The ability of certain species of parasitic nematodes to survive desiccation for considerable periods is a fascinating example of adaptation to the demands of fluctuating environments that occasionally can become extreme and life threatening. Behavioural and morphological adaptations associated with desiccation survival serve primarily to reduce the rate of drying, either to prolong the time taken for the nematode's water content to reach lethal low levels or, in true anhydrobiotes, to enable the structural and biochemical changes required for long-term survival to take place. Examples of these adaptations are reviewed, together with information on the factors involved in rehydration that ensure successful exit from the dormant state. Information on desiccation survival is central to effective management and control options for parasitic nematodes. It is also required to assess the feasibility of enhancing the longevity of commercial formulations of entomopathogenic nematodes, both before and after application; current research and future prospects for enhancing survival of these bio-insecticides are discussed.

The survival capacities of Muellerius capillaris (Nematoda: Protostrongylidae) free-living larval stages (L1) of an Israeli desert isolate (Nubian isolate) (92 mm rain per annum) and a French isolate from a temperate habitat (650 mm rain per annum) were compared under humid and extreme dry conditions. Under the dry conditions (7% relative humidity [r.h.] and 33% r.h. at 23 °C), both isolates exhibited the same remarkable survival capacity for periods of 17 and 28 days, respectively (>92% survival, P>0·1). However, during and after recovery from the anhydrobiotic state, different behaviour patterns of L1 were observed in each isolate. Under humid conditions (97% r.h.) for 10 days, both isolates exhibited similarly low survival percentages (13·4±4·6%, S.E., survival for the Nubian isolate, 3±3% for the French isolate, P>0·05). In water, the French isolate was less active, survived better (52±8·4% vs 28·7±3·7, by day 28, P<0·05) and retained a better morphological appearance (shrunk less) than the Nubian isolate. Larval development of the French isolate in the land snail Theba pisana was significantly faster than that of the Nubian isolate, reaching a higher proportion of infective stages (L3) on day 36 post-infection (78·6% vs 56% P>0·05). However, similar infection intensities were recovered in snails infected with either of the isolates (59·8±10·3, S.E., n=14, for the French isolate, 53±6, n=13, for the Nubian isolate, P<0·05).

Anhydrobiosis, or life without water, is the remarkable ability of certain types of plants and animals to survive almost total dehydration. This phenomenon requires a coordinated series of events within the cells of anhydrobiotes that protect their cellular components, particularly proteins and lipid membranes, from damage caused by the removal of water. Much of what is now understood about preserving biological samples during drying was learned by studying naturally desiccation-tolerant organisms and extended using model systems such as phospholipid vesicles. Most anhydrobiotic organisms accumulate disaccharides in their cells and tissues during the dehydration process. These carbohydrates, usually sucrose or trehalose, satisfy two criteria that appear to be necessary for protecting membranes during desiccation and during storage in the dry state. These requirements include: (1) depression of the gel-to-liquid crystalline phase transition temperature (Tm) in the dehydrated lipid to a temperature at or near that of the hydrated lipid, a process that appears to require a direct interaction between the carbohydrates and the lipid molecules of the membrane; and (2) formation of a carbohydrate glass with a relatively high glass transition temperature, leading to inhibition of fusion between the vesicles.

Microbes, particularly Archaea, are well-known for their superb adaptation to extreme environments. However, amazing adaptations to extreme stresses do not only occur in microbes, but also in many multicellular organisms. Examples include tardigrades and their capability to survive freezing to near absolute zero, the Pompeii worm withstanding temperatures of up to 105°C, the Crucian carp remaining active in anoxic conditions for several months, and resurrection plants, which can survive near-complete desiccation. Here, we review some of the cryptobiotic strategies and adaptations exhibited in multicellular organisms, and point out their relevance to astrobiology.

Osmotic priming reduces the longevity of Vigna radiata (mung bean) seeds during subsequent dry storage. The present study has investigated the relationship between volumetric changes, modified water sorption properties and the decrease in seed longevity after priming. Volumetric analysis of control embryos revealed two major dehydration-related volumetric contractions in the biopolymer matrix, reflecting structural reorganization in biopolymer matrix during drying. These contractions occurred at hydration from 0.29 to 0.23 g g–1 dw and from 0.18 to 0.11 g g–1 dw, respectively. Volumetric contractions were reduced significantly after priming and also occurred at higher water contents (i.e. 0.48–0.32 g g–1 dw and 0.21–0.11 g g–1 dw). Consequently, at the same water content, primed seed embryos had higher specific biopolymer volume and lower specific density than control embryos. Water sorption study showed that priming did not change the monolayer hydration value, but altered the properties of multilayer water sorption in seed axes. The analysis of water clustering function suggested that priming enhanced the water–water association in seed axes, but not in cotyledon tissues. Solid-state 1H-NMR (nuclear magnetic resonance) investigation confirmed that priming did not cause a significant change of water dynamic properties in cotyledon tissues at water contents below 0.20 g g–1 dw. The relevance of volumetric modifications of the biopolymer matrix and changes in water clustering function to the reduced seed longevity after priming is discussed. It is proposed that priming-induced increases in surface reactivity may enhance deteriorative chemical reactions in re-dried seed tissues.

