The significance of the moult cycle to cold tolerance in the Antarctic collembolan Cryptopygus antarcticus M.R. Worland *, P. Convey British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, United Kingdom 1. Introduction Springtails are among the most successful terrestrial animals to have colonised polar terrestrial habitats making them ideal model organisms for low temperature survival studies (Sinclair et al., 2001, 2003). Without exception, all species of cold tolerant springtails so far examined are freezing intolerant and have evolved mechanisms to reduce the ice nucleation temperature of their body fluids below that of their environment (Block, 1990). The Antarctic springtail Cryptopygus antarcticus is classified as ‘chill tolerant’ (Bale, 1993, 1996) with a high proportion (>80%) surviving winter temperatures below À15 8C(Block, 1982). For this class of arthropods the temperature of crystallisation (=supercooling point—SCP) is considered to be a reliable proxy for lower lethal temperature (Cannon and Block, 1988). Studies of seasonal fluctuations in SCPs of field populations of C. antarcticus show strong cycles with mean values ranging from À6 8C in summer to À30 8C in winter (Block, 1990). Increased winter cold hardiness is achieved by accumulation of cryoprotec- tants (Montiel, 1998), reduced water content (Block and Convey, 2001; Block and Zettel, 2003) and the removal of ice-nucleating material from the gut (Sømme and Block, 1982). Rather than individual SCPs being smoothly distributed along a continuum of temperature, studies of a range of arthropods, including C. antarcticus, have described characteristic strongly bimodal dis- tributions (Block and Rothery, 1992; Hawes, 2006), indicative of two distinct cold tolerance ‘states’. More recently, rapid switching between these two states has been described in C. antarcticus in both experimental manipulations and field observations (Worland and Convey, 2001), although the mechanism by which this is achieved remains unclear. A feature of these bimodal distributions is that summer populations of C. antarcticus include a proportion of animals with low SCPs (Sømme and Block, 1982), which appears to be inconsistent with the levels of biological activity (i.e. feeding) that is concentrated within the short Antarctic summer. During warm summer periods, high SCPs are largely a con- sequence of the presence of ice-nucleating material in the animal’s gut (Worland and Lukes ˇova ´ , 2000). This includes fragments of food, and bacteria that form part of the animal’s natural gut flora. Although there is evidence that clearing material from the gut Journal of Insect Physiology 54 (2008) 1281–1285 ARTICLE INFO Article history: Received 11 January 2008 Received in revised form 19 June 2008 Accepted 23 June 2008 Keywords: Ecdysis Feeding Springtail Supercooling Survival ABSTRACT Research into the ecophysiology of arthropod cold tolerance has largely focussed on those parts of the year and/or the life cycle in which cold stress is most likely to be experienced, resulting in an emphasis on studies of the preparation for and survival in the overwintering state. However, the non-feeding stage of the moult cycle also gives rise to a period of increased cold hardiness in some microarthropods and, as a consequence, a proportion of the field population is cold tolerant even during the summer active period. In the case of the common Antarctic springtail Cryptopygus antarcticus, the proportion of time spent in this non-feeding stage is extended disproportionately relative to the feeding stage as temperature is reduced. As a result, the proportion of the population in a cold tolerant state, with low supercooling points (SCPs), increases at lower temperatures. We found that, at 5 8C, about 37% of the population are involved in ecdysis and exhibit low SCPs. At 2 8C this figure increased to 50% and, at 0 8C, we estimate that 80% of the population will have increased cold hardiness as a result of a prolonged non-feeding, premoult period. Thus, as part of the suite of life history and ecophysiological features that enable this Antarctic springtail to survive in its hostile environment, it appears that it can take advantage of and extend the use of a pre-existing characteristic inherent within the moulting cycle. ß 2008 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +44 1223221553; fax: +44 1223221259. E-mail address: mrwo@bas.ac.uk (M.R. Worland). Contents lists available at ScienceDirect Journal of Insect Physiology journal homepage: www.elsevier.com/locate/jinsphys 0022-1910/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jinsphys.2008.06.009