Current status of the tardigrada: evolution and ecology

The Tardigrada are bilaterally symmetrical micrometazoans withfour pairs of lobopod legs terminating in claws or sucking disks.They occupy a diversity of niches in marine, freshwater, andterrestrial environments throughout the world. Some have a cosmopolitandistribution, while others are endemic. About 900 species havebeen described thus far, but many more species are expectedas additional habitats are investigated. Most are less than1 mm in body length and are opaque or translucent, exhibitingcolors such as brown, green, orange, yellow, red, or pink inthe cuticle and/or gut. Marine species are more variable inbody shape and overall appearance and generally exhibit lowpopulation density with high species diversity. Reproductivemodes include sexual reproduction and parthenogenesis, but muchremains to be known about development. Tardigrades have a hemocoel-typeof fluid-filled body cavity, a complete digestive tract, anda lobed dorsal brain with a ventral nerve cord with fused ganglia.Recent molecular analyses and additional morphological studiesof the nervous system have confirmed the phylogenetic positionof tardigrades as a sister group of the arthropods. The abilityof tardigrades to undergo cryptobiosis has long intrigued scientists.Although tardigrades are active only when surrounded by a filmof water, they can enter latent states in response to desiccation(anhydrobiosis), temperature (cryobiosis), low oxygen (anoxybiosis),and salinity changes (osmobiosis). Cryptobiotic states aid indispersal.     

Anhydrobiosis in tardigrades and its effects on longevity traits

Living in harsh and variable environments that are prone to periodic desiccation, tardigrades exhibit remarkable tolerance against physical extremes through a state known as anhydrobiosis. To study the effect of this state on the longevity and hence the lifecycle in the taxon Tardigrada for the first time, we exposed a tardigrade species, Milnesium tardigradum , to alternating periods of drying and active feeding periods in a hydrated state. Compared with a hydrated control, the periodically dried animals showed a similar longevity, indicating that the time spent in anhydrobiosis was ignored by the internal clock. Thus, desiccation can produce a time shift in the age of tardigrades similar to the model described for rotifers that has been termed 'Sleeping Beauty'.     

Welcome by the Dean Henrik Jeppesen at the 8th International Symposium on Tardigrada

Mr. Chairman. Distinguished ladies and gentlemen. As Dean of Science it is a great honour on behalf of the University of Copenhagen and the Faculty of Science to welcome all of you here to the 8th International Symposium on Tardigrada. We are especially happy to have you here at the August Krogh Institute (named after our well-known Nobel Prize winner in Physiology), because on September 1st we celebrate the establishment of the Faculty. So coming here and honouring our 150 year anniversary jubilee help us to promote the importance of science in our society. The University was founded in 1479 as a theological catholic school. It broke down in 1530 and was reestablished in 1537 after the reformation. Right from the start in 1479 there was science thought of at the university. Mathematics and Astronomy. And Zoology became a subject over the centuries together with other subjects which are today regarded as science. But only in 1850 did we become an independent Faculty thanks to the effort and progress done by the Danish Chemist H.C. Ørsted.The animals, which you study, are marvellous in the sense that they can survive under severe conditions for centuries. Under extreme dry conditions in Sahara, in extreme cold conditions (they can survive minus 273 °C, or survive in vacuum). This has practical implications for people who need to excuse their scientific interest, for medicine if we can freeze human tissue, or for space study how to survive under extreme conditions. The study of Tardigrada is an important field here at the Institute of Zoology, at the Zoological Museum, and at the University of Roskilde, 30 km west of Copenhagen. Some of our most distinguished zoologists take part in this research. That might be the reason why you have chosen to have the symposium here in Copenhagen. They are doing research on tardigrades in marine areas, and in Greenland on the ice cap. Especially interesting are the studies done in the Ikka Fjord in Greenland, where the unique Ikkaite Tufa columns made of calcium carbonate hexahydrate originating from alkaline cold springs at the bottom of the fjord create very specific environments with nearly brackish conditions in the center and sea water salinity on the outside. And this creates varied conditions for different species of Tardigrada.We also celebrate this year the 50th anniversary of the 2. Galathea expedition which went round the world and specifically looked for deep sea fauna. There are Tardigrades here. It has been interesting to look through the 54 abstracts in the programme and read the names and work places for the 65 participants listed. In English tardigrades are called water bears, in Danish “bjørnedyr” meaning bear animals. I prefer the Danish version, this sounds more like pet bears.The symposium is followed by a field trip to the faculty's research station on Disko in Greenland. In 1994 I arrived on the new research vessel “Porsild” to Disko to deliver the new boat some of you will sail in during the workshop up there. I stayed there some days, and there was this man Professor Reinhardt Møbjerg Kristensen, looking into his microscope. It was fascinating to see the joy which he expressed explaining his animals. His engagement was so impressive and his talk so marvellous. It was really his pet animals he caressed all day and night. If all of you are looking on the water bears with the same fascination and engagement, then this will be one of the most entertaining symposiums ever held. One can fear that you are so engaged that you will forget everything around you, even to listen to the contributions of the others, and to be careful that maybe a new group will be announced.I wish you some very good days here at the Faculty of Science and some very fruitful days. I should like to thank the sponsors of the meeting, The Danish Science Foundation, The Carlsberg Foundation and Dr. Bøje Benzon Foundation. I would like to express my gratitude to the organizing committee for attracting the conference here and making the programme so wide and interesting. I can promise the committee will do all their best to help you all way through. And for those going to Disko — you will have a most splendid experience.I shall ask my colleagues at my own institute, Geography, to arrange some bad weather except on Thursday where you join the excursion. This to prevent you from sneaking away and enjoy the wonders of Copenhagen.By this once again welcome and a wish for a fruitfulconference.     

