The distribution of saline-alkali water is extensive and is increasing globally each year. Fully utilizing saline-alkali water for aquaculture can help alleviate the scarcity of freshwater resources in global fisheries. As a major economic fish species, the largemouth bass (Micropterus salmoides) holds significant potential for aquaculture in saline-alkali water. In the present study, we evaluated its tolerance to different salinities (0 ppt, 6 ppt, 9 ppt, 12 ppt, 15 ppt, and 18 ppt) and investigated tissue pathology, serum biochemical indicators, enzyme activities of osmolality and antioxidant, and the relative expression of Na-K-2Cl 1a cotransporter (NKCC1a) under different saline stress (0 ppt, 6 ppt, 9 ppt, and 12 ppt). The largemouth bass 96 h mortality rate increased with increasing salinity, and the LC₅₀ for 96 h was 14.28 ppt based on the mortality results. High salinity group (12 ppt) caused gill and intestinal damage, including necrosis and cell shedding, while 6 ppt had no adverse effects, and the 9 ppt between the two salinities showed an adaptive change histologically. Serum osmolality, Na⁺, Cl⁻, and cortisol levels of the high salinity group were significantly higher than of the low salinities (p < 0.05). Similarly, Na⁺/K⁺-ATPase (NKA), Ca²⁺-Mg²⁺-ATPase (CMA), and superoxide dismutase (SOD) activities of 12 ppt peaked at 24 h (15.7 U/mgprot, 11.5 U/mgprot, and 243 U/mgprot), which is significantly different compared to the other three groups (p < 0.05). The expression of NKCC1a was significantly upregulated at 9 ppt and 12 ppt, suggesting its role in osmoregulation. Furthermore, the expression of NKCC1a in the gill is 2–4 times higher than that in the intestine. These results suggested that largemouth bass can be cultured at 6 ppt and selectively bred for tolerance at 9 ppt. NKA activity, cortisol levels, and NKCC1a expression can be used as a marker of salinity suitability. These findings provide insight into the adaptive mechanisms underlying the physiological responses to acute salinity stress and will contribute to improving aquaculture in saline waters.

Affected by the continuously rising temperature, thermal stress leads to a delinked growth rate and resistance to stress in cultured largemouth bass (Micropterus salmoides, LMB) in China. Identification of LMB with better thermal resistance will benefit the breeding of new varieties. However, there has been limited reporting on the evaluation to identify LMB with better thermal resistance. LMB consists of the northern LMB (Micropterus salmoides salmoides, NLMB) and the Florida LMB (Micropterus salmoides floridanus, FLMB). Due to their different geographical distributions, it has been suggested that FLMB exhibit better thermal resistance compared to NLMB. In this study, NLMB and FLMB were subjected to thermal stress for 3 h (acute) and 60 d (chronic) at 33 °C, respectively. Subsequently, the variations of 12 candidate biomarkers between NLMB and FLMB were analyzed. Exposure to acute thermal stress significantly increased plasma cortisol, blood glucose, and lactate levels; activities of superoxide dismutase (SOD), glutathione peroxidase (GPX), catalase (CAT), glucose kinase (GK), pyruvate kinase (PK), lactate dehydrogenase (LDH), and glucose 6 phosphatase (G6Pase); and the expressions of hsp70 and hsp90 in both NLMB and FLMB (p < 0.05). Compared to NLMB, FLMB exhibited a lower plasma cortisol level and a higher expression of hsp90 under acute thermal stress (p < 0.05). Exposure to chronic thermal stress significantly increased plasma cortisol and blood glucose levels, as well as activities of GK, PK, LDH, and G6Pase, as well as expressions of hsp70 and hsp90 in both NLMB and FLMB (p < 0.05). Additionally, FLMB showed a lower expression of hsp70 compared to NLMB (p < 0.05). In conclusion, our results showed that LMB with lower plasma cortisol level and higher expression of hsp90 under acute thermal stress, as well as lower expression of hsp70 under chronic thermal stress were suggested to have better thermal resistance. Our study provides valuable information for identifying and breeding LMB varieties with better thermal resistance in the future.

Fatty liver, characterized by excessive lipid droplet (LD) accumulation in hepatocytes, is a common physiological condition in humans and aquaculture species. Lipid mobilization is an important strategy for modulating the number and size of cellular LDs. Cyclooxygenase (COX)-mediated arachidonic acid derivatives are known to improve lipid catabolism in fish; however, the specific derivatives remain unknown. In the present study, we showed that serum starvation induced LD degradation via autophagy, lipolysis, and mitochondrial energy production in zebrafish hepatocytes, accompanied by activation of the COX pathway. The cellular concentration of PGF2α, but not other prostaglandins, was significantly increased. Administration of a COX inhibitor or interference with PGF2α synthase abolished serum deprivation-induced LD suppression, LD–lysosome colocalization, and expression of autophagic genes. Additionally, exogenous PGF2α suppressed the accumulation of LDs, promoted the accumulation of lysosomes with LD and the autophagy marker protein LC3A/B, and augmented the expression of autophagic genes. Moreover, PGF2α enhanced mitochondrial accumulation and ATP production, and increased the transcript levels of β-oxidation- and mitochondrial respiratory chain-related genes. Collectively, these findings demonstrate that the COX pathway is implicated in lipid degradation induced by energy deprivation, and that PGF2α is a key molecule triggering autophagy, lipolysis, and mitochondrial development in zebrafish hepatocytes.

