Microbial communities associated with fish mucosal tissues play important roles in host health and environmental adaptation. However, the effects of contrasting aquaculture conditions on the microorganisms of the large yellow croaker (Larimichthys crocea, L. crocea) remain poorly understood. In this study, 16S rRNA gene sequencing was used to compare the gut and gill microbiota of L. crocea cultured in a marine system and a saline–alkaline system, together with the corresponding surrounding water samples. The results showed that both host tissue type and rearing environment significantly influenced microbial community structure. Water samples generally exhibited higher alpha diversity than host-associated samples, whereas the gut and gill communities were clearly separated from the surrounding water microbial community. The intestine showed stronger culture system differentiation than the gill, suggesting greater niche selectivity, while the gill retained a closer association with the rearing water. Across multiple taxonomic levels, microbial composition differed markedly among water, gut, and gill samples, and the divergence between the marine culture and saline–alkaline culture system became more pronounced at finer taxonomic resolution. Differentially enriched taxa further revealed clear tissue- and habitat-associated microbial patterns. In addition, several dominant taxa were significantly correlated with environmental variables, especially salinity and nutrient-related factors, indicating that physicochemical heterogeneity contributed substantially to microbial community assembly. Overall, this study shows that host mucosal niche and rearing environment are associated with distinct gut and gill microbiota patterns in L. crocea and provides preliminary ecological information for future microbiome-informed aquaculture management, functional validation of candidate taxa, and the development of land-based culture systems for marine fish.

Background: The visual system plays critical roles in fish feeding, predator avoidance, and reproduction, with the phototransduction pathway serving as the core molecular mechanism of visual signal transduction. This study represents the first systematic analysis of phototransduction-related genes in large yellow croaker (Larimichthys crocea). Methods: Bioinformatics, transcriptomic analysis, and RT-qPCR were used to identify phototransduction-related genes in large yellow croaker and analyze their developmental expression patterns and responses to different light spectra. Results: Thirty phototransduction-related genes were identified, including genes involved in light perception, signal amplification, and signal termination and recovery. Structural analyses indicated that these genes were highly conserved in teleosts. Most genes showed low expression during 1–7 days post-hatching (dph), increased expression during 7–24 dph, and stable expression during 24–35 dph. Collectively, these results identify 15–24 dph as a critical developmental window for visual system maturation in large yellow croaker larvae. Under different spectral treatments at 15 dph, phototransduction-related genes exhibited wavelength-dependent expression patterns. Under the present experimental conditions, relatively higher expression levels of most phototransduction-related genes were observed under blue- and green-light treatments, whereas relatively lower expression levels were observed under the red-light treatment. Key downstream genes, including gucy2f, guca1d, calml4a, and rgs9b, were significantly affected by different light spectra. Conclusions: This study revealed the developmental dynamics and spectral response characteristics of phototransduction-related genes in the large yellow croaker. These findings provide new insights into teleost visual development and light adaptation and provide a molecular basis for future studies aimed at optimizing light conditions during larval rearing in large yellow croaker aquaculture.

Temporary holding water in live aquatic-product markets represents a short-term, animal-associated aquatic microenvironment, but its bacterial community dynamics remain poorly understood. In this study, 16S rRNA gene amplicon sequencing was used to characterize bacterial communities in holding water of Anguilla japonica, Sinonovacula constricta, and Ruditapes philippinarum. Samples were collected in the morning and evening to evaluate taxon-associated differences and short-term temporal shifts. The bacterial communities showed clear differentiation among the six animal–time groups. Anguilla japonica holding water showed distinct bacterial assemblages from the two bivalve-associated systems, whereas S. constricta and R. philippinarum holding water exhibited more pronounced morning-to-evening changes. In particular, R. philippinarum holding water had higher bacterial richness and a larger proportion of group-specific taxa in the morning, suggesting greater initial community heterogeneity. In addition, the occurrence–abundance analysis showed that taxa with broader distributions tended to have higher mean relative abundances, whereas Rud_8 contained a comparatively large proportion of low-occurrence taxa. Overall, this study demonstrates that live aquatic-product holding water harbors taxon-specific and time-responsive bacterial communities. These findings provide preliminary ecological insight into microbial dynamics in market-associated artificial aquatic microenvironments and highlight the need to consider aquatic animal type and sampling time in future water-quality monitoring.

