Temperature is a critical environmental factor that influences the growth, development, metabolism, and overall physiological performance of fish. Eleutheronema tetradactylum is an economically significant fish species; however, its molecular mechanism’s response to long-term cold stress is still unclear. In this study, we investigated the physiological responses of the liver in E. tetradactylum exposed to a constant temperature of 18 °C for durations of both 7 and 14 days, utilizing liquid chromatography–mass spectrometry (LC-MS), metabolomics, and conventional biochemical assays. The antioxidant status, liver histology, and metabolite profiles were examined at different time points. Our results revealed that, following sustained cold exposure, the activities of key antioxidant enzymes—superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)—initially increased and then decreased. Additionally, levels of malondialdehyde (MDA), a marker of oxidative damage, significantly elevated after 7 and 14 days of cold stress. Histopathological examination of liver tissues showed varying degrees of vacuolation and nuclear atrophy in hepatocytes, indicating oxidative damage. Metabolomic profiling identified 87 and 116 differentially expressed metabolites in the liver on days 7 and 14, respectively. Pathway enrichment analysis revealed significant alterations in pathways related to carbohydrate digestion and absorption, glutathione metabolism, and glycerolipid metabolism. These findings suggest that mechanisms regulating cell membrane fluidity, energy metabolism, autophagy, and antioxidant defense are crucial for the adaptation of E. tetradactylum to cold stress. Overall, this study provides valuable insights into the molecular and physiological adaptations of E. tetradactylum to low temperature, highlighting the activation of protective antioxidant responses and modifications of metabolic pathways in the liver.

The effects of Bacillus coagulans (BC) T-21 on growth performance, intestinal digestive enzyme activity, intestinal morphology, and intestinal microbiota of juvenile fourfinger threadfin (Eleutheronema tetradactylum) were investigated in the present study. Healthy juvenile E. tetradactylum with an initial body weight of 4.2 ± 0.5 g were fed a basal diet sprayed with 1 × 10⁸ cfu/g B. coagulans for eight weeks, and the growth parameters, intestinal digestive enzyme activities, HE-stained intestinal sections, and intestinal microbiota of the juvenile fish were measured. The differences in the feed conversion ratio between the experimental and control groups (fed the basal diet) were extremely significant (p < 0.01), whereas the differences in weight gain rate, specific growth rate, survival rate, and condition factor were significant (p < 0.05). In the experimental group, trypsin and amylase activities decreased significantly, whereas there were no significant differences in lipase activity between the two groups. Compared to the control group, the height of the intestinal villi was greater. No significant differences were observed in the diversity of intestinal microbiota and microbial species at the genus level (p > 0.05). Based on the function prediction analysis, the count values for the glycan biosynthesis, metabolism, and digestive system pathways were significantly increased in the experimental group (p < 0.05). However, there were no significant differences in the counts of other functional pathways (p > 0.05). These results indicate that dietary B. coagulans supplementation is effective in promoting the growth performance and intestinal health of juvenile E. tetradactylum.

Vibrio parahaemolyticus is an important foodborne and aquaculture-associated pathogen, and its increasing antimicrobial resistance threatens shrimp production, seafood safety, and public health. Although bacteriophages have emerged as a potential solution to the growing challenge of antimicrobial resistance, whether antibiotic resistance is associated with phage tolerance remains unclear. This study characterized antimicrobial resistance profiles and resistance genes in Vibrio parahaemolyticus isolates from Zhanjiang shrimp aquaculture farms and evaluated their association with bacteriophage susceptibility. A total of 132 isolates were tested against 18 antibiotics, while molecular and phage-related analyses were performed on 132 isolates. High resistance was observed against kanamycin (95.5%), sulfamethoxazole (94.7%), cefazolin (78.8%), imipenem (78.8%), amoxicillin (70.5%), and ampicillin (65.2%). Multidrug resistance was detected in 131/132 isolates (99.24%), indicating an extensive resistance burden. PCR analysis showed high detection rates of aphA (100.00%), sul1 (99.24%), blaTEM (98.48%), blaOXA (73.48%), blaCTX-M (53.03%), qnrA (45.45%), tetA (37.88%), and tetB (21.21%). Phage response analysis showed that 70.45% and 68.18% of isolates were phage-resistant at 4 h and 5 h, respectively. Genes potentially associated with phage–host interactions were also detected, with luxS showing the highest frequency (85.61%). Association analysis revealed significant links between tetA, tetB, blaOXA, and bacteriophage responses. These findings suggest that antimicrobial resistance and reduced phage susceptibility can co-occur among V. parahaemolyticus isolates recovered from shrimp farms in Zhanjiang, highlighting the need for regional resistance surveillance and careful phage selection.