Tetraploid oysters are artificially produced oysters that do not exist in nature. The successful breeding of 100% triploid oysters resolved the difficulties of traditional drug-induced triploids, such as the presence of drug residues and a low triploid induction rate. However, little is known concerning the biochemical composition and nutrient contents of such tetraploids. Therefore, we investigated compositional differences among diploid, triploid, and tetraploid Crassostrea gigas as well as between males and females of diploids and tetraploids. The findings indicated that glycogen, EPA, ∑PUFA, and omega-3 contents were significantly higher in triploid oysters than in diploids or tetraploids; tetraploid oysters had a significantly higher protein content, C14:0, essential amino acid, and flavor-presenting amino acid contents than diploids or triploids. For both diploid and tetraploids, females had significantly higher levels of glutamate, methionine, and phenylalanine than males but lower levels of glycine and alanine. In addition, female oysters had significantly more EPA, DHA, omega-3, and total fatty acids, a result that may be due to the fact that gonadal development in male oysters requires more energy to sustain growth, consumes greater amounts of nutrients, and accumulates more proteins. With these results, important information is provided on the production of C. gigas, as well as on the basis and backing for the genetic breeding of oysters.

Ploidy level exerts profound influences on the phenotypic and physiological traits of Crassostrea gigas. Compared to diploids, triploids exhibit desirable characteristics such as sterility, a faster growth rate, and improved meat quality. In contrast, tetraploids often suffer from slow growth, yet the mechanisms underlying these polyploid-associated traits remain unclear. This study aimed to elucidate these mechanisms by comparing differences in growth-related phenotypes and gene expression among diploid, triploid, and tetraploid oysters. We identified 1533 differentially expressed genes (DEGs) between diploids and triploids, 946 DEGs between triploids and tetraploids, and 1326 DEGs between diploids and tetraploids. Through trend analysis, we clustered genes with similar expression changes across ploidy levels and conducted functional enrichment analysis on these gene clusters. The results revealed that genes associated with the innate immune response were significantly up-regulated in tetraploids, whereas genes related to biomineralization and metabolism were markedly up-regulated in triploids. These findings suggest that tetraploid oysters may mount a stronger innate immune response compared to diploids and triploids, while triploids demonstrate superior growth performance. This study provides valuable resources for investigating the functional aspects of genes related to polyploid phenotype differences.

Near-infrared spectroscopy (NIR) has become an essential tool for non-destructive analysis in various fields, including aquaculture. This study presents a pioneering application of portable NIR spectrometers to analyze glycogen content in the gonadal tissues of the Pacific oyster (Crassostrea gigas), marking the first instance of developing quantitative models for glycogen in tetraploid C. gigas. The research also provides a comparative analysis with models for diploid and triploid oysters, underscoring the innovative use of portable NIR technology in aquaculture. Two portable NIR spectrometers were employed: the Micro NIR 1700 (908–1676 nm) and the Micro PHAZIR RX (1624–2460 nm). Near-infrared spectra were acquired from the gonadal tissues of diploid, triploid, and tetraploid C. gigas. Quantitative models for glycogen content were developed and validated using cross-validation methods. Additionally, qualitative models for different ploidies and genders were established. For the Micro NIR 1700, the cross-validation correlation coefficients (Rcv) and cross-validation relative predictive errors (RPDcv) for glycogen were 0.949 and 3.191 for diploids, 0.915 and 2.498 for triploids, and 0.902 and 2.310 for tetraploids. The Micro PHAZIR RX achieved Rcv and RPDcv values of 0.781 and 2.240 for diploids, 0.839 and 2.504 for triploids, and 0.717 and 1.851 for tetraploids. The Micro NIR 1700 demonstrated superior quantitative performance, with RPD values exceeding 2, indicating its effectiveness in predicting glycogen content across different ploidy levels. Qualitative models showed a performance index of 91.6 for diploid and 95 for tetraploid genders using the Micro NIR 1700, while the Micro PHAZIR RX achieved correct identification rates of 99.79% and 100% for diploid and tetraploid genders, respectively. However, differentiation of ploidies was less successful with both instruments. This study’s originality lies in establishing the first quantitative models for glycogen content in tetraploid C. gigas using portable NIR spectrometers, highlighting the significant advancements in non-destructive glycogen analysis. The applicability of these findings is substantial for oyster breeding programs focused on enhancing meat quality traits. These models provide a valuable phenotyping tool for selecting oysters with optimal glycogen content, demonstrating the practical utility of portable NIR technology in aquaculture.

Selenium-enriched yeast is an important organic selenium source for improving the nutritional value and physiological function of aquatic products. However, whether ploidy affects the molecular responses of Pacific oysters (Crassostrea gigas) under selenium supplementation conditions remains unclear. In this study, diploid, triploid, and tetraploid Pacific oysters were fed a selenium-enriched diet for 10 days. Hepatopancreas tissues were collected for antioxidant enzyme assays and transcriptomic analysis. The results showed that triploid oysters exhibited higher superoxide dismutase, glutathione peroxidase, and catalase activities, together with a relatively lower malondialdehyde level, suggesting stronger antioxidant defense under selenium supplementation conditions. Transcriptomic analysis revealed substantial numbers of differentially expressed genes across all groups compared, indicating pronounced ploidy-dependent transcriptional responses. Functional enrichment analysis showed that these genes were mainly associated with metabolic processes, oxidative stress responses, membrane components, endocrine-related pathways, lipid metabolism, and signal transduction. Several redox-related genes, including glutathione S-transferase family genes, CHAC1, superoxide dismutase family genes, and NRX1, showed ploidy-dependent expression patterns. In particular, triploid oysters appeared to rely more on enzymatic antioxidant defense and glutathione turnover, whereas tetraploid oysters showed a stronger tendency toward redox signal integration and homeostatic regulation. These findings provide new insights into ploidy-dependent selenium responses in Pacific oysters and offer a theoretical basis for the development and utilization of selenium-enriched oyster products.

Near-infrared (NIR) spectroscopy is a vital non-destructive analytical tool in the food and aquaculture industries. This study pioneers the application of portable NIR spectrometers for evaluating selenium (Se) content in the Pacific oyster (Crassostrea gigas). We developed quantitative and qualitative models to predict selenium levels in oyster tissue, representing a novel application for monitoring trace elements in marine organisms. Quantitative models were developed using partial least squares (PLS) regression on spectra collected with two portable spectrometers (Micro NIR 1700, Micro PHAZIR RX) and a benchtop FT-NIR instrument, with validation via cross-validation and an independent set. Qualitative models were also constructed to categorize Se content into three levels: 0–1, 1–3, and >3 mg/kg. For quantitative analysis, the Micro NIR 1700 model performed robustly in external validation (RP = 0.932; RMSEP = 0.392; RPD = 2.46). The Micro PHAZIR RX model achieved the highest RC (0.988) and the lowest RMSEC (0.233), yet cross-validation indicated a potential risk of overfitting. In contrast, the FT-NIR instrument yielded the best external predictive ability for powdered samples (RP = 0.954, RPD = 2.60), highlighting its high precision under laboratory conditions. For qualitative discrimination, the Micro PHAZIR RX’s classification module achieved a 100% correct recognition rate (AUC = 0.937). The models based on the Micro NIR 1700 and FT-NIR instruments showed cumulative contribution rates (CCR) of 98.61% and 97.59%, respectively, with high performance indices (PI) of 89.3 and 90.2, confirming their effective discrimination capability. The models established in this study enable the rapid, on-site detection of Se content in oyster samples, underscoring the significant potential of portable NIR spectroscopy for selenium analysis in shellfish.