Vitamin C Promoted Feeding and Growth Rate of Apostichopus japonicus by Increasing Its Digestive Capacity Through a 5-Hydroxytryptamine-Mediated Signaling Pathway

To investigate the effects of dietary vitamin C (VC) supplementation at different concentrations (0, 5000, and 15,000 mg/kg) on the growth and feeding performance of sea cucumbers (Apostichopus japonicus), a 60-day feeding trial was conducted. The results demonstrated that supplementation with 5000 mg/kg VC significantly enhanced growth performance, evidenced by a higher specific growth rate and weight gain, along with a shorter feeding initiation time and increased tentacle feeding frequency. Furthermore, the activities of digestive enzymes—including amylase, lipase, and pepsin—were markedly elevated in the intestines of A. japonicus fed 5000 mg/kg VC, accompanied by increased mucus secretion in the oral tentacles. Correspondingly, intestinal 5-hydroxytryptamine (5-HT) levels and the expression of tryptophan hydroxylase (AjTPH) and 5-HT receptor (Aj5-HT4R) genes were significantly upregulated in the same group. In contrast, relatively lower growth performance was observed in the control (0 mg/kg) and high-dose (15,000 mg/kg) VC groups. Histological analysis further revealed that 5000 mg/kg VC increased villus height and width and thickened the intestinal muscle layer. Overall, dietary supplementation with 5000 mg/kg VC enhanced feeding activity, digestive function, and intestinal morphology, thereby promoting growth. However, excessive supplementation (15,000 mg/kg) failed to provide similar benefits and was associated with impaired performance.