Recently, a new pearl oyster Pinctada fucata martensii strain has begun to be cultured as seafood. In the present study, the seasonal variation (February and June) in biochemical composition and flavor compounds in two P. f. martensii strains (strain for pearl production was abbreviated to PP, and seafood was abbreviated to PE) were detected to compare the nutritional and flavor differences between them, and to provide a reference for the seasonal preference of consumers for eating P. f. martensii. The ratio of soft tissues in PE-Feb was significantly higher than that in PP-Feb (p < 0.05). The contents of ash, crude protein, and crude lipid were higher in PP than those in PE in the same season, while significantly higher contents of glycogen in the PE strain were observed compared to the PP strain in the same season (p < 0.05). The major amino acids (such as Glu and Asp) and PUFA (such as DHA and EPA) were almost the same in two P. f. martensii strains in the same season, while the contents of these nutrients were significantly higher in February compared to June (p < 0.05). Taurine content in PE-Feb was the highest (19.58 mg/g wet weight), followed by PP-Jun, PP-Feb, and PE-Jun. The umami and sweet FAA contents of the same P. f. martensii strain in February were significantly higher than those in June (p < 0.05). The AMP content in PP-Jun was the highest (64.17 mg/100 g wet weight), followed by PP-Feb, PE-Jun, and PE-Feb. Succinic acid was the major organic acid, and its content in February was significantly higher than in June (p < 0.05). The betaine content in PP-Feb was the highest (23.02 mg/g of wet weight), followed by PE-Feb (20.43 mg/g of wet weight), PP-Jun (16.28 mg/g of wet weight), and PE-Jun (12.33 mg/g of wet weight), and significant differences were observed among these four groups (p < 0.05). In conclusion, the edible P. f. martensii strain harvest in February is rich in protein, glycogen, PUFA (DHA and EPA), taurine, succinic acid, and betaine, which could provide healthy nutrition and a good flavor for humans.

Crassostrea hongkongensis (C. hongkongensis) is one of the three most commonly cultivated oyster species in China. Seasonal hypoxia is one of the most serious threats to its metabolism, reproductive behavior, and survival. To investigate the effects of hypoxia stress on the antioxidant capacity and energy metabolism of C. hongkongensis, the total antioxidant capacity (T-AOC), glycogen content, and enzyme activities (phosphofructokinase, PFK; pyruvate kinase, PK; phosphoenolpyruvate carboxykinase, PEPCK) of oysters were determined under normoxic (DO 6 ± 0.2 mg/L) and hypoxic (DO 1.5 mg/L) conditions at 0 h, 6 h, 48 h, and 72 h. We also determined the T-AOC, glycogen content, and enzyme activities of oysters under reoxygenation (recovered to normoxia for 24 h). To further examine the potential molecular regulatory mechanism of hypoxic adaptation, a transcriptome analysis was conducted on the gill of C. hongkongensis under normoxia (N, 72 h), hypoxia (H, 72 h), and reoxygenation (R). After being exposed to hypoxia for 6 h, the T-AOC, glycogen content, and enzyme activities of PK, PFK, and PEPCK in C. hongkongensis were significantly decreased. However, after prolonging the duration of hypoxia exposure for 72 h, the T-AOC, glycogen content, and enzyme activities increased compared to that of 48 h. After 24 h reoxygenation, the T-AOC, glycogen content, and enzyme activity of PK and PFK returned to close to initial levels. In addition, a transcriptome analysis discovered 6097 novel genes by mapping the C. hongkongensis genome with the clean reads. In total, 352 differentially expressed genes (DEGs) were identified in the H vs. N comparison group (235 upregulated and 117 downregulated genes). After recovery to normoxia, 292 DEGs (134 upregulated and 158 downregulated genes) were identified in the R vs. N comparison group, and 632 DEGs were identified (253 upregulated and 379 downregulated genes) in the R vs. H comparison group. The DEGs included some hypoxia-tolerant genes, such as phosphoenolpyruvate carboxykinase (PEPCK), mitochondrial (AOX), tyramine beta-hydroxylase (TBH), superoxide dismutase (SOD), glutathione S-transferase (GST), and egl nine homolog 1 isoform X2 (EGLN1). Additionally, DEGs were significantly enriched in the KEGG pathways that are involved in hypoxia tolerance, including the metabolism of xenobiotics by cytochrome P450 pathways and the HIF-1 signaling pathway. Then, we selected the five hypoxic-tolerant candidate DEGs for real-time quantitative polymerase chain reaction (RT-qPCR) validation, and the results were consistent with the transcriptome sequencing data. These discoveries have increased our understanding of hypoxia tolerance, recovery ability after reoxygenation, and molecular mechanisms governing the responses to hypoxia in C. hongkongensis.