Grass carp (Ctenopharyngodon idella) is a key aquaculture species, and understanding its adaptation mechanisms to saline environments is crucial for addressing the global freshwater salinization challenge. In this study, juvenile grass carp were acclimated to three salinity levels (0, 4, and 8 ppt) for 30 days, after which gill and intestinal tissues were sampled to quantify cortisol concentrations and conduct RNA-seq. Results showed that cortisol levels exhibited a salinity-dependent increase, with significantly higher concentrations in gill tissues than in intestinal tissues, suggesting that cortisol plays an important role in the salt adaptation of grass carp. RNA-seq revealed that ion transport-related genes were upregulated in gills, whereas biosynthesis, oxygen transport, and energy metabolism genes were downregulated. In the intestine, genes involved in taurine transport and intercellular junctions were highly expressed, while immune-related genes showed reduced expression. These findings suggest that high salinity suppresses respiration and energy metabolism efficiency, with ion exchange primarily occurring in gills. Functional annotation identified seven candidate genes (LOC127513882, aqp9b, ca4a, ca5a, igfbp1b, slc12a2, and slc12a4) as key regulators of salinity adaptation. Overall, our study provides valuable insights into the mechanisms underlying the salt tolerance of grass carp.

Meat quality critically influences product value and consumer preferences. Here, the effect of long-term saline adaptation on flesh nutritional quality, sensory value, texture, and flavor was evaluated in a new Micropterus salmoides salt-tolerant population. The results showed that the salt-tolerant population exhibited decreased lipid, saturated fatty acid, and long-chain polyunsaturated fatty acid contents but increased monounsaturated fatty acid content, accompanied by upregulated elongase 5 and fatty acid desaturase 6 mRNA levels. The meat color of the new population was brighter and fresher, with a slightly less red tint, and the increased 2,3-butanedione content resulted in a desirable creamy aroma. 3-Pentanone was the most abundant volatile substance in both populations. Regarding taste parameters, the salt-tolerant population had improved hardness, stickiness, chewiness, resilience, cooking loss, myocyte space, and collagen content. Alanine, proline, and histidine were the main amino acids responsible for flavor presentation. The salt-tolerant population was found to have lower bitter and higher sweet amino acid contents. Higher umami nucleotides and lower pH intensified umami and sourness in salt-tolerant meat. The saltiness of the new-population meat was enhanced. This study comprehensively evaluated the flesh quality of a salt-tolerant M. salmoides population with the potential for cultivation, thereby providing a reference for its potential development as an alternative aquaculture strain.

This study investigated the role of trpc3 in osmoregulation under salt stress in grass carp (Ctenopharyngodon idella). Tissue expression analysis showed trpc3 was highest in the brain, followed by gills, skin, and muscle. In gills and kidney, its expression increased with salinity. RNA interference using synthesized trpc3-dsRNA (2 μg/g) significantly reduced its expression. Under early salt stress, serum osmolality, Na⁺, Ca²⁺, and renal NKA activity were comparable between the interference and non-interference groups. However, gill CaN, NKA, and Ca²⁺-ATPase, as well as renal CaN and Ca²⁺-ATPase activities were significantly lower in the interference group. During late salt stress, the interference group exhibited significantly higher serum osmolality, Na⁺ concentration, and renal NKA activity than the non-interference group. Serum Ca²⁺ concentration remained unchanged between groups. Conversely, gill CaN, NKA, and Ca²⁺-ATPase, as well as renal CaN and Ca²⁺-ATPase activities were significantly lower in the interference group. Gene expression analysis revealed that early plcxd1 was higher, while trpc3, calm1a, aqp3a, nka beta 1b subunit, IL-1β, hsp70, and gill nkcc1 variant X1 were lower in the interference group. Late-stage expression showed nka beta 1b subunit (gills/kidney) and renal trpc3, nkcc1 variant X1 were higher; gill trpc3, plcxd1, aqp3a, nkcc1 variant X1, IL-1β and renal plcxd1, aqp3a, nkcc1 variant X1, hsp70 were comparable; and gill calm1a, hsp70 and renal calm1a, IL-1β remained lower in the interference group. In summary, grass carp trpc3 may mediate Ca²⁺ influx to regulate ion transport in the gills and kidney, playing a key role in restoring osmotic homeostasis.