Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Antarctica is busy enough in summer, but in winter it's virtually deserted. Male emperor penguins huddle together incubating their eggs, their mates hundreds of kilometres away at sea. Some Weddell seals also hang around, and there is plenty of life in the water beneath the ice sheets. But the continent itself is close to uninhabitable during the long polar night.
However, an unlikely long-term resident can be found lurking in the snow of west Antarctica. The Antarctic midge is the only insect to spend its entire life on the continent, despite being regularly frozen, dried out and sprayed with salt water.
They are so resilient, in fact, that they can survive even if they lose over two-thirds of their body water. That's almost certainly a record for an insect, and allows them to claim a unique title: no insect lives further south.
Two years of refrigeration
Well, actually one does, but it is cheating. A flea called Glaciopsyllus antarcticus has been found further south, but that's because it's a parasite that lives on seabirds. Antarctic midges do not stow away on the skin of warm-blooded animals: they survive the hard way.
They spend most of their lives as larvae, typically taking two years to reach adulthood (see photo). The 5-millimetre-long adults eat nothing and survive only 10 to 14 days, during which time they must mate and (if they're female) lay eggs. Males outnumber females six to one, so the competition for mates is fierce. The adults must have access to liquid water and a reasonably moist habitat or they die.
It is the larvae that are the tough nuts. They must survive not just the Antarctic summer, which is quite cold enough, but also the depths of the winter. Nicholas Teets of Ohio State University in Columbus is trying to find out how they cope.
During the three-month Antarctic summer, life is good. The larvae feed on flourishing algae and other microorganisms. They make their homes near penguin and seal colonies, which produce a lot of effluent for the algae to eat. Many sites are close to the shore, so the larvae are often sprayed with bracing sea water.
The larvae can cope with a wide range of stressors, many of which would kill other insects. Instead, the stress toughens them up: tipping them into sea water helps them tolerate freezing and dehydration, while dehydrating them in turn helps them shrug off both heat and deep freezing.
Teets says he once tried to obtain samples from some larvae by placing them in a toxic solvent that both kills cells and preserves them. But to his horror the experiment was ruined when the larvae failed to die. "24 hours later they were still swimming around in it," he says.
When a larva feels a chill coming, it has two options. It can stay unfrozen but rapidly shed body water, or it can conserve water but risk damage from ice crystals by letting itself freeze. Most insects can follow only one of these strategies, but Antarctic midge larvae have both at their disposal.
If the larva opts to freeze it hardens its body, which protects it – although Teets says it's not clear how. It also produces a cocktail of "antifreeze" chemicals that protects its vital organs. These include the sugar trehalose, which is also found in the tissues of plants and animals that can tolerate dehydration. Despite the gathering cold, the larva doesn't completely shut down until the temperature drops below zero.
Being frozen is not that big a risk for the animals, Teets says, because they spend the winter under rocks where it doesn't get all that cold. The real problem is loss of drinking water as it freezes. Humans die if we lose 15 per cent of our body water, but the larvae can survive a 70 per cent loss.
The larvae cope by clustering together, which minimises water loss. As dehydration sets in they start expressing different genes and change the proteins that control the shapes of their cells. Special proteins called aquaporins shuttle water around and help protect the larva's vulnerable guts.
However, Teets has found that surviving dehydration does have its costs. The larvae must burn their stores of carbohydrate to make protective chemicals, and these stores are depleted with each dehydration. In his lab, Teets found that the larvae could happily survive four cycles of dehydration and rehydration, but five was too many and killed over 35 per cent of them.
In the wild, the larvae may have to undergo several bouts of dehydration over the course of the winter. Apparently five times is too many – tough they may be, but a larva's got to know its limitations.
Journal reference: Journal of Insect Physiology, DOI: 10.1016/j.jinsphys.2011.11.011
Read previous Zoologger columns: 'Werewolf birds' hook up by the full moon, Cannibal shrimp shows its romantic side, The only cross-dressing bird of prey, The biggest spider web in the world, Slime killer hagfish feasts in rotten flesh, Female monkeys indulge in synchronised sex, The toad that's part clone, part love child, The first reptile with a true placenta, The fearsome jaws of a mini movie monster, Stealth millipede wears living camouflage, Dozy hamsters reverse the ageing process, World's nicest bird murders chicks, Architect mouse builds a food mansion.
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