Although the expression of miRNAs has been widely applied to investigate on gonads, the role of miRNAs in the gonadal development of white Pacific shrimp (Litopenaeus vannamei) remains unknown. In this study, we performed high-throughput sequencing to identify the sex-related microRNAs (miRNAs) that elucidated the regulatory mechanisms on the gonadal differentiation of L. vannamei. We obtained a total of 29,671,557 and 28,526,942 raw reads from the ovaries and testes library, respectively. We then mapped 26,365,828 (92.73%) of the ovarian clean sequences and 23,694,294 (85.65%) of the testicular clean sequences for a transcriptome reference sequence of L. vannamei. After blasting the miRNA sequences against the miRBase database, we identified 153 significantly differentially expressed miRNAs between the ovaries and testes. To confirm the high-throughput sequencing results, we used a reverse transcriptase–quantitative polymerase chain reaction (RT-qPCR) to verify the expression patterns of the seven most differentially expressed miRNAs (i.e., novel_mir23, miR-92b-3p_3, miR-12-5p_2, novel_mir67, miR-279_1, let-7-5p_6, miR-263a-5p_1). According to the results of RT-qPCR, most of the miRNAs were expressed consistently with the high-throughput sequencing results. In addition, the target genes significantly enriched several Kyoto Encyclopedia of Genes and Genome (KEGG) pathways that were closely related to gonadal differentiation and development, including extracellular matrix–receptor interaction, Hedgehog signaling pathway, protein digestion and absorption and cell adhesion molecules (CAMs). This study revealed the first miRNAs sequencing of L. vannamei gonads. We identified sex-related differentially expressed miRNAs and KEGG pathways, which will be helpful to facilitate future research into the regulatory mechanism on the gonadal differentiation of L. vannamei.

The use of an appropriate amount of anesthetic can prevent the sacrifice of oysters during experimental sampling or seedling rearing. In this study, we assessed the effects of air exposure and three concentrations of magnesium chloride (10, 40, and 70 g L^–1) on the anesthetic efficiency for Crassostrea hongkongensis. We examined the recuperative capacity of the oysters post-tissue sampling. The results indicated that 36 h of air exposure could significantly enhance the anesthetic efficiency of C. hongkongensis while ensuring the oysters’ survival. A 40 g L^–1 solution of magnesium chloride had the optimal anesthetic effect on C. hongkongensis across all oyster sizes. In addition, smaller oysters (119.10 g) had a higher relaxation rate than larger oysters (191.00 g). The oysters anesthetized with 40 g L^–1 magnesium chloride also demonstrated the best recovery. No deaths were observed among the non-anesthetized oysters, anesthetized-recovered oysters, or non-recovered oysters during a seven-day post-farming period. In terms of live sampling, the control group (100 %), gonad group (100 %), mantle group (98.67 %), gill group (89.33 %), and adductor muscle group (97.33 %) all showed high survival rates after anesthesia. Observation of tissue growth post-sampling revealed the presence of focal green areas in tissues between days 7 and 14. By day 30, the adductor muscle and mantle tissues had fully healed, and the regrown tissue in the gills had approached its original length. A 40 g L^–1 magnesium chloride solution had no significant effects on gametes in terms of fertilization rate, growth rate, and attachment rate. The present findings indicated that C. hongkongensis could be effectively anesthetized using a 40 g L^–1 magnesium chloride solution following 36 h of aerial exposure. Post-anesthesia tissue and gamete sampling did not result in significant mortality, and the tissues had fully recovered within 30 days post-sampling. The anesthesia did not have any adverse effects on the progeny.

The tri-spine horseshoe crab, Tachypleus tridentatus, is an evolutionarily ancient marine chelicerate of considerable ecological and conservation importance in the Indo-West Pacific. Although several mitochondrial genomes have been reported, comprehensive comparative mitogenomic analyses encompassing all extant limulid genera remain limited. In this study, we sequenced and characterized the complete mitochondrial genome of an F1 Tachypleus tridentatus individual derived from wild-caught broodstock from the Beibu Gulf, northern South China Sea, and subsequently maintained at an aquaculture facility, using high-throughput sequencing, and performed an integrated comparative mitogenomic analysis across the three extant genera of Limulidae. The circular mitochondrial genome was 15,006 bp in length and contained the typical set of 37 mitochondrial genes, including 13 protein-coding genes (PCGs), 22 transfer RNA genes, and two ribosomal RNA genes. The newly sequenced Tachypleus tridentatus mitogenome showed highly conserved genome organization and nucleotide composition relative to previously published conspecific mitogenomes, confirming established mitochondrial characteristics of the species. The principal contribution of this study is the addition of a complete Beibu Gulf mitogenome and its integration with 12 additional complete mitochondrial genomes representing all extant limulid species, together with four mitochondrial markers, K2P genetic-distance analyses, and independent BI and ML phylogenetic reconstruction. The concordance among these mitochondrial datasets provides an integrated assessment of genetic divergence and mitochondrial phylogenetic relationships within Limulidae and expands the geographic representation of complete mitochondrial genomic resources for Tachypleus tridentatus.