Cold tolerance of microarthropods

first_img1. 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 maintenance of microarthropod populations of polar land habitats seem not to have required the evolution of any novel features with respect to cold tolerance.last_img read more