A 70-day feeding experiment was performed to investigate the effects of dietary vitamin E at different addition levels (0, 100, 200, and 400 mg/kg) on the growth, collagen content, antioxidant capacity, and expressions of genes related to the transforming growth factor beta (TGF-β)/Sma- and Mad-related protein (SMAD) signaling pathway in sea cucumbers (Apostichopus japonicus). The results showed that the A. japonicus in the group with 200 mg/kg vitamin E exhibited significantly higher growth rates, hydroxyproline (Hyp) and type III collagen contents, and superoxide dismutase (SOD) activity, as well as the upregulation of genes related to Tenascin, SMAD1, and TGF-β. Additionally, the A. japonicus in the group with 100 mg/kg vitamin E exhibited significantly higher body-wall indexes, denser collagen arrangements, improved texture quality, higher activities of glutathione peroxidase (GSH-Px) and peroxidase (POD), as well as the upregulation of genes related to collagen type I alpha 2 chain (COL1A2), collagen type III alpha 1 chain (COL3A1), and Sp-Smad2/3 (SMAD2/3). In contrast, the A. japonicus in the group with 400 mg/kg vitamin E showed a decrease in the growth rates, reduced Hyp contents, increased type I collagen contents, collagen fiber aggregation and a harder texture, along with the downregulation of genes related to the TGF-β/SMAD signaling pathway. Furthermore, the A. japonicus in the group with 400 mg/kg exhibited oxidative stress, reflected by the lower activities of SOD, GSH-Px, and POD. These results indicated that A. japonicus fed diets with the addition of 100–200 mg/kg vitamin E had improved collagen retention and texture quality by increasing the activities of antioxidant enzymes and the expressions of genes in the TGF-β/SMAD signaling pathway. However, the excessive addition of vitamin E (400 mg/kg) induced oxidative stress, which could increase the collagen degradation and fibrosis and pose a threat to the growth and texture quality of A. japonicus.

Sea cucumber (Apostichopus japonicus) is an important economically cultured species in the northern coastal regions of China. Its body wall is rich in collagen, which directly determines product quality and market value. However, with the expansion of aquaculture scale, issues such as insufficient collagen deposition have led to inconsistent quality among cultured individuals. Therefore, there is an urgent need to improve growth performance and body wall quality through nutritional regulation. As functional nutrients, amino acids play key roles in collagen synthesis, yet relevant research on A. japonicus remains limited. This study was conducted to investigate the effects of dietary proline on the growth performance, body wall collagen deposition and organoleptic quality of sea cucumber (initial body weight: 30.20 ± 2.02 g). Three kinds of feed with equal levels of nitrogen and other lipids, and supplemented with different concentrations of proline (0%, 1.5%, 3%) in the basal diet, were named P0, P1.5, and P3, and the experiment was conducted for 60 days. The results showed that supplementation with 3% proline significantly increased final body weight and weight gain rate (p < 0.05), reaching 66.39 g and 115.30%, respectively. Collagen content in the body wall increased by approximately 18.5% compared to the control group. Histological analysis of the body wall showed that the collagen fibers in the P1.5 and P3 groups were thicker, with an increased proportion of type I collagen. Texture profile analysis indicated that hardness, cohesiveness, and chewiness were significantly improved in the P3 group (p < 0.05). In summary, supplementation with 1.5% and 3% proline effectively enhanced growth, collagen deposition, and body wall quality. Compared to the P0 group, the relative expression levels of collagen type I alpha 2 chain (COL1A1), Sma- and Mad-related protein 1(SMAD1), and sp-smad2/3 (SMAD2/3) in the body wall tissue were significantly upregulated in both the P1.5 and P3 groups (p < 0.05).