Welcome by the Dean Henrik Jeppesen at the 8th International Symposium on Tardigrada

Mr. Chairman. Distinguished ladies and gentlemen. As Dean of Science it is a great honour on behalf of the University of Copenhagen and the Faculty of Science to welcome all of you here to the 8th International Symposium on Tardigrada. We are especially happy to have you here at the August Krogh Institute (named after our well-known Nobel Prize winner in Physiology), because on September 1st we celebrate the establishment of the Faculty. So coming here and honouring our 150 year anniversary jubilee help us to promote the importance of science in our society. The University was founded in 1479 as a theological catholic school. It broke down in 1530 and was reestablished in 1537 after the reformation. Right from the start in 1479 there was science thought of at the university. Mathematics and Astronomy. And Zoology became a subject over the centuries together with other subjects which are today regarded as science. But only in 1850 did we become an independent Faculty thanks to the effort and progress done by the Danish Chemist H.C. Ørsted.The animals, which you study, are marvellous in the sense that they can survive under severe conditions for centuries. Under extreme dry conditions in Sahara, in extreme cold conditions (they can survive minus 273 °C, or survive in vacuum). This has practical implications for people who need to excuse their scientific interest, for medicine if we can freeze human tissue, or for space study how to survive under extreme conditions. The study of Tardigrada is an important field here at the Institute of Zoology, at the Zoological Museum, and at the University of Roskilde, 30 km west of Copenhagen. Some of our most distinguished zoologists take part in this research. That might be the reason why you have chosen to have the symposium here in Copenhagen. They are doing research on tardigrades in marine areas, and in Greenland on the ice cap. Especially interesting are the studies done in the Ikka Fjord in Greenland, where the unique Ikkaite Tufa columns made of calcium carbonate hexahydrate originating from alkaline cold springs at the bottom of the fjord create very specific environments with nearly brackish conditions in the center and sea water salinity on the outside. And this creates varied conditions for different species of Tardigrada.We also celebrate this year the 50th anniversary of the 2. Galathea expedition which went round the world and specifically looked for deep sea fauna. There are Tardigrades here. It has been interesting to look through the 54 abstracts in the programme and read the names and work places for the 65 participants listed. In English tardigrades are called water bears, in Danish “bjørnedyr” meaning bear animals. I prefer the Danish version, this sounds more like pet bears.The symposium is followed by a field trip to the faculty's research station on Disko in Greenland. In 1994 I arrived on the new research vessel “Porsild” to Disko to deliver the new boat some of you will sail in during the workshop up there. I stayed there some days, and there was this man Professor Reinhardt Møbjerg Kristensen, looking into his microscope. It was fascinating to see the joy which he expressed explaining his animals. His engagement was so impressive and his talk so marvellous. It was really his pet animals he caressed all day and night. If all of you are looking on the water bears with the same fascination and engagement, then this will be one of the most entertaining symposiums ever held. One can fear that you are so engaged that you will forget everything around you, even to listen to the contributions of the others, and to be careful that maybe a new group will be announced.I wish you some very good days here at the Faculty of Science and some very fruitful days. I should like to thank the sponsors of the meeting, The Danish Science Foundation, The Carlsberg Foundation and Dr. Bøje Benzon Foundation. I would like to express my gratitude to the organizing committee for attracting the conference here and making the programme so wide and interesting. I can promise the committee will do all their best to help you all way through. And for those going to Disko — you will have a most splendid experience.I shall ask my colleagues at my own institute, Geography, to arrange some bad weather except on Thursday where you join the excursion. This to prevent you from sneaking away and enjoy the wonders of Copenhagen.By this once again welcome and a wish for a fruitfulconference.     

