Seasonal variations in biochemical composition and nutritional quality of Crassostrea hongkongensis, in relation to the gametogenic cycle.

Variations in the biochemical composition and nutritional quality with annual changes in gonad development were investigated to identify the optimal harvesting time of C. hongkongensis. The glycogen levels in the mantle, muscle, and gonad-visceral mass were significantly lower in June than in December, associated with changes in the expressions of ChGS and ChGP. Protein content consistently exceeded 52% of dry weight. The only significant change in protein levels was an increase between April and June in the gonad-visceral mass, which was associated with the gonadal transition from proliferation to maturation. Moreover, C. hongkongensis consistently had a well-balanced essential amino acid profile, meeting the essential amino acid requirements of preschool children. The lipid content and fatty acid composition of C. hongkongensis varied with the reproductive cycle, but the omega-3:omega-6 ratio was consistently higher than those of C. gigas and C. virginica. In summary, the optimal harvest time of C. hongkongensis was during the inactive stage of most gonads (from August to February at Beihai).

Tetraploid induction of Crassostrea hongkongensis and C. sikamea by inhibiting the polar body 1 release in diploid fertilized eggs.

The production of an all-triploid population by mating tetraploid males with diploid females is the best and most fundamental method for the large-scale production of triploid oysters. Obtaining a stable tetraploid population is essential for guaranteed production in industrialized triploid cultivation. C. hongkongensis and C. sikamea are important oyster breeding species in southern China, and have great economic value. However, there are not any published data on inducing tetraploid C. hongkongensis or C. sikamea. Therefore, we investigated tetraploid induction in these two oyster species by inhibiting the PB1 release in diploid fertilized eggs using Cytochalasin B (CB) under 31 °C, 15 ‰ salinity. The results confirmed that the optimal tetraploid induction conditions for C. hongkongensis were a CB concentration of 0.50 mg/L with induction starting at 9.0 min after fertilization, and stopping at 21.0 min after fertilization; the induction efficiency index reached 0.123 under these conditions. The optimal tetraploid induction conditions for C. sikamea were a CB concentration of 0.50 mg/L, with induction starting at 7.5 min after fertilization and stopping at 18 min after fertilization; the induction efficiency index could be as high as 0.281 under these conditions. However, we confirmed that the tetraploid rate decreased with larval growth, and no tetraploids were detected in the juvenile period of either C. hongkongensis or C. sikamea. This may be attributed to the very low survival of the tetraploid larvae induced by this method, especially as most tetraploid larvae died during the first three days. In summary, it is simple to directly induce tetraploid C. hongkongensis and C. sikamea larvae by inhibiting the PB1 release of diploid zygotes, but the low survival rate makes it challenging to obtain viable juvenile tetraploids.

Comparison of the Biochemical Composition and Nutritional Quality Between Diploid and Triploid Hong Kong Oysters, Crassostrea hongkongensis.

This study is the first systematic comparison of the biochemical composition and nutritional quality between diploid and triploid Hong Kong oysters, Crassostrea hongkongensis. Results showed that in the reproductive season, the glycogen content in five tissues (gill, mantle, adductor muscle, labial palps and gonad) was significantly higher (P < 0.05) in triploids than in diploids, with odds ratios (ORs) of 96.26, 60.17, 72.59, 53.56, and 128.52%, respectively. In the non-reproductive phase, significant differences in glycogen content (P < 0.05) between diploid and triploid oysters existed only in gill and gonad. In both diploid and triploid Hong Kong oysters, quantitative real-time PCR analysis of the glycogen synthesis gene (ChGS) and glycogen phosphorylase gene (ChGP) showed that the gene expression patterns matched the pattern of variation in glycogen content. Moreover, in both the reproductive and the non-reproductive phases, triploid Hong Kong oysters had a well balance of essential amino acids and were thus a well source of high-quality protein. Surprisingly, in both phases, significantly higher (P < 0.05) percentages of four essential fatty acids (α-linolenic acid, linoleic acid, eicosapentaenoic acid, and docosahexaenoic acid) were observed in triploids than in diploids. Additionally, the ratio of n-3/n-6 polyunsaturated fatty acids (PUFAs) was much higher in triploids than that in diploids. Variations in Biochemical composition were consistent with the relative expression of the citrate synthase gene (ChCS) and the α-ketoglutarate dehydrogenase gene (ChKD), which are key enzyme genes of the tricarboxylic acid cycle. Overall, the triploid Hong Kong oyster has a better nutritional value and taste than the diploid in terms of glycogen content, protein quality and fatty acid content.

Analysis of in situ Transcriptomes Reveals Divergent Adaptive Response to Hyper- and Hypo-Salinity in the Hong Kong Oyster, Crassostrea hongkongensis.

Crassostrea hongkongensis, a commercially valuable aquaculture species dwelling in estuaries along the coast of the South China Sea, is remarkable for its eurysalinity traits that enable its successful colonization of diverse osmotic niches ranging from near freshwater to seawater. In order to elucidate how this oyster copes with coastal waters with immense salinity differences, we performed in situ transcriptomic analysis (RNA-seq) to characterize the global expression patterns of oysters distributed across naturally formed salinity gradients in Zhenhai Bay along the northern coast of the South China Sea. Principal component analysis reveals distinct expression profiles of oysters living in the extreme conditions of hypo-salinity and hyper-salinity. Compared with the situation of optimal salinity for oyster growth, hypo-salinity mainly regulated expression of genes involved in FoxO and oxytocin signaling, tight junction and several immune pathways, while hyper-salinity altered gene expression implicated in amino acid metabolism, AMPK and PI3K-AKt signaling pathways, demonstrating the complexity and plasticity of transcriptomic expression underpinning oyster eurysalinity. Furthermore, the expression patterns of several genes correlated with salinity gradients reveals the fine-tuned coordination of molecular networks necessary for adaptive homeostasis in C. hongkongensis. In conclusion, a striking capacity and distinct patterns of transcriptomic expression contribute to eurysalinity adaptation in C. hongkongensis, which provides new mechanistic insights into the adaptive plasticity and resilience of marine mollusks.