This study represents the first systematic investigation of long-term anhydrobiotic survival in tardigrades, rotifers and nematodes inhabiting mosses and lichens. Sixty-three different samples from public and private collections, kept dry for 9–138 years, were examined. Rotifers of the genus Mniobia and the eutardigrade Ramazzottius oberhaeuseri (hatched from eggs) were found alive from one of the samples (9 years old). These observations represent the longest record for rotifers in the anhydrobiotic state. For tardigrades, our results confirm previous reports on the upper limit of anhydrobiotic survival under atmospheric oxygen conditions. This study suggests the possibility that tardigrade eggs are able to withstand longer periods in anhydrobiosis than animals. Some problems related to the evaluation of long-term anhydrobiotic survival, such as contamination and chemical treatments of samples, are reported. The possible role of the microenvironment in which the anhydrobiotic animals are kept is discussed.

Tardigrades have achieved a widespread reputation for an ability to survive more than a century in an inactive, ametabolic state called cryptobiosis. However, a closer look at the empirical evidence provides little support for the claim that tardigrades are capable of century-long survival. Instead, current evidence suggests that a decade may more realistically represent the upper limit of cryptobiotic survival in the most resistant tardigrades.

The present study describes a novel heat-stable, water-stress-related protein with a molecular mass of 47 kDa (designated Desc47) in the entomopathogenic nematode Steinernema feltiae (IS-6). The protein was accumulated about 10-fold (from 7·84±1·85 to 74·09±4·35% relative content level [RCL]) in dehydrated clumps of infective juveniles (IJs), which had lost 34·4% of their initial water content (from 65·1±1·7% to 42·7±0·72%) in a desiccation-tolerance-inducing treatment (97% relative humidity [RH] for 3 days). The appearance of Desc47 was accompanied by trehalose accumulation (from 300 to 600 mg trehalose/g protein) during the process of inducing the IJs into a quiescent anhydrobiotic state. A second cycle of IJ dehydration did not alter the RCL of Desc47 (79·3% for the first cycle and 73·3% for the second cycle). Desc47 retained its high RCL (69·7%) in rehydrated active IJs for 3 days, reaching 51·2% of its initial RCL only after a week. No homology to other known proteins was found by mass-spectrometry electrospray-ion-trap analysis. However, of the 5 sequences obtained from the protein (ranging from 11 to 21 amino acids), the 21-amino-acid peptide NVASDAVETVGNAAGQAG(D/T)AV showed excellent homology (74% identity in 19 amino acids) to the cold-responsive protein COR14b (g6564861) from Triticum aestivum. In the Caenorhabditis elegans predicted proteome database search, the N21 yielded the first-best identity score (59% identity in 17 amino acids) to the CE-LEA homologue protein (g2353333). In plants, COR and LEA are related proteins, heat-stable, which are expressed in response to both dehydration and cold acclimation. The implication of the involvement of Desc47 and the osmoprotectant trehalose in the desiccation-tolerance mechanisms of S. feltiae is discussed.

Depression of metabolic rate has been recorded for virtually all major animal phyla in response to environmental stress. The extent of depression is usually measured as the ratio of the depressed metabolic rate to the normal resting metabolic rate. Metabolic rate is sometimes only depressed to approx. 80% of the resting value (i.e. a depression of approx. 20% of resting); it is more commonly 5–40% of resting (i.e. a depression of approx. 60–95% of resting); extreme depression is to 1% or less of resting, or even to an unmeasurably low metabolic rate (i.e. a depression of approx. 99–100% of resting). We have examined the resting and depressed metabolic rate of animals as a function of their body mass, corrected to a common temperature. This allometric approach allows ready comparison of the absolute level of both resting and depressed metabolic rate for various animals, and suggests three general patterns of metabolic depression.

