Cold Tolerance of Microarthropods

Abstract

Summary 1. Microarthropods (Acari and Collembola) are dominant components of the terrestrial fauna in the Antarctic. Their cold tolerance, which forms the mainspring of their adaptational strategy, is reviewed against a background of their structure and function, and by comparison with other arthropods. 2. Two species, the isotomid collembolan Cryptopygus antarcticus Willem and the oribatid mite Alaskozetes antarcticus (Michael), are examined in detail, and afford a comparative approach to the mechanisms underlying cold tolerance in insect and arachnid types. 3. All microarthropods appear to be freezing‐susceptible (unable to tolerate tissue ice), and they utilize varying levels of supercooling to avoid freezing. Gut contents are considered to be the prime nucleation site in most arthropods when supercooled, particularly for Antarctic species. Moulting also increases individual supercooling ability especially in Collembola, and the activity of ice‐nucleating bacteria in cold‐hardy arthropods may be important. 4. Sources of ice nucleators are many and varied, originating externally (motes) or internally (ice‐nucleating agents). They act either extracellularly (mainly in the haemolymph) to promote freezing in ice‐tolerant life stages, or intracellularly in freezing‐susceptible forms. Thermal hysteresis proteins, acting colligatively, occur in many arthropods including Collembola; they depress both the freezing point of body fluids and the whole‐body supercooling point of freezing‐ susceptible and freezing‐tolerant species. 5. Bimodal supercooling point distributions are a feature of microarthropods and water droplets. Samples of field populations of Antarctic mites and springtails show significant seasonal changes in these distributions, which in some respects are analogous to purely physical systems of water droplets. Supercooling points are confirmed as accurate measures of cold‐hardiness and survival for Antarctic species, but not necessarily for other arthropods. The effects of constant sub‐zero temperatures approaching the limit of the supercooling ability of arthropods require study. 6. Desiccation and dehydration influence microarthropod physiology in several ways; in Alaskozetes it triggers glycerol synthesis. Glycerol may aid binding of water in severely dehydrated insects, but the relationship of such ‘bound’ water to cold‐hardiness is unclear. 7. Sugar alcohols (polyols) and sugars are accumulated as potential cryoprotectants in many arthropods at low temperatures, and antifreeze systems may be single or multi‐component in structure. Cryoprotectant synthesis and regulation have been studied principally in insects, and fresh weight concentrations of 0–3‐5 M of polyols have been found. Trehalose accumulation may also influence cold‐hardiness. 8. Microarthropods fall within the spectrum of cold tolerance observed for arthropods and other invertebrates. No special adaptations are found in Antarctic species, and similar strategies and mechanisms are present in both insects and arachnids. The colonization and ...

Cold hardening processes in the Antarctic springtail, Cryptopygus antarcticus: Clues from a microarray

“…whether it is an adaptation to environmental conditions or part of the animals natural lifecycle. It has been correlated in some studies with the presence or absence of gut contents (Sømme & Conradi-Larsen, 1977;Young & Block, 1980;Sømme, 1981;Block, 1982;Cannon, 1983;Cannon & Block, 1988;Rothery & Block, 1992). It has also been suggested that some insects retain a low SCP during the summer, providing insurance against unexpected freezing temperatures (Chown and Klok, 1998).…”

“…One of the most amenable and studied group of organisms is the arthropoda where such physiological processes are well documented (Salt, 1961;Lee and Denlinger, 1991;Sømme, 1995;Sinclair et al, 2003a). These organisms have evolved two distinct strategies to survive sub-zero temperatures termed freeze tolerance and freeze avoidance (Cannon and Block, 1988;Block, 1990 see also Bale, 1993 for more ecologically-refined schemes of classificiation). A third mechanism, protective dehydration, has more recently been described but is perhaps less common.…”

“…Freeze avoidance is the main strategy used by Antarctic terrestrial microarthropods (Block 1990(Block , 1991Cannon and Block, 1988;Convey, 1996), which routinely have to survive over wintering temperatures below -20°C. For these organisms the SCP is equal to their lower lethal temperature.…”

“…Other factors such as the cooling rate can affect the measured SCP and the lower lethal temperature (Worland, 2005). Cold tolerance also varies seasonally, with higher mean values in the summer for active feeding animals than for winter acclimated, cold hardened individuals (Cannon and Block, 1988). Seasonal cold-hardening involves a number of physiological and biochemical processes that slowly increase supercooling ability and enhance survival at low temperatures.…”

“…In general, species typified by a bimodal SCP distribution show a population shift from high to low SCPs as winter approaches (Cannon and Block, 1988), but SCPs in some Antarctic species can vary or be altered diurnally (Sinclair, 2003b) or even within hours (Worland and Convey, 2001) in response to changing environmental conditions. This "rapid cold hardening" is almost certainly a critical factor in the ability of Antarctic terrestrial arthropods to survive variable and unpredictable summer temperatures (Convey, 1997).…”