Damage to Photosynthetic Membranes in Chilling-Sensitive Plants: Maize,a Case Study

Abstract

Biotechnology may soon take greater advantage of extremophiles — microorganisms that grow in high salt or heavy metal concentrations, or at extremes of temperature, pressure, or pH. These organisms and their cellular components are attractive because they permit process operation over a wider range of conditions than their traditional counterparts. However, extremophiles also present a number of challenges for the development of bioprocesses, such as slow growth, low cell yield, and high shear sensitivity. Difficulties inherent in designing equipment suitable for extreme conditions are also encountered. This review describes both the advantages and disadvantages of extremophiles, as well as the specialized equipment required for their study and application in biotechnology.     

缓步动物休眠现象研究进展

本文对缓步动物休眠现象的研究历史和现状作了简要的回顾和总结。休眠现象是一个集合名词,指的是缓步动物为克服不利的环境条件而出现的新陈代谢活动减弱甚至暂停的生命状态。最新的观点将其划分为两类,即隐生和滞育。根据导致隐生的环境因子的不同,又可分为低湿隐生、低温隐生、高压隐生、低氧隐生四种形式。滞育包括包囊和休眠卵两种形式。缓步动物的三种休眠状态（桶状、包囊和休眠卵）在其一生中任何一个阶段都能够出现,以度过极端不利的环境,并以此延长生物个体的寿命。总之,休眠现象存缓步动物的生态和进化方面具有不可估量的作用。     

Phylogenetic position of the Tardigrada based on the 18S ribosomal RNA gene sequences

The phylogenetic position of the Tardigrada remains uncertain. This is due to the limited information available, and the uncertainty of whether some characters are homologous or analogous with other taxa. Based on some morphological characters, current discussion centres on whether the taxon branches from the annelid-arthropod lineage, or lies within the arthropod complex. The molecular data presented here from an analysis of the 18S rRNA gene sequences are used to test the validity of these two hypotheses. Phylogenetic inference by the maximum parsimony and distance (neighbour-joining) methods suggests that the Tardigrada is a sister group of the major protostome eucoelomate assemblage that emerged before the arthropods, annelids, molluscs, and sipunculids evolved. The tardigrade clade also appears as an independent lineage separate from the nematode clade, thus supporting the current idea that tardigrades do not have a close aschelminth relationship. The molecular data also imply that several morphological features, considered significant in determining the phylogenetic relationships of tardigrades, are not synapomorphic characters.     

Tardigrades survive exposure to space in low Earth orbit

Vacuum (imposing extreme dehydration) and solar/galactic cosmic radiation prevent survival of most organisms in space [1]. Only anhydrobiotic organisms, which have evolved adaptations to survive more or less complete desiccation, have a potential to survive space vacuum, and few organisms can stand the unfiltered solar radiation in space. Tardigrades, commonly known as water-bears, are among the most desiccation and radiation-tolerant animals and have been shown to survive extreme levels of ionizing radiation [2-4]. Here, we show that tardigrades are also able to survive space vacuum without loss in survival, and that some specimens even recovered after combined exposure to space vacuum and solar radiation. These results add the first animal to the exclusive and short list of organisms that have survived such exposure.     

A survey of the terrestrial Tardigrada of the Vestfold Hills,Antarctica

A survey of the terrestrial tardigrades inhabiting growths of algae, lichens and mosses in the Vestfold Hills, Antarctica, was carried out at 11 and 35 sites during the austral summers of 1980 and 1982, respectively. In all, 24 species of plants were collected from which four genera and four species of Tardigrada were recovered. A key to the tardigrades of the area is presented. The distribution and associational patterns of the tardigrades are discussed in the context of other studies of antarctic Tardigrada.     