Firstly, metabolic depression to approx. 0.05–0.4 of rest is a common and remarkably consistent pattern for various non-cryptobiotic animals (e.g. molluscs, earthworms, crustaceans, fishes, amphibians, reptiles). This extent of metabolic depression is typical for dormant animals with ‘intrinsic’ depression, i.e. reduction of metabolic rate in anticipation of adverse environmental conditions but without substantial changes to their ionic or osmotic status, or state of body water. Some of these types of animal are able to survive anoxia for limited periods, and their anaerobic metabolic depression is also to approx. 0.05–0.4 of resting. Metabolic depression to much less than 0.2 of resting is apparent for some ‘resting’, ‘over-wintering’ or diapaused eggs of these animals, but this can be due to early developmental arrest so that the egg has a low ‘metabolic mass’ of developed tissue (compared to the overall mass of the egg) with no metabolic depression, rather than having metabolic depression of the entire cell mass. A profound decrease in metabolic rate occurs in hibernating (or aestivating) mammals and birds during torpor, e.g. to less than 0.01 of pre-torpor metabolic rate, but there is often no intrinsic metabolic depression in addition to that reduction in metabolic rate due to readjustment of thermoregulatory control and a decrease in body temperature with a concommitant Q10 effect. There may be a modest intrinsic metabolic depression for some species in shallow torpor (to approx. 0.86) and a more substantial metabolic depression for deep torpor (approx. 0.6), but any energy saving accruing from this intrinsic depression is small compared to the substantial savings accrued from the readjustment of thermoregulation and the Q10 effect.

Secondly, a more extreme pattern of metabolic depression (to <0.05 of rest) is evident for cryptobiotic animals. For these animals there is a profound change in their internal environment – for anoxybiotic animals there is an absence of oxygen and for osmobiotic, anhydrobiotic or cryobiotic animals there is an alteration of the ionic/osmotic balance or state of body water. Some normally aerobic animals can tolerate anoxia for considerable periods, and their duration of tolerance is inversely related to their magnitude of metabolic depression; anaerobic metabolic rate can be less than 0.005 of resting. The metabolic rate of anhydrobiotic animals is often so low as to be unmeasurable, if not zero. Thus, anhydrobiosis is the ultimate strategy for eggs or other stages of the life cycle to survive extended periods of environmental stress.

Thirdly, a pattern of absence of metabolism when normally hydrated (as opposed to anhydrobiotic or cryobiotic) is apparently unique to diapaused eggs of the brine-shrimp (Artemia spp., an anostracan crustacean) during anoxia. The apparent complete metabolic depression of anoxic yet hydrated cysts (and extreme metabolic depression of normoxic, hypoxic, or osmobiotic, yet hydrated cysts), is an obvious exception to the above patterns.

In searching for biochemical mechanisms for metabolic depression, it is clear that there are five general characteristics at the molecular level of cells which have a depressed metabolism; a decrease in pH, the presence of latent mRNA, a change in protein phosphorylation state, the maintenance of one particular energy-utilizing process (ion pumping), and the down-regulation of another (protein synthesis). Oxygen sensing is now the focus of intense investigation and obviously plays an important role in many aspects of cell biology. Recent studies show that oxygen sensing is involved in metabolic depression and research is now being directed towards characterising the proteins and mechanisms that comprise this response. As more data accumulate, oxygen sensing as a mechanism will probably become the sixth general characteristic of depressed cells.

The majority of studies on these general characteristics of metabolically depressed cells come from members of the most common group of animals that depress metabolism, those non-cryptobiotic animals that remain hydrated and depress to 0.05–0.4 of rest. These biochemical investigations are becoming more molecular and sophisticated, and directed towards defined processes, but as yet no complete mechanism has been delineated. The consistency of the molecular data within this group of animals suggests similar metabolic strategies and mechanisms associated with metabolic depression.

The biochemical ‘adaptations’ of anhydrobiotic organisms would seem to be related more to surviving the dramatic reduction in cell water content and its physico-chemical state, than to molecular mechanisms for lowering metabolic rate. Metabolic depression would seem to be an almost inevitable consequence of their altered hydration state.

The unique case of profound metabolic depression of hydrated Artemia spp. cysts under a variety of conditions could reflect unique mechanisms at the molecular level. However, the available data are not consistent with this possibility (with the exception of a uniquely large decrease in ATP concentration of depressed, hydrated Artemia spp. cysts) and the question remains: how do cells of anoxic and hydrated Artemia spp. differ from anoxic goldfish or turtle cells, enabling them so much more completely to depress their metabolism?