To address the quality deterioration of Japanese flounder (Paralichthys olivaceus) muscle characterized by the formation of a paste-like texture during frozen storage at −20 °C, we investigated the antifreeze mechanism and optimal concentration of oregano (Origanum vulgare) essential oil. Healthy P. olivaceus with a body mass of (700±50) g were used as experimental subjects, and four groups were established: a control group (Soaked in normal saline) and three oregano essential oil soaking groups at concentrations of 100, 500, and 1000 mg·kg⁻¹ (Mass fractions of the key components: carvacrol ≥5.0%, thymol ≥5.0%, eugenol ≥2.5%, cinnamaldehyde ≥4.0%). The samples were stored at −20 °C for 28 d, and indicators including texture properties, moisture-related parameters, myofibrillar protein oxidation and denaturation, antioxidant capacity, and microstructure were measured every 7 d. The results show that after 28 d of frozen storage, the control group exhibited significant quality deterioration (p<0.05): hardness, springiness, and chewiness decreased by 46.21%, 27.73%, and 38.86%, respectively; the drip loss reached (17.49±1.01)%; water-holding capacity (WHC) decreased to (84.36±0.85)%; the carbonyl mass fraction of myofibrillar protein increased by 82.74%; the activities of superoxide dismutase (SOD) and catalase (CAT) decreased by 62.90% and 65.56%, respectively; malondialdehyde (MDA) concentration increased to (3.98±0.19) nmol·mg⁻¹; the muscle fiber structure was disordered with severe degradation of the endomysium; and collagen mass fraction decreased to (0.251±0.021) mg·g⁻¹ (A 58.96% reduction compared with the initial value). The 100 and 500 mg·kg⁻¹ oregano essential oil groups significantly ameliorated the above deterioration (p<0.05), among which the 500 mg·kg⁻¹ group showed the best effect: on 28^th day, its hardness was 24.97% higher than that of the control group, Ca²⁺-ATPase activity increased by 26.69%, and muscle fibers still maintained a partial hexagonal arrangement. The 100 mg·kg⁻¹ group had a 17.56% lower drip loss than that of the control group, and its collagen mass fraction was 26.2% higher than that of the control group (p<0.05). In contrast, the 1 000 mg·kg⁻¹ group exhibited inferior protective effects compared with the medium-low concentration groups. In conclusion, medium-low concentrations (100, 500 mg·kg⁻¹) of oregano essential oil can alleviate the quality deterioration of frozen P. olivaceus muscle, possibly by regulating ice crystal morphology, inhibiting oxidation, and reducing myofibrillar protein degradation.

The management of effluent generated by industrial marine aquaculture constitutes a significant challenge for the sustainable development of the sector. In this study, we employed juvenile Urechis unicinctus as a model organism to assess their efficacy in treating effluent derived from industrial Paralichthys olivaceus aquaculture, with an additional focus on elucidating the underlying physiological adaptation mechanisms and intestinal microbial interactions. Four stocking densities (450, 900, 1 350, and 1 800 ind·m⁻²) were implemented for both short-term (168 h) and long-term (30 d) dynamic monitoring. The findings indicated that juvenile U. unicinctus effectively reduced concentrations of suspended solids (SS), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), ammonia nitrogen ( ${\mathrm{N}\mathrm{H}}_4^{+}\text{-}\mathrm{N}$), nitrite ( ${\mathrm{N}\mathrm{O}}_2^{-}\text{-}\mathrm{N}$), and phosphate ( ${\mathrm{P}\mathrm{O}}_4^{3-}\text{-}\mathrm{P}$) in the effluent. Among the tested densities, the group stocked at 900 ind·m⁻² demonstrated superior treatment performance, achieving removal rates of 35.63% for SS and 26.70% for TP, with an optimal hydraulic retention time identified as 24 h. In a 30-day long-term experiment, under conditions of 900 ind·m⁻², the survival rate of the larvae reached 96.23%, the body mass gain rate reached 20.94%, and the system maintained a stable removal efficiency for the aforementioned water quality parameters. Mechanistic investigations revealed that activities of intestinal digestive enzymes (Lipase, α-amylase, and trypsin) increased markedly during the initial treatment phase before stabilizing. Metagenomic sequencing analysis showed that the intestinal microbiota was predominantly composed of Proteobacteria (65.16%) and Vibrio (46.27%), with species diversity peaking on Day 15. Functional predictions suggested a significant upregulation of genes associated with energy and amino acid metabolism, alongside a downregulation of those involved in carbohydrate and lipid metabolism, indicating an adaptive metabolic shift within the microbial community. Collectively, this study systematically elucidates the synergistic adaptation mechanisms of juvenile U. unicinctus to aquaculture effluent through the regulation of digestive enzymes and functional remodeling of the intestinal microbiome. Furthermore, it confirms that these juveniles can achieve satisfactory growth while effectively purifying effluent, thereby presenting a novel candidate species and providing a theoretical basis for the biological treatment of marine aquaculture wastewater.