DEHYDRATION AND COLD-HARDINESS IN THE ARCTIC COLLEMBOLAN ONYCHIURUS ARCTICUS TULLBERG 1876

Specimens of the Arctic Collembolon Onychiurus arcticus were exposed t o desiccation at several subzero temperatures over ice and at 0.5 degr ees C over NaCl solutions. The effects of desiccation on water content (WC), body fluid melting point (MP), supercooling point (SCP) and sur vival were studied at several acclimation temperatures and relative hu midities. Exposure to temperatures down to -19.5 degrees C caused a su bstantial and increasing dehydration. At the lowest exposure temperatu re unfrozen individuals lost 91.6% of the WC at full hydration but mor e than 80% of the individuals survived when rehydrated. Exposure at 0. 5 degrees C to decreasing relative humidities (RH) from 100% to 91.3% caused increasing dehydration and increasing mortality. Survival of eq ually dehydrated individuals was higher at subzero temperatures than a t 0.5 degrees C. Concurrent with the decline in WC a lowering of the M P was observed. Animals exposed to -3 degrees C and -6 degrees C over ice for 31 days had a MP of -3.8 and < -7.5 degrees C, respectively. S pecimens from a laboratory culture had a mean SCP of -6.1 degrees C, a nd acclimation at 0 or -3 degrees C had little effect on SCPs. Exposur e at -8.2 degrees C over ice for 8 days, however, caused the mean SCP to decline to -21.8 degrees C due to the severe dehydration of these i ndividuals. Dehydration at 0.5 degrees C in 95.1 and 93.3% RH also cau sed a decline in SCPs to about -18 degrees C. Individuals that had bee n acclimated over ice at -12.4 degrees C or at lower temperatures appa rently did not freeze at all when cooled to -30 degrees C, probably be cause all freezeable water had been lost. These results show that O. a rcticus will inevitably undergo dehydration when exposed to subzero te mperatures in its natural frozen habitat. Consequently, the MP and SCP of the Collembola are substantially lowered and in this way freezing is avoided. The increased cold hardiness by dehydration is similar to the protective dehydration mechanism described in earthworm cocoons an d Arctic enchytraeids.