ABSTRACT Ozobranchus jantseanus is the largest metazoan known to survive in liquid nitrogen without pretreatment to date; however, the mechanism underlying this tolerance remains unclear. In this study, the first analyses of histological and morphological changes in normal, frozen, and dehydrated states were performed. Adults survived after direct placement in liquid nitrogen for 96 h, with a survival rate of approximately 86.7%. The leech could withstand rapid desiccation and its survival rate after rehydration was 100% when its water loss was below about 84.8%. After freezing, desiccation, and ethanol dehydration, the leech immediately formed a hemispherical shape. Particularly during drying, an obvious transparent glass-like substance was observed on surface. Scanning electron microscopy revealed many pores on the surface of the posterior sucker, creating a sponge-like structure, which may help to rapidly expel water, and a hemispherical shape may protect the internal organs by contraction and folding reconstruction in the anterior–posterior direction. A substantial amount of mucopolysaccharides on the surface and acid cells and collagen fibers in the body, all of which contained substantial polysaccharides, may play a key protective role during freezing. Our results indicate that the resistance of leeches to ultra-low temperatures can be explained by cryoprotective dehydration/vitrification strategies. This article has an associated First Person interview with the first author of the paper.