Tardigrades entering a state of anhydrobiosis (cryptobiosis) show considerable interspecific variation in desiccation tolerance, lower lethal humidities for initial desiccation ranging from 78 to 53 %. Species most tolerant of rapid initial drying also show the most rapid acquisition of tolerance to low humidities (25–31 %) following drying in high humidity.
Surface area reduction during tun formation shows a significant positive regression against desiccation tolerance in the Eutardigrada. The most desiccation-tolerant species thus infold the largest areas of cuticle. By comparison, the heterotardigrade Echiniscus testudo shows a very poor capacity for surface area reduction. The thick dorsal plates may restrict cuticle intucking in this species.
When hydrated tardigrades are desiccated in 80 % relative humidity they show a characteristic profile of mass loss, dehydrating rapidly in the first few minutes and then showing an abrupt reduction in transpiration (the ‘permeability slump'). This applies to living animals, which form tuns when desiccated, and to dead animals remaining extended. The permeability slump is not, therefore, a metabolic phenomenon and is not related to tun formation. Subsequent transpiration rates are very low and decline exponentially. The permeability slump allows animals to retain considerable amounts of internal water when desiccated, although less water is retained if the desiccation rate is increased. This may determine upper tolerable desiccation rates if a certain minimum quantity of water is required for the metabolic synthesis of membrane protectants. A significant positive regression between water retention and desiccation tolerance supports this hypothesis.
- © 1989 by Company of Biologists