EVAPORATIVE WATER LOSS BY TARDIGRADES UNDER CONTROLLED RELATIVE HUMIDITIES

¹ Department of Zoology, University of California, Davis, California 95616

The effects of relative humidity on the survival of desiccation by the tardigrade, Macrobiotus areolatus, were investigated. The most survivals were obtained when the animals were dried at relative humidities greater than 70% at 20° C. At these high humidities the animals form tuns, while at lower humidities they become flattened or crumpled. Anesthetized animals do not form tuns at any humidity.

The rate of evaporative water loss from tuns in air was investigated by recording weight loss from animals kept at known relative humidities. Tuns formed by active animals lose water during the early stages of dehydration at a rate approximately 0.3 times the rate of anesthetized animals. Anesthetized animals equilibrate with the surrounding air within one hour, while tuns require more than 100 hours to equilibrate. At the end of 100 hours, the water content of tuns at 80% RH is 10-25%. During dehydration the permeability coefficient of tuns decreases a hundredfold (from 2.0-5.0 x 10^-4 cm hr^-11 atm^-1 to 1.6-6.0 x 10^-6 cm hr^-1 atm^-1). The tardigrades can be reduced to 2-3% water content in dry air without killing them, if they are first dehydrated to about 20% water content at 70-95% RH.

The cuticle was investigated as the most likely site of control of evaporative water loss. Studies with dye and heavy metal tracers demonstrated that during tun formation high permeability areas of the cuticle are removed from contact with the air. After extensive dehydration the entire cuticle becomes less permeable to water, possibly due to a lipid phase change.

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