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First published online January 31, 2007
Journal of Experimental Biology 210, i-a (2007)
Copyright © 2007 The Company of Biologists Limited
doi: 10.1242/jeb.02721
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HOW MITES COPE WITH LIFE IN THE FREEZER
laura{at}biologists.com
While a cold and exposed rock face in Antarctica is the last place many animals would choose to live, Antarctic lichen mites, Halozetes belgicae, are quite at home. They are often found near gull `rubbish dumps', or middens, grazing on the lichens encrusting the rocks. How the mites survive this harsh environment interests Tim Hawes and his colleagues at the University of Birmingham and the British Antarctic Survey. They tested their idea that the mites' extreme environment – low temperatures and extreme variation in temperature – makes them very responsive, or `plastic', when it comes to acclimating to new temperatures (p. 593).
To survive in Antarctica's cold conditions, arthropods like the lichen mite must adapt to colder temperatures, especially when the mercury drops well below freezing. To do this, they empty their guts, concentrate their body fluids and accumulate small molecule anti-freezes in their tissues so that the liquid in their bodies cools down to below water's freezing point, but without forming ice crystals which wreak havoc in delicate tissues.
To find out how mites acclimated at different temperatures would cope in the cold, the team exposed animals to temperatures of 5°C or 10°C for one week. They measured their cold hardiness by determining their supercooling point, which is the temperature where ice crystals start to form in body fluids. They found that the supercooling points were lower for mites acclimated at 5°C than mites who had spent a week at 10°C, showing that acclimation at colder temperatures makes mites more cold hardy.
Having shown that the mites could adapt to changes in temperature over the course of a week, the team tested how the mites responded to temperature changes over the course of a month. They put groups of mites feeding on rocks in containers and then measured how their cold hardiness changed as the temperature changed outside. They found that over a month, cold hardiness improved as the temperature dropped, showing that the mites could adjust to colder temperatures over longer timescales too. Knowing that starvation can affect cold hardiness, they found that starving the animals acclimated at 10°C and 5°C made them more cold hardy, probably because starved mites have no material in their guts to provide sites for the formation of damaging ice crystals.
Then to find out how the animals would cope when they rapidly dropped the temperatures, they looked at their rapid cold hardening response, where animals become more cold hardy if they have been exposed to a brief cold snap beforehand. Dropping the temperature to 0°C, –5°C or –10°C didn't increase the cold hardiness of 5°C acclimated mites, because they already had low supercooling points; however, when they repeated the experiment on mites acclimated to 10°C, cold hardiness improved after 2 h at their new temperatures; the supercooling point became up to 15°C colder. This is the fastest rapid cold hardening response seen in any animal, showing that the mites are not just plastic but `superplastic' when it comes to acclimating to the cold. Not only that, but they didn't lose this hardiness once they were returned to their acclimation temperature.
But what is causing the mites to cope so well in the cold? Analysing the mites' gut contents, they found that acclimated mites did indeed have emptier guts than those who weren't acclimated. This means that they can supercool without ice forming in their guts, and have a fighting chance of survival in their extreme environment.
References
Hawes, T. C., Bale, J. S., Worland, M. R. and Convey, P.
(2007). Plasticity and superplasticity in the acclimation
potential of the Antarctic mite Halozetes belgicae (Michael).
J. Exp. Biol. 210,593
-601.
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