Rising environmental temperatures increase stress on marine organisms, causing physiological damage or death in extreme cases. In this study, we examined the crab species Xenograpsus testudinatus from a shallow hydrothermal vent area in Kueishan Island, off northeast Taiwan. This crab species belongs to the monotypic family Xenograpsidae and is a well-known inhabitant of the shallow hydrothermal vents of Kueishan Island. The maximum temperature tolerance of X. testudinatus and the effects of temperature on this crab species remain unclear. Our laboratory experiments demonstrated the effects of elevated seawater temperatures on the survival and enzymatic function of X. testudinatus. Its mortality rate increased with the elevation of seawater temperature from 25 °C to 40 °C. Survival duration was short (<
4 h) when the crabs were reared at 40 °C, and the lethal temperature (LT