The market value of sea urchin gonads is determined by the specific characteristics associated with gonad size and texture. Formulated feeds can effectively promote the gonad growth of sea urchins but cannot assure essential gonad texture traits. The objective of this study was to investigate the impact of vitamin C (VC) on the gonad growth, texture, collagen content, and the expression of genes involved in the collagen synthesis of sea urchins (Mesocentrotus nudus). Graded amounts of VC (0, 3000 and 6000 mg/kg) were supplemented to make three formulated feeds. Fresh kelp (Saccharina japonica) was used as the control diet. Each diet was randomly distributed to three tanks of M. nudus. The results indicated that the gonadosomatic index (GSI) and texture traits of M. nudus fed C3000 were significantly greater than those fed C0 and C6000. Collagen type I (Col I) in the gonads of M. nudus fed C3000 showed significantly greater areas than those fed C0 and C6000. Consistently, the expression levels of collagen alpha-1 (colp1α) of M. nudus fed C3000 were significantly higher than those fed C0 and C6000. As for the transforming growth factor beta (tgf-β)/Smads pathway, the expression levels of collagen synthesis genes (tgf-β receptor 1 and 2, smad nuclear-interacting protein 1 (snip1) and prolyl 4-hydroxylase subunit beta (p4hβ)) in the C3000 group were significantly greater than those in the C0, C6000 and kelp groups. On the contrary, the expression levels of collagen degradation genes (lysyl oxidase-like 2 (loxl2) and matrix metalloproteinase 14 (mmp14)) in the C3000 group were significantly lower than those in the C0, C6000 and kelp groups. In conclusion, VC at an addition level of 3000 mg/kg significantly increased the gonad texture and collagen contents of M. nudus, which could be accomplished by increasing collagen synthesis and inhibiting collagen degradation through the tgf-β/Smads pathway. These results could contribute to better understanding the beneficial effects of VC addition on the gonad texture quality of M. nudus.

Rapid and coordinated test growth is crucial for maintaining the normal body shape of Strongylocentrotus intermedius juveniles. In total, 270 S. intermedius (1.19 ± 0.01 g) were randomly assigned to 18 floating cages. Three cages of sea urchins were fed kelp (Saccharina japonica) (control diet) or one of five formulated feeds with different carbohydrate-to-lipid ratios (C/L) (1, 2, 4, 8, and 16) for 90 days. The results suggested that the weight gain rate of S. intermedius fed C/L4 was markedly greater than that fed C/L1 and C/L16 except for kelp, C/L2, and C/L8. The test diameter (TD) and test height (TH) gain rates of S. intermedius fed C/L4 were markedly greater than those fed other dry feeds except for C/L2. The TH/TD of S. intermedius fed kelp was markedly greater than that fed dry feeds, except for C/L1 and C/L2. Juvenile S. intermedius fed C/L2 showed higher test magnesium content, larger holes, and longer and thinner trabeculae than those fed other dry feeds. Overall, juvenile S. intermedius fed C/L4 had the highest body weight gain rate and test growth rate among the formulated feed groups. Juveniles fed C/L2 had the most coordinated test growth reflected by TD/TH, which is comparable to those fed kelp. Therefore, the optimal C/L for rapid and coordinated test growth of juvenile S. intermedius should be higher than 2 but lower than 4.