Engineering considerations for the application of extremophiles in biotechnology

Biotechnology may soon take greater advantage of extremophiles--microorganisms that grow in high salt or heavy metal concentrations, or at extremes of temperature, pressure, or pH. These organisms and their cellular components are attractive because they permit process operation over a wider range of conditions than their traditional counterparts. However, extremophiles also present a number of challenges for the development of bioprocesses, such as slow growth, low cell yield, and high shear sensitivity. Difficulties inherent in designing equipment suitable for extreme conditions are also encountered. This review describes both the advantages and disadvantages of extremophiles, as well as the specialized equipment required for their study and application in biotechnology.     

A review of acquired thermotolerance, heat-shock proteins, and molecular chaperones in archaea

Abstract: Acquired thermotolerance, the associated synthesis of heat-shock proteins (HSPs) under stress conditions, and the role of HSPs as molecular chaperones under normal growth conditions have been studied extensively in eukaryotes and bacteria, whereas research in these areas in archaea is only beginning. All organisms have evolved a variety of strategies for coping with high-temperature stress, and among these strategies is the increased synthesis of HSPs. The facts that both high temperatures and chemical stresses induce the HSPs and that some of the HSPs recognize and bind to unfolded proteins in vitro have led to the theory that the function of HSPs is to prevent protein aggregation in vivo. The facts that some HSPs are abundant under normal growth conditions and that they assist in protein folding in vitro have led to the theory that they assist protein folding in vivo; in this role, they are referred to as molecular chaperones. The limited research on acquired thermotolerance, HSPs, and molecular chaperones in archaea, particularly the hyperthermophilic archaea, suggests that these extremophiles provide a new perspective in these areas of research, both because they are members of a separate phylogenetic domain and because they have evolved to live under extreme conditions.     

Extremophiles: radiation resistance microbial reserves and therapeutic implications

Micro‐organisms with the ability to survive in extreme environmental conditions are known as ‘extremophiles’. Currently, extremophiles have caused a sensation in the biotechnology/pharmaceutical industries with their novel compounds, known as ‘extremolytes’. The potential applications of extremolytes are being investigated for human therapeutics including anticancer drugs, antioxidants, cell cycle‐blocking agents, anticholesteric drugs, etc. It is hypothesized that the majority of ultraviolet radiation (UVR)‐resistant micro‐organisms can be used to develop anticancer drugs to prevent skin damage from UVR. The metabolites from UVR‐resistant microbes are a great source of potential therapeutic applications in humans. This article aims to discuss the potentials of extremolytes along with their therapeutic implications of UVR extremophiles. The major challenges of therapeutic development using extremophiles are also discussed.     

Crystal structure of secretory abundant heat soluble protein 4 from one of the toughest “water bears” micro‐animals Ramazzottius Varieornatus

Though anhydrobiotic tardigrades (micro‐animals also known as water bears) possess many genes of secretory abundant heat soluble (SAHS) proteins unique to Tardigrada, their functions are unknown. A previous crystallographic study revealed that a SAHS protein (RvSAHS1) from one of the toughest tardigrades, Ramazzottius varieornatus, has a β‐barrel architecture similar to fatty acid binding proteins (FABPs) and two putative ligand binding sites (LBS1 and LBS2) where fatty acids can bind. However, some SAHS proteins such as RvSAHS4 have different sets of amino acid residues at LBS1 and LBS2, implying that they prefer other ligands and have different functions. Here RvSAHS4 was crystallized and analyzed under a condition similar to that for RvSAHS1. There was no electron density corresponding to a fatty acid at LBS1 of RvSAHS4, where a putative fatty acid was observed in RvSAHS1. Instead, LBS2 of RvSAHS4, which was composed of uncharged residues, captured a putative polyethylene glycol molecule. These results suggest that RvSAHS4 mainly uses LBS2 for the binding of uncharged molecules.     

Tardigrades — Are They Really Miniaturized Dwarfs?

Tardigrades are animals of small body size which is often regarded to be a secondary phenomenon. This interpretation makes sense in the traditional concept that tardigrades are closely related to Onychophora, Euarthropoda and Annelida. A large body size in the ancestor of this common taxon (Articulata) is probable. Small size and the absence of organs such as a dorsal heart, segmental coelomic cavities and metanephridia must then be interpreted as derived in tardigrades. However, when Cycloneuralia are taken as an outgroup instead of Annelida (taxon Ecdysozoa), an interpretation of small body size as a primary feature is plausible. This also accounts for the absence of heart, coelom and nephridia.The choice of outgroup influences hypotheses about sister-group relationships within Panarthropoda, with either Onychophora (Articulata-concept) or Tardigrada (Ecdysozoa-concept) being basal.     

