resources. Existing research suggests that the responses of aquatic animals to the combined stresses of high salinity, high pH, and complex ionic compositions depend on a highly coordinated physiological compensation network. To uphold internal homeostasis, organisms must modulate the activity of ion-transport enzymes and acid–base regulatory proteins via key organs such as the gills, kidneys, and intestine. This energy-intensive process compels animals to restructure their energy metabolism strategies and is accompanied by significant alterations in oxygen consumption and ammonia excretion patterns.

Simultaneously, in response to the oxidative stress and immune challenges that ensue, cultured animals have developed protective mechanisms centered around enzymatic antioxidant systems and non-specific immune defenses, mitigating tissue damage through the scavenging of reactive oxygen species (ROS) and the activation of immune factors. This cross-system coordination, which encompasses osmoregulation, energy allocation, and immune defense, collectively forms the physiological foundation for the survival of aquatic animals in extreme saline–alkaline habitats.

Although the adaptive mechanisms at the physiological and biochemical levels have been relatively well-characterized, further endeavors are necessary to clarify the molecular regulatory networks underlying saline–alkaline tolerance and to advance research on germplasm innovation. Future studies should integrate multi-omics approaches to thoroughly explore the key functional genes that regulate osmotic balance and acid–base homeostasis, and to examine the roles of epigenetic modifications such as DNA methylation in environmental memory and trait inheritance. In particular, a systematic elucidation of the unique tolerance mechanisms of indigenous species such as Gymnocypris przewalskii and Leuciscus spp. is required to enrich the adaptive gene pools.

On this basis, it is essential to establish molecular marker-assisted breeding systems based on genome-wide association studies (GWAS), and to shift the research focus from the analysis of acute stress induced by single factors to the examination of chronic adaptation under multifactorial coupled stresses. Such efforts will facilitate the construction of precise water quality regulation models and nutritional enhancement strategies, providing robust theoretical support for the breeding of high-quality saline–alkaline-tolerant varieties and for promoting the green and high-quality development of saline–alkaline fisheries.