Life in Antarctica seems like it might be stressful for an insect because of the cold temperatures. But recent work by Nicholas Teets and his colleagues published in the Journal of Insect Physiology demonstrates that the lack of water in that habitat can also demand a stiff toll.
During much of the year, water in Antarctica is frozen, so the availability of water is highly variable and insects are vulnerable to dehydration. Yet, insects survive desiccation by accumulating osmoprotectants such as trehalose and glycerol, which the authors hypothesized would exact an energetic cost. As physiological responses to cold stress can alter as the number of exposures increases, Teets and his co-workers tested whether there were differences in the energetic costs between repeated and prolonged dehydration stress in the midge Belgica Antarctica – the only insect endemic to the Antarctic continent.
The authors travelled to Palmer Station on the Antarctic Peninsula and collected larvae during the austral summer from the adjacent islands. They then exposed larvae either to five bouts of rapid dehydration followed by rehydration, or to a single 10 day exposure to a slower dehydration followed by rehydration. After each rehydration they measured whole-animal and midgut cell survival, water content and metabolite concentrations (lipids, osmoprotectants and glycogen), to quantify the cost of each type of exposure.
The team found that repeated bouts of dehydration were stressful for the larvae, causing significantly more mortality than a single prolonged dehydration. In addition, the survival of midgut cells was reduced in the repeatedly dehydrated larvae, although one dehydration cycle was just as damaging as five. What the authors didn’t expect was that while water content was lower after the dehydration phase of the first few cycles, over the course of the five dehydration/rehydration cycles, the larvae were able to retain more and more of their water during dehydration. This suggested that there were adaptive changes in the insects’ ability to prevent water loss during dehydration after multiple occurrences.
They also found that the energetic costs of dehydration differed between larvae that received repeated and prolonged exposures. Repeatedly dehydrated larvae had dramatically decreased glycogen and trehalose reserves, and therefore a lowered carbohydrate energy content after five cycles of dehydration. By contrast, larvae that had been exposed to prolonged dehydration had slightly decreased glycogen levels but their lipid content was also much lower, which resulted in a significantly lower total energy content relative to the larvae that received repeated exposures. The authors suggest this decrease may be due to the insects having a higher metabolic rate during the prolonged dehydration period.
Taken together, these results show that dehydration stress is energetically costly for B. antarctica and that the type and amount of energetic cost depends on the frequency and intensity of that stress. As moisture regimes on the Antarctic Peninsula may change as a result of climate change, the costs of dehydration may form an important limit to B. antarctica fitness in a changing world.
