Habitat space is crucial for animals. This experiment explored the effect of four different farming acreages (0.5 m2, 1 m2, 10 m2, and 100 m2) on survival rate, growth performance, and muscle nutrition composition of swimming crabs (Portunus trituberculatus) at the farming density of 2 ind/m2. The results showed that the survival rate of the crabs that live in the 10 m2 group and live in 0.5 m2 alone had the highest survival rate of 20.50% and 20%, better than the 1 m2 group and the 100 m2 group with the lowest of 12.00% and 13% ( P < 0.05 ). There was no significant difference between final body weight and final carapace width among the 4 groups ( P   > 0.05 ). The crude lipid content was the highest in 1 m2, which was significantly higher than in 0.5 m2 ( P   < 0.05 ). The crude protein content in the 1 m2 and 10 m2 groups was significantly higher than that in the 0.5 m2 and 100 m2 groups ( P   < 0.05 ). The results of the three conventional nutrients showed that there was no positive correlation between the muscle conventional nutrients and the farming area. The polyunsaturated fatty acid part of the 100 m2 and 10 m2 group was significantly different from the 1 m2 and 0.5 m2 groups ( P   < 0.05 ). The results show that farming crab alone is not better than together, and a small aquaculture acreage helps to improve the muscle nutrition composition of the swimming crabs, while a large aquaculture acreage is more conducive to balance the development of nutrition. Overall, 10 m2 of habitat space was more economically feasible in practice.

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.