Anhydrobiosis in bdelloid species, populations and individuals.

Bdelloid rotifers are aquatic microinvertebrates common in water bodies and in unstable "terrestrial" habitats, such as mosses and lichens. The key to the adaptability to live in unstable habitats is their capacity to tolerate habitat desiccation through anhydrobiosis, that is assumed apomorphic to the taxon. The life history traits of some "moss" and "water" species of bdelloid are compared, showing that the water species have shorter life span, higher fecundity and earlier age at first reproduction than the moss species. These traits are discussed in the light of current life history theories. Contrary to the assumptions of the models, anhydrobiosis of bdelloids does not appear to imply energy demand. Past research on bdelloid anhydrobiosis is briefly reviewed, focusing on the factors that affect anhydrobiosis success, like morphological and physiological adjustments, and on the effect of events of anhydrobiosis during life time. Desiccation produces a time shift on the age of the bdelloid, which disregards the time spent as anhydrobiotic, following the so-called "Sleeping Beauty" model. Average fecundity is never found to decrease as a consequence of anhydrobiosis, but is either equal or even higher than that of a hydrated rotifer. Bdelloid populations seem to benefit from anhydrobiosis; fitness of a bdelloid is found to decline, if populations are maintained hydrated for several generations, but not if populations are cyclically desiccated. We hypothesize that anhydrobiosis can be an essential event for long-term survival of bdelloid populations.