Abstract:

Cryptocaryon irritans is a highly infectious and lethal parasitic ciliate affecting marine fishes. The intestine, as a vital organ for nutrition and immunity in fish, is directly exposed to waterborne pathogens through the oral cavity. To investigate the impact of C. irritans infection on intestinal structure and function in fish, this study used the small yellow croaker (Larimichthys polyactis) as a model and systematically compared differences in intestinal microstructure, immune parameters, and microbial composition between healthy control and infected groups. The results showed that C. irritans infection caused structural damage and reduced the density of intestinal microvilli, while significantly increasing the levels of interleukin 1β (IL-1β) and complement 3 (C3) in the intestine. In terms of microbial composition, the infected group exhibited significant decreases in both Chao1 and Shannon indices, indicating reduced microbial diversity and richness. Specifically, the relative abundances of Vibrio, Alloprevotella, and Bacteroides increased significantly, whereas that of Neisseria was significantly reduced. KEGG analysis further revealed enhanced activities of the GnRH signaling pathway and FcγR-mediated phagocytosis pathway in the infected group, while metabolic pathways such as caffeine metabolism, mineral absorption, and retinol metabolism were significantly suppressed. In summary, C. irritans infection disrupts the intestinal microstructure of L. polyactis, triggers a strong immune response, reduces microbial diversity and richness, and impairs fundamental metabolic functions. This study elucidates the effects of C. irritans on the small yellow croaker intestine from histological, immunological, and microecological perspectives, providing a theoretical basis for future strategies in fish health management through intestinal microbiota modulation.

Key words: Larimichthys polyactis, Cryptocaryon irritans, microbial diversity, immune factor