Hunting slugs on the tundra
Visitors to Herschel Island Territorial Park last summer might have been puzzled by the sight of Yukon government biologist Dorothy Cooley and others lifting scraps of cardboard off the tundra and peering intently at the undersides.
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A Herschel Island slug stretches out beside a metre stick.
(photo: Dorothy Cooley)
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The explanation was even more puzzling -- a slug hunt.
Most of us think of slugs as large slimy creatures that invade coastal gardens. But slugs on an island off the Yukon's north coast?
In fact, slugs occur across much of northern Canada, says Susan Kutz, the Saskatchewan researcher who instigated the Herschel Island slug hunt.
"They are extremely difficult to find," she says. "More than likely they are widespread on the mainland, but I was surprised that they are on the islands."
One reason the slugs are hard to find is size. Northern slugs (Deroceras laeve) are the size of a pinhead as babies and a centimetre or so long as adults.
"Our trophy slug was 35 millimetres," says Kutz.
The other reason they're hard to find is low density. They live in places were the tundra is consistently damp -- but not in large numbers.
The trick to finding slugs is to cater to their taste for wet, dark places. A sheet of wet cardboard dropped over tundra vegetation makes a perfect slug hide-out. The slugs crawl onto the bottom of the cardboard, the slug hunter lifts up the cardboard and, if the hunting is good, counts slugs.
Fascinating as they find the tiny slugs, Kutz and her colleague Emily Jenkins are more interested in what's inside some of them.
For several years, Kutz has been researching parasites that show up in muskoxen as lungworm. In 1999, her work led to the research of Jenkins, whose PhD thesis is on lungworm and muscleworm infections in Dall's sheep.
"Slugs and snails are essential parts of the life cycle of this family of parasites," says Kutz, adding that the same group of parasites is also common in caribou.
"If it is a muscleworm, the females lodged in the muscles stick their butt ends in the blood vessels and lay their eggs," explains Kutz. "The eggs are transported by the blood stream to the lungs where they hatch, burst into the airways, and then up the trachea."
Lungworm adults are already in the lungs and lay their eggs there. Their larvae also travel up the host animal's trachea, are swallowed, progress through the digestive tract, and are dropped onto the tundra in the animal's feces.
Then luck takes a hand. To develop further, the larva must come into contact with the right kind of snail or slug -- and only a few species