It is essential to formulate artificial diets for Cyclina sinensis to satisfy the requirements of industrial aquaculture. We evaluated the effects of different dietary sources on the growth, survival, nutritional components, digestive performance and nonspecific immunity of C. sinensis. The diets of the three experimental groups were as follows: natural bait (control group, CG), fermented Enteromorpha prolifea (EP) compound diet (Diet #1), and fermented Pyropia yezoensis (PY) compound diet (Diet #2). Feeding these diets did not affect the individuals' final weight, specific growth rate and survival rate. The total amino acid and total monounsaturated fatty acid of individuals fed with Diet #1 were significantly higher than those provided with the other diets (p < 0.05). There were no significant differences in the activities of α‐amylase, lipase, trypsin, total superoxide dismutase, lysozyme and acid phosphatase in the clam fed with different diets. In conclusion, the two fermented seaweed compound diets prepared herein can replace natural bait and meet the growth and developmental needs of C. sinensis. The fermented EP compound diet can improve the nutritional quality of the clam. This study lays a preliminary foundation for developing new artificial diets for clams and elucidates the further utilization of low‐value seaweed resources.

Polychlorinated biphenyls (PCBs) are environmentally persistent and highly toxic organochlorine compounds that may cause toxic effects on aquatic animals. In this study we assess the toxic effect of PCBs on a bivalve used in aquaculture, the clam Cyclina sinensis. To this end, individuals of C. sinensis were exposed for 72 h at two PCB concentrations (1 ng/L and 10 ng/L) and control (absence of PCBs). At the end of the exposure, the hemolymph, hepatopancreas, and gills samples of C. sinensis were harvested for analysis of the enzyme activity and histology. The results showed that acute PCBs exposure decreased the survival rate of C. sinensis compared to the control. Acute PCBs exposure up-regulated the enzymatic activity of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) and the content of malondialdehyde (MDA) in the hemolymph of C. sinensis, while down-regulated the non-specific enzymatic activity of alkaline phosphatase (AKP). For the hepatopancreas, 1 ng/L PCBs exposure up-regulated the enzymatic activity of SOD while down-regulated the enzymatic activity of CAT of C. sinensis. In the gill, the enzymatic activity of CAT decreased significantly and the MDA content increased of C. sinensis after 10 ng/L PCBs exposure. Moreover, histological observations showed that acute exposure to PCBs caused loss of gill filaments and lateral cilia and shortening of their length, in the studied organism. The present study will provide valuable reference data for marine shellfish aquaculture and toxicology research.

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The clam Cyclina sinensis is one of the important economical aquaculture shellfish in China. However, the mechanisms of sex determination and differentiation in C. sinensis have not been fully studied. In this study, full-length cDNAs of DMRT3 and FOXL2 were cloned and functionally characterized. The ORF region of CsDMRT3 consists of 1137 nucleotides, which encode 378 amino acids contains a conserved DM domain of DMRT family. The ORF region of CsFOXL2 is 1245 bp, encodes 414 amino acids, and contains a conserved FH domain. Tissue-specific expression results showed that the higher expression level of CsDMRT3 and CsFOXL2 was found in the ovary and testis of C. sinensis. The expression levels of CsDMRT3 and CsFOXL2 also peaked at the maturation stage of male and female gonadal development, respectively. Moreover, the expression levels of CsDMRT3 and CsFOXL2 were significantly higher in the trochophore and D-larval stages than in other stages. The transcript levels of CsDMRT3 reached the highest level at 11 months of age, while the CsFOXL2 reached the highest level at 7 months of age. In estradiol-treated experiments, the expression levels of CsDMRT3 and CsFOXL2 in the gonads were highest at 5 µg/L estradiol treatment, and histologically, it was observed that the oocytes diameters became larger with increasing estradiol concentration. These results suggest that CsDMRT3 and CsFOXL2 play an important role in gonadal development and sex differentiation of C. sinensis.

Residual chlorine is a common by-product of warm drainage in coastal nuclear power plants. when accumulating to some limit, it may threaten marine ecosystem especially for benthic clam. However, there are few studies on the molecular mechanisms related to immunity and antioxidant of residual chlorine stress on clams. In this study, the clam (Cyclina sinensis) was exposed for 96 h at different concentrations (0, 50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 mg/L) of residual chlorine to observe its mortality, measure the activity of antioxidant and immune-related enzymes, and analyses the gene expression level in the hepatopancreas by using the transcriptome sequencing. The results showed that the mortality rate increased with the increase of stress time and concentration, and the mortality rate in the 400, 450 and 500 mg/L groups reached 100% at 96 h. The tolerance to residual chlorine of C. sinensis decreased with the increase of chlorine dioxide concentration, and the LC50 of 96 h was 217.6 mg/L by linear regression method. After residual chlorine stress, the activity of antioxidant-related enzymes (T-AOC and SOD) in the hepatopancreas showed a trend of first increase and then decrease with the extension of stress time. The immune-related enzyme activities of AKP and LZM showed a downward trend between 0 and 96 h, while the ACP enzyme activity showed a trend of first rising and then decreasing. Transcriptome analysis showed that residual chlorine stress significantly changed the expression levels of immune-related molecules associated with signal transduction, prophenoloxidase cascade, cell apoptosis and pattern recognition protein/receptor. Moreover, glutathione S-transferase (GST), heat shock protein (HSP) and other antioxidant-related genes were significantly affected under residual chlorine stress. This study provided valuable information for understanding the effects of residual chlorine stress on survival, physiological metabolism and molecular mechanisms of immune and antioxidant functions of C. sinensis.