Effects of Dietary Vitamin A on the Growth Performance, Nonspecific Immune Response, Shell Microbiota and Red Spotted Disease Resistance of Juvenile Sea Urchin (<i>Strongylocentrotus intermedius</i>)

A 114‐day feeding trial was used to investigate the influence of vitamin A (VA) on growth performance, nonspecific immune responses and shell microbiota in juvenile sea urchin ( Strongylocentrotus intermedius ). Graded levels of VA (0, 4000, 8000, 16,000, 32,000 and 64,000 IU/kg) were added to make six experimental feeds. Each feed was allocated to three parallel tanks of sea urchins (initial weight 0.87 ± 0.05 g and initial test diameter 1.83 ± 0.57 mm). The data revealed that the weight gain rate (WGR) and gonadosomatic index (GSI) rose markedly as VA addition level increased from 0 to 4000 IU/kg and then reached a plateau with further increase of dietary VA levels. As VA addition level increased, nonspecific immune response of S. intermedius first increased and then decreased, with those fed diets with relatively higher addition of VA (32,000 IU/kg) exhibiting significantly greater phagocytic activity (PA) and acid phosphatase (ACP) activities, as well as upregulated expression of several immune‐related genes such as tumour necrosis factor α ( TNF-α ), antimicrobial peptides ( AMPs ), toll‐like receptors ( TLRs ) and lysozyme ( LYZ ). The abundance of Firmicutes, Bacteroidota, Bacteroides and Faecalibacterium increased, but that of Proteobacteria and Leucothrix decreased in the shell of S. intermedius as VA addition level increased. The percentage of sea urchins with severe red spotted disease decreased from 64.44% to13.33% as VA addition level increased to 32,000 IU/kg and subsequently increased to 42.22% with further increase of VA addition level. On the contrary, the percentage of sea urchins with mild red spotted disease increased from13.33% to 55.55% as VA addition level increased to 32,000 IU/kg and subsequently decreased to 31.11% with further increase of VA addition level. These results demonstrated that a low addition level of VA (4000 IU/kg) can help S. intermedius achieve ideal growth performance. However, relatively higher addition levels of VA (32,000 IU/kg) enhanced nonspecific immunity and red spotted disease resistance of S. intermedius , which could be accomplished by promoting immune gene expression and optimizing the shell microbiota composition.