AbstractTardigrades are well known for their ability to survive extreme conditions such as desiccation and freezing by entering cryptobiosis, a state in which metabolism becomes immeasurable. Within tardigrades, cryptobiosis has been investigated almost entirely in eutardigrades and echiniscoideans living in semi-terrestrial habitats (predominantly mosses); information regarding cryptobiosis in the neglected marine arthrotardigrades remains sporadic, with the current understanding that arthrotardigrades are unable to undergo cryptobiosis. In the present study we have investigated two marine arthrotardigrade species, Styraconyx haploceros (Halechiniscidae) and Batillipes pennaki (Batillipedidae), inhabiting the tidal zone of Roscoff, France. These two species are likely candidates for cryptobiotic arthrotardigrades as they may experience large variations in environmental parameters. Our results show that S. haploceros, which live on the lichen Lichina, is able to withstand complete desiccation. Upon exposure to high salinities, this species enters a tun state, characterized by a longitudinal contraction of the body and a withdrawal of head and limbs. Furthermore, it can withstand very low salinities and distilled water for extended periods. Batillipes pennaki, which lives in sandy sediments, is vulnerable to both desiccation and distilled water, and can only survive extreme conditions for brief periods of time.