Δ9 fatty acyl desaturase (Δ9 FAD) exerts important regulatory effects on fatty acid metabolism and membrane fluidity. However, its existence and the effects of nutritional regulation and ambient salinity on its expression are currently unknown in mud crab (Scylla paramamosain). In this study, the full cDNA sequence of Δ9 fad was isolated from the hepatopancreas of mud crab, which consists of 1468 bp and encodes a polypeptide of 347 amino acids. Sequence analysis revealed that the protein possessed typical characteristics of Δ9 FAD including three histidine-rich domains (HRLWSH, HRVHH and HNYHH), four transmembrane regions and three conserved motifs (FFSHMGW, QRKYY and PWDY). The results of tissue distribution indicated that the mud crab’s Δ9 fad expression was enriched in the hepatopancreas, with relatively lower levels found in the thoracic ganglia and cranial ganglia. Δ9 fad expression in the hepatopancreas was significantly influenced by dietary fatty acid profiles, and the replacement of fish oil with soybean oil markedly downregulated the transcriptional levels of hepatopancreatic Δ9 fad. Furthermore, the transcriptional levels of Δ9 fad were also significantly affected by ambient salinity. Under chronic salinity stress, the 7‰, 12‰, 17‰ and 22‰ salinity groups showed higher transcriptional levels of Δ9 fad in the hepatopancreas than the 27‰ salinity group, and the 7‰ salinity group significantly differed from the 27‰ salinity group. Under the acute salinity stress condition, the Δ9 fad transcriptional levels of muscle in the 14‰ and 4‰ salinity groups were higher than those in the 24‰ salinity group at 6 h and 12 h. By contrast, Δ9 fad in the hepatopancreas exhibited a slow response in reaction to low salinity stress, and only the 4‰ salinity group at 96 h had a higher value than the 24‰ salinity group. In addition, as for the starvation stress experiment, Δ9 fad expression in the starvation group was dramatically reduced when compared with the feeding group. These findings improve our understanding of the physiological roles of Δ9 fad and its regulatory mechanisms in crustaceans.

Gut microbiome development during fish growth involves not only taxonomic turnover but also changes in the persistence and ecological dominance of microbial members. Here, we characterized the gut microbiota of D. maruadsi across three successive growth-associated body-size groups: the early-growth group (BLEGC; 4.98–6.81 g), middle-growth group (BLMG; 16.67–19.63 g), and late-growth group (BLLGC; 27.30–28.66 g). The three groups were successively sampled from the same cohort continuously reared in the same culture system. Fish received the same diet formulation throughout cultivation, with feeding quantity adjusted according to growth stage. At each stage, 15 fish were sampled, and rinsed intestinal tissues from five individuals were pooled to generate one composite sample, yielding three pooled intestinal samples per growth stage. Gut bacterial communities differed significantly among growth stages (ANOSIM: R = 0.2593, p = 0.005), while Pseudomonadota remained the dominant phylum across all groups. Complementary PERMANOVA supported an overall stage-associated difference, although PERMDISP also indicated heterogeneous multivariate dispersion among stages. Growth-associated restructuring became more evident at finer taxonomic levels and was accompanied by a pronounced contraction of stage-specific microbial membership. The number of unique ASVs declined from 361 in BLEGC to 286 in BLMG and 69 in BLLGC, whereas 84 ASVs were shared across all three stages. Forty ASVs met the global persistent criterion of detection in at least eight of the nine pooled samples, and their abundance contribution increased from 58.43 ± 15.08% in BLEGC to 79.54 ± 6.38% in BLMG and 90.14 ± 2.14% in BLLGC. When mean abundance was calculated only among samples in which each ASV was detected, prevalence remained positively associated with conditional mean abundance. Collectively, these results indicate that growth-associated gut microbiome succession in D. maruadsi involves not only taxonomic turnover but also progressive redistribution of community abundance toward highly prevalent and persistent bacterial members.

Estrogen deficiency induces bone loss and is accompanied by lipid metabolic disturbances. This study investigated the protective effects of Larimichthys crocea-derived calcium-chelating peptides (LCP) in a leuprorelin-induced estrogen-deficient mouse model. LCP attenuated tibial trabecular deterioration, increased serum procollagen type I N-terminal propeptide (PINP), osteocalcin (OCN), and bone-specific alkaline phosphatase (BALP), and reduced C-terminal telopeptide of type I collagen (CTX-I). Histological analyses further showed increased trabecular area and OCN expression and reduced marrow adiposity. LCP also attenuated alterations in serum lipid profiles and circulating levels of leptin and adiponectin, while reducing adipogenesis-related gene expression in femoral tissue and white adipose tissue. Exploratory assays using group-pooled cultures of bone marrow mesenchymal stem cells (BMSCs) showed differentiation-related patterns that were directionally consistent with the in vivo findings, but these descriptive observations require confirmation using independent biological replicates. Correlation analysis revealed associations among bone mass, osteogenesis-related indicators, and lipid metabolism-related parameters. Collectively, LCP attenuated estrogen deficiency-induced bone loss together with favorable changes in indicators related to bone formation, adipogenesis, and lipid metabolism. These findings support further investigation of LCP as a food-derived nutritional ingredient for supporting bone health under estrogen-deficient conditions.