Abstract: 【Objective】To provide a reference for exploring the immune function of Onchidium reevesii skin and developing natural antimicrobial drugs,ultrastructural observation of O. reevesii abdominal skin and analysis of the antimicro-bial properties of skin mucus were performed.【Method】Collected abdominal skin (foot and side of foot) and skin mucus from O. reevesii,observed skin structure,glandular and glandular cell characteristics by light microscopy and transmis-sion electron microscopy,detection of glycoconjugates in mucous cells by histochemical staining technique was con-ducted and seven pathogenic bacteria were treated with skin mucus of O. reevesii,the growth curve method and zone of inhibition were used to determine the antimicrobial activity of the mucus,and the antimicrobial mechanism of the mucus was studied in depth with Staphylococcus aureus and Vibrio harveyi.【Result】The results showed that the mucous cells had a higher content of acidic mucopolysaccharides;the mucus glands in the foot were round,and mucus containing a mixture of mucopolysaccharides was stored in the central lumen of the glands;and the granular glands in the skin of side of foot secreted acidic mucopolysaccharides.According to the morphology and density of the secretory vesicles,there were six and three types of glandular cells in the skin of the foot and in the lateral skin of the foot respectively.And the se-cretory vesicles were attached to the cell membrane and secreted into the environment via exocytosis.O. reevesii skin mu-cus had an inhibitory effect on the growth curves of all seven pathogens,with broad-spectrum antibacterial activity,and the results of the zone of inhibition test showed the optimal inhibitory effect on S.aureus and V.harveyi.Scanning electron microscope results showed that after being treated with skin mucus,the morphology of S.aureus and V.harveyi appeared malform and crack,cellular contents leaked out.Meanwhile,the change in OD₂₆₀ reflected that skin mucus could increase the permeability of bacterial cell membranes,indicating that skin mucus could effectively disrupt the integrity of bacterial cell membranes.In addition,the activity of key enzymes of tricarboxylic acid cycle and ATP content in S.aureus and V.harveyi were significantly decreased (P<0.05,the same below) and the activity of the Na⁺/K⁺-ATPase significantly in-creased compared to the control group after co-cultivation of mucus with bacteria.This suggests that mucus was able to disrupt the energy metabolism process of the bacteria,which led to the inhibition of bacterial growth and reproduction.【Conclusion】The abdominal skin of O. reevesii has several types of glands and glandular cells which secrete mucus ca-pable of disrupting the integrity of bacterial cell membranes as well as energy metabolism processes,thus exhibiting a broad-spectrum antibacterial activity.