Evolution of the Reproductive Mechanisms in Tardigrades — A Review

Although tardigrades can reproduce only through gametes they have exploited several modes of reproduction, which may be determined by their environment. Marine species (mainly heterotardigrades) are gonochoristic; hermaphroditism is only cited once, and parthenogenesis is unknown. In many cases females mature one egg at a time throughout adult life, whereas males are semelparous. Gonochorism is still present in limno-terrestrial species, while sporadic hermaphroditism occurs in several eutardigrade families. Thelytoky is the most common mode of reproduction in non-marine Tardigrada. Females are iteroparous, laying groups of eggs (free or in the exuvium), while males are semelparous (in a limnic species) or iteroparous with a continuous or cyclical maturation of the spermatozoa (in species from moss and leaf litter). Self-fertilisation appears to characterise hermaphroditic species, found in freshwater, mosses, leaf litter and soil. Egg maturation in these species is similar to that of the gonochoristic species, while spermatozoa mature in appreciable numbers before the oocytes, subsequently maturing continuously but in small numbers over the life of the animal. Parthenogenesis in limno-terrestrial tardigrades always appears continuous. In many species only females occur, but morpho-species populations may be found with both bisexual amphimictic (diploid) and unisexual thelytokous (often but not always polyploid) cytotypes. We can hypothesise that with the evolution of cryptobiosis and passive dispersal unstable and isolated habitats may favour parthenogenesis and self-fertilisation, as both reproductive modes allow colonisation of a new territory by a single individual. Parthenogenesis and hermaphroditism do not occur in the same species, and we can surmise that self-fertilisation will only evolve where parthenogenesis has never occurred.     

Phylum Tardigrada: an “individual” approach

Phylum Tardigrada consists of ～1000 tiny, hardy metazoan species distributed throughout terrestrial, limno‐terrestrial and oceanic habitats. Their phylogenetic status has been debated, with current evidence placing them in the Ecdysozoa. Although there have been efforts to explore tardigrade phylogeny using both morphological and molecular data, limitations such as their few morphological characters and low genomic DNA concentrations have resulted in restricted taxonomic coverage. Using a protocol that allows us to identify and extract DNA from individuals, we have sequenced 18S rDNA from 343 tardigrades from across the globe. Using maximum parsimony and Bayesian analyses we have found support for dividing Order Parachela into three super‐families and further evidence that indicates the traditional taxonomic perspective of families in the class Eutardigrada are nonmonophyletic and require re‐working. It appears that conserved morphology within Tardigrada has resulted in conservative taxonomy as we have found cases of several discrete lineages grouped into single genera. Although this work substantially adds to the understanding of the evolution and taxonomy of the phylum, we highlight that inferences gained from this work are likely to be refined with the inclusion of further taxa—specifically representatives of the nine families yet to be sampled. © The Willi Hennig Society 2008.     

Nature,Source and Function of Pigments in Tardigrades: In Vivo Raman Imaging of Carotenoids in Echiniscus blumi

Tardigrades are microscopic aquatic animals with remarkable abilities to withstand harsh physical conditions such as dehydration or exposure to harmful highly energetic radiation. The mechanisms responsible for such robustness are presently little known, but protection against oxidative stresses is thought to play a role. Despite the fact that many tardigrade species are variously pigmented, scarce information is available about this characteristic. By applying Raman micro-spectroscopy on living specimens, pigments in the tardigrade Echiniscus blumi are identified as carotenoids, and their distribution within the animal body is visualized. The dietary origin of these pigments is demonstrated, as well as their presence in the eggs and in eye-spots of these animals, together with their absence in the outer layer of the animal (i.e., cuticle and epidermis). Using in-vivo semi-quantitative Raman micro-spectroscopy, a decrease in carotenoid content is detected after inducing oxidative stress, demonstrating that this approach can be used for studying the role of carotenoids in oxidative stress-related processes in tardigrades. This approach could be thus used in further investigations to test several hypotheses concerning the function of these carotenoids in tardigrades as photo-protective pigments against ionizing radiations or as antioxidants defending these organisms against the oxidative stress occurring during desiccation processes.     

