Low-temperature survival of the Antarctic midge, Belgica antarctica, is promoted by alternative strategies of freeze tolerance and freeze avoidance. Larvae are freeze tolerant year round. Yet, provided they avoid inoculative freezing, they could remain unfrozen by either supercooling (during acute exposure) or cryoprotective dehydration (during prolonged exposure). The first two projects in this dissertation compared each of these two strategies of freeze-avoidance to that of freeze-tolerance.
The purpose of the first project was to compare the induction of the rapid cold-hardening (RCH) response in frozen and supercooled larvae. At the same induction temperature, RCH occurred more rapidly and conferred a greater cryoprotection in frozen versus supercooled larvae. Since the primary difference between these two groups is cellular dehydration, and dehydration without chilling significantly increased larval cold tolerance, it was speculated that cellular dehydration caused by freeze concentration promoted the rapid development of cryoprotection in frozen larvae.
The second investigation examined the alternative overwintering strategies of freezing and cryoprotective dehydration. Freezing had little effect on larval body water content and hemolymph osmolality. In contrast, cryoprotective dehydration resulted in a progressive loss of body water, causing a four-fold increase in hemolymph osmolality. Also, freezing and cryoprotective dehydration produced distinctly different patterns of glycogen breakdown. However, post-recovery levels of glycogen were similar in these two groups as were total lipids. In summary, freezing and cryoprotective dehydration were both effective in promoting winter survival of larvae, with only minor differences in energetic costs.
The use of cryoprotective dehydration as an overwintering strategy in B. antarctica is constrained by inoculative freezing from environmental ice. Thus, the last project investigated the effect of different microhabitat substrates on larval overwintering by freezing or cryoprotective dehydration. When tested at ecologically-relevant hydration levels, all types of substrate created environmental conditions that were too moist to permit avoidance of freezing by inoculation. Consequently, it is likely that most larvae would likely be forced to overwinter in a frozen state. Yet, dehydrated larvae were found in the field. Therefore, spatial and temporal variations in microhabitat conditions can expose larvae to dehydration stress and potentially allow them to overwinter by cryoprotective dehydration.