Staying young and fit? Ontogenetic and phylogenetic consequences of animal anhydrobiosis

Although gradual deterioration of life functions with age is not a fundamental rule, it is pervasive among living organisms, regardless of their mode of reproduction and the number of constituent cells. However, deterioration can be temporarily arrested or slowed down due to the process of anhydrobiosis. Two modes of anhydrobiosis can be distinguished for the developmental and adult stages of animals. Developmental resting stages are reported for different animals, including sponges (Porifera), stingers (Cnidaria), flatworms (Platyhelminthes), insects (Insecta), copepods (Copepoda) and branchiopods (Branchiopoda). However, anhydrobiosis occurring at any stage of animal life, including adults, is found only in a few invertebrate phyla, namely roundworms (Nematoda), wheel animals (Rotifera) and water bears (Tardigrada). Notably, in the second group anhydrobiosis has been proposed to eliminate or slow-down aging symptoms. This, in turn, may correlate with higher fitness and fecundity, and increased offspring longevity. We present available data concerning anhydrobiosis of tardigrades, bdelloid rotifers and nematodes, the only animals known to be capable of anhydrobiosis as adult individuals. The impact of anhydrobiosis on animal aging is illustrated by two models based on experimental data, namely the "Sleeping Beauty" and "Picture of Dorian Grey" models. According to the "Sleeping Beauty" model, anhydrobiotic organisms do not age during anhydrobiosis, whereas the "Picture of Dorian Grey" model predicts that the anhydrobiotic organism ages, at least during the initial stage of anhydrobiosis. Finally, we discuss possible implications of these models for individual longevity and survival as well as phenotypic diversity of taxa and their evolution. A better understanding of life strategies of anhydrobiotic animals both at the ontogenetic and phylogenetic levels can provide answers to many fundamental questions and useful practical outputs in branches of applied sciences.