An upgraded comprehensive multilocus phylogeny of the Tardigrada tree of life

Providing accurate animals’ phylogenies rely on increasing knowledge of neglected phyla. Tardigrada diversity evaluated in broad phylogenies (among phyla) is biased towards eutardigrades. A comprehensive phylogeny is demanded to establish the representative diversity and propose a more natural classification of the phylum. So, we have performed multilocus (18S rRNA and 28S rRNA) phylogenies with Bayesian inference and maximum likelihood. We propose the creation of a new class within Tardigrada, erecting the order Apochela (Eutardigrada) as a new Tardigrada class, named Apotardigrada comb. n. Two groups of evidence support its creation: (a) morphological, presence of cephalic appendages, unique morphology for claws (separated branches) and wide‐elongated buccopharyngeal apparatus without placoids, and (b) phylogenetic support based on molecular data. Consequently, order Parachela is suppressed and its superfamilies erected as orders within Eutardigrada, maintaining their current names. We propose a new classification within the family Echiniscidae (Echiniscoidea, Heterotardigrada) with morphological and phylogenetic support: (a) subfamily Echiniscinae subfam. n., with two tribes Echiniscini tribe n. and Bryodelphaxini tribe n.; (b) subfamily Pseudechiniscinae subfam. n., with three tribes Cornechiniscini tribe n., Pseudechiniscini tribe n. and Anthechiniscini tribe n.; and (c) subfamily Parechiniscinae subfam. n., with two tribes Parechiniscini tribe n. and Novechiniscini tribe n. Reliable biodiversity selection for tardigrades in broad phylogenies is proposed due to biased analyses performed up to now. We use our comprehensive molecular phylogeny to evaluate the evolution of claws in the clawless genus Apodibius and claw reduction across the Tardigrada tree of life. Evolutionary consequences are discussed.     

The Biology of Tardigrades: An Introduction to the 9th International Symposium on Tardigrada*

The 9th International Symposium on Tardigrada took place in Tampa, Florida, USA from 28 July to 1 August 2003. Fifty-four participants representing thirteen countries attended and there were fifty-two presentations of which fourteen were chosen for the publication in these proceedings. Topics include cryptobiosis, ecology, taxonomy and systematics of tardigrades. * This symposiumvolume is dedicated to Nigel Marley (Fig. 4) for his courage and persistence in pursuing research on tardigrades, despite ongoing medical challenges. His optimism and positive attitude are an inspiration to all of us, and his willingness to help other tardigradologists is gratefully acknowledged and appreciated.     

On dormancy strategies in tardigrades

In this review we analyze the dormancy strategies of metazoans inhabiting “hostile to life” habitats, which have a strong impact on their ecology and in particular on the traits of their life history. Tardigrades are here considered a model animal, being aquatic organisms colonizing terrestrial habitats. Tardigrades evolved a large variety of dormant stages that can be ascribed to diapause (encystment, cyclomorphosis, resting eggs) and cryptobiosis (anhydrobiosis, cryobiosis, anoxibiosis). In tardigrades, diapause and cryptobiosis can occur separately or simultaneously, consequently the adoption of one adaptive strategy is not necessarily an alternative to the adoption of the other. Encystment and cyclomorphosis are characterized by seasonal cyclic changes in morphology and physiology of the animals. They share several common features and their evolution is strictly linked to the molting process. A bet-hedging strategy with different patterns of egg hatching time has been observed in a tardigrade species. Four categories of eggs have been identified: subitaneous, delayed-hatching, abortive and diapause resting eggs, which needs a stimulus to hatch (rehydration after a period of desiccation). Cryptobiotic tardigrades are able to withstand desiccation (anhydrobiosis) and freezing (cryobiosis) at any stage of their life-cycle. This ability involves a complex array of factors working at molecular (bioprotectans), physiological and structural levels. Animal survival and the accumulation of molecular damage are related to the time spent in the cryptobiotic state, to the abiotic parameters during the cryptobiotic state, and to the conditions during initial and final phases of the process. Cryptobiosis evolved independently at least two times in tardigrades, in eutardigrades and in echiniscoids. Within each evolutionary line, the absence of cryptobiotic abilities is more related to selective pressures to local habitat adaptation than to phylogenetic relationships. The selective advantages of cryptobiosis (e.g. persistency in “hostile to life” habitats, reduction of competitors, parasites and predators, escaping in time from stressful conditions) could explain the high tardigrade species diversity and number of specimens found in habitats that dry out compared to freshwater habitats.