Experimentally Induced Repeated Anhydrobiosis in the Eutardigrade Richtersius coronifer.

Michaela Czernekova,K Ingemar Jönsson,Bob Goldstein

doi:10.1371/journal.pone.0164062

Abstract

Tardigrades represent one of the main animal groups with anhydrobiotic capacity at any stage of their life cycle. The ability of tardigrades to survive repeated cycles of anhydrobiosis has rarely been studied but is of interest to understand the factors constraining anhydrobiotic survival. The main objective of this study was to investigate the patterns of survival of the eutardigrade Richtersius coronifer under repeated cycles of desiccation, and the potential effect of repeated desiccation on size, shape and number of storage cells. We also analyzed potential change in body size, gut content and frequency of mitotic storage cells. Specimens were kept under non-cultured conditions and desiccated under controlled relative humidity. After each desiccation cycle 10 specimens were selected for analysis of morphometric characteristics and mitosis. The study demonstrates that tardigrades may survive up to 6 repeated desiccations, with declining survival rates with increased number of desiccations. We found a significantly higher proportion of animals that were unable to contract properly into a tun stage during the desiccation process at the 5th and 6th desiccations. Also total number of storage cells declined at the 5th and 6th desiccations, while no effect on storage cell size was observed. The frequency of mitotic storage cells tended to decline with higher number of desiccation cycles. Our study shows that the number of consecutive cycles of anhydrobiosis that R. coronifer may undergo is limited, with increased inability for tun formation and energetic constraints as possible causal factors.

Highlights

Water availability is one of the most important ecological factors and evolutionary pressures on terrestrial life
Despite the fact that water is crucial for all life forms, numerous organisms survive temporary drying to equilibrium with the air humidity by entering a highly stable and reversible state called anhydrobiosis, a special form of the ametabolic life state known as cryptobiosis [1,2,3,4,5]
The highest survival rate was recorded after the first desiccation (98.5%) and the lowest non-zero survival rate was recorded after the sixth desiccation (28.6%)

Summary

Introduction

Water availability is one of the most important ecological factors and evolutionary pressures on terrestrial life. Tardigrades are microscopic aquatic animals found in a variety of habitats worldwide [9], and they are common in semi-terrestrial microhabitats such as mosses, lichens and leaf litter In these environments they are exposed to periods of desiccation that varies in frequency and length, and the anhydrobiotic capacity of semi-terrestrial tardigrades is an evolutionary adaptation to survive under such conditions. Their ability to enter anhydrobiosis is well documented [5,10,11], and the anhydrobiotic state may be entered recurrently and at any stage of their life cycle (so-called “holo-anhydrobiosis”; [12])

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