To elucidate the community structure of phytoplankton, the primary producers, and associated environmental drivers in the Rushan oyster aquaculture area, field investigations were conducted at 20 fixed stations during four cruises in Oct. 2023, Feb. 2024, May 2024, and Aug. 2024, which represented autumn, winter, spring, and summer, respectively. Qualitative samples were collected via vertical tows using a Shallow Water Ⅲ plankton net, while quantitative samples were obtained from surface waters (0.5 m) using a 5 L organic glass water sampler. All samples were fixed with Lugol′s solution. Environmental parameters, including water temperature, salinity, pH, and dissolved oxygen, were measured in situ using a YSI professional plus portable multiparameter meter, and secchi depth was determined with a 30 cm Secchi disk. Chlorophyll a was analyzed by acetone extraction-spectrophotometry, and nutrients (NH₄⁺-N, NO₃^--N, NO₂^--N, PO₄^3--P, and SiO₃^2--Si) were determined spectrophotometrically. Community diversity indices were calculated using PRIMER 6.0, and relationships between environmental factors and phytoplankton assemblages were analyzed using DCA and RDA in Canoco 5.0. A total of 67 species belonging to 34 genera and 2 phyla were identified, with an average abundance of 25.39×10³ cells/L. Diatoms (Bacillariophyta) dominated the species composition of phytoplankton (91.04% of the total). Seasonal analysis revealed significant fluctuations in species richness and abundance. The richness peaked in autumn 2023 (37 species) but bottomed out in summer (23 species). The abundance also peaked in autumn (59.50×10³ cells/L), which was significantly higher than that of other seasons (P < 0.01). Diversity indices remained consistently high (Shannon-Wiener H′=2.51±0.48, Margalef d=2.63±0.93, Pielou′s J=0.86±0.07), indicating that high diversity and even distribution were significantly variable seasonally (P < 0.01). Twelve dominant species, all diatoms were identified weirdly (Y≥0.02). Coscinodiscus granii dominated autumn through spring while Pseudo-nitzschia pungens dominated winter through summer. Pearson correlation and RDA revealed that phytoplankton community structure was driven by multiple physicochemical factors which included chlorophyll a, dissolved oxygen, temperature, nutrients (N, P, Si), salinity and COD_Mn, and the dominant driver(s) shifted seasonally. These findings provide a scientific basis for ecological conservation and sustainable management of the Rushan aquaculture area.

To investigate the optimal stocking density and dietingredient of tetraploid Crassostrea gigas larvae, in this study, we used diploid larvae as a control group and established three larval density gradients (4, 8, and 12 individuals/mL) and four diet treatments, Total Isochrysis galbana (Iso) group (T-Iso group, 100% Iso), Mixed α group (70% Iso+30% Tetraselmis subcordiformis (Tet)), Mixed β group (30% Iso+70% Tet) and Total Tet group (T-Tet group, 100% Tet). The effects of stocking density and diet on larval growth and survival were analyzed. The results demonstrated that both growth and survival of tetraploid larvae were significantly lower than those of diploids, with tetraploids being more sensitive to high-density stress and having higher dietary requirements. The density experiment revealed that a moderately higher density (8 individuals/mL) showed an advantage at the early developmental stage, while a low density (4 individuals/mL) was most conducive to the growth and survival of tetraploid larvae in the later stage, yielding the maximum shell height ((270.79±15.07)μm) and survival rate ((30.55±4.44)%) in 20 days post-fertilization. In contrast, high density (12 individuals/mL) led to growth retardation and a significant decrease in survival. The diet experiment indicated that the mixed diet (Mixed α group) was the most effective, resulting in the greatest shell height ((330.87±15.19)μm) and the highest survival rate ((70.30±2.81)%) of tetraploid larvae in 20 days. The T-Iso group and the Mixed β group were less effective while the single diet of Tet performed the poorest. In summary, under the experimental conditions applied, a moderately higher stocking density (8 individuals/mL) is more favorable for the growth and survival of C. gigas tetraploid larvae at the early developmental stage, while a lower density (4 individuals/mL) is more significantly advantageous in the later phase. After reaching the umbo larval stage, Tet can be supplemented as part of the diet at a recommended feed ratio of 70% Iso and 30% Tet.

To investigate the tetraploid induction effect between two allotetraploid oysters (GGAG—GGA ♀×GG ♂ and AAGG—AAG ♀×GG ♂) under varying cytochalasin B (CB) concentration (0.2, 0.4, 0.6, 0.8 mg/L), salinity gradient (6, 8, 10, 12) and induction duration (10, 15, 20, 25 min), we analyzed the cleavage rate, hatching rate, tetraploidy rate and induction efficiency. At the same time, the growth and survival of the early larvae were analyzed. The results showed that the optimal method of inducing tetraploid of GGAG was 0.6 mg/L of CB and 15 min. The tetraploidy rate of (42.75± 2.89)% was obtained with the method. The salinity 8 and 15 min was also the optimal method, with which the tetraploidy rate of (32.28±1.97)% was obtained. For AAGG, 0.6 mg/L of CB and 20 min was optimal, with which the tetraploidy rate of (47.03±2.66)% was obtained. The salinity 10 and 15 min was the optimal method, with which the tetraploidy rate of (33.06±1.18)% was obtained.

To explore the optimal conditions for inducing triploid oyster (Crassostrea nippona), the effects of different induction methods (cytochalasin B and hypotonic treatment), induction durations, and induction times on cleavage rate, hatching rate, triploid rate and comprehensive evaluation index were examined in this study. Additionally, the impact of the two induction methods on larval growth and survival was analyzed. When cytochalasin B(CB) concentration was 0.5 mg/L, the induction duration was 16 min, and the induction was initiated when 30% of the first polar body(PB1) was released, the triploid rate reached (69.71±2.59)% at which the comprehensive evaluation index was the highest (42.39%). For the hypotonic treatment, at salinity 7, with the same induction duration and time (30% PB1 release), the triploid rate was (41.82±1.79)% with a comprehensive evaluation index of 24.82%. Regarding survival rate, both CB and hypotonic groups exhibited significantly lower survival rates than the control. The survival rate of the hypotonic group was higher than that of the CB group within the first 6 days but dropped below it after 8 days. In summary, the optimal condition for inducing triploid C.nippona is 0.5 mg/L CB concentration, initiating induction at 30% PB1 release, and maintaining treatment for 16 min. With the optimized method, the highest triploid rate, survival rate and comprehensive evaluation index were observed.