Survival in extreme environments – on the current knowledge of adaptations in tardigrades

Abstract

Tardigrades are microscopic animals found worldwide in aquatic as well as terrestrial ecosystems. They belong to the invertebrate superclade Ecdysozoa, as do the two major invertebrate model organisms: Caenorhabditis elegans and Drosophila melanogaster. We present a brief description of the tardigrades and highlight species that are currently used as models for physiological and molecular investigations. Tardigrades are uniquely adapted to a range of environmental extremes. Cryptobiosis, currently referred to as a reversible ametabolic state induced by e.g. desiccation, is common especially among limno-terrestrial species. It has been shown that the entry and exit of cryptobiosis may involve synthesis of bioprotectants in the form of selective carbohydrates and proteins as well as high levels of antioxidant enzymes and other free radical scavengers. However, at present a general scheme of mechanisms explaining this phenomenon is lacking. Importantly, recent research has shown that tardigrades even in their active states may be extremely tolerant to environmental stress, handling extreme levels of ionizing radiation, large fluctuation in external salinity and avoiding freezing by supercooling to below -20 °C, presumably relying on efficient DNA repair mechanisms and osmoregulation. This review summarizes the current knowledge on adaptations found among tardigrades, and presents new data on tardigrade cell numbers and osmoregulation.

Genetic diversity in the parthenogenetic reproducing tardigrade Echiniscus testudo (Heterotardigrada: Echiniscoidea)

“…The presence of a film of free water is a prerequisite for active tardigrades. Many species of tardigrades, primarily those living in mosses and lichens, are famous for their ability to enter cryptobiosis, a form of latent life, during desiccation (Wright et al, 1992;Møbjerg et al, 2011).…”

General environmental heterogeneity as the explanation of sexuality? Comparative study shows that ancient asexual taxa are associated with both biotically and abiotically homogeneous environments

“…arbuscular mycorrhizal fungi of the order Glomales (Croll & Sanders, 2009), tardigrades (Mobjerg et al., 2011; Pilato, 1979), nematode genus Meloidogyne (Castagnonesereno et al., 1993), ostracods Heterocypris incongruens (Ramdohr) and Eucypris virens (Jurine) (Butlin et al., 1998; Martens, 1998), bristle fern Trichomanes intricatum (Farrar) (Farrar, 1992), basidiomycete fungal families Lepiotaceae and Tricholomataceae (Currie, Mueller, & Malloch, 1999; Currie, Scott, Summerbell, & Malloch, 1999), ambrosia fungi Ophiostomatales (Farrell et al., 2001), or brine shrimp “ Artemia parthenogenetica ” (Bowen & Sterling) (Vanhaecke, Siddall, & Sorgeloos, 1984)—and their adaptations are similar to those of the AAs included in this study (see below).…”

“…Similar durable stages could also be found in some contested AAs, namely “ Artemia parthenogenetica ” (Vanhaecke et al., 1984) and tardigrades (Mobjerg et al., 2011). Moreover, AA Lasaea is able to become mostly inactive and rests during the adverse conditions for some time as well (Morton, Boney, & Corner, 1957).…”

Element analysis of the eutardigrades Richtersius coronifer and Milnesium cf. asiaticum using Particle Induced X-ray Emission (PIXE)

“…Tardigrades inhabiting semi-terrestrial habitats are well-known for their tolerance to extreme environmental conditions such as desiccation, freezing, and radiation (Wright et al, 1992;Jönsson, 2003;Møbjerg et al, 2011). These abilities are shared with some species from a few other animal groups such as arthropods, nematodes and rotifers, and represent adaptations (or by-products of adaptations) to survive under very dry or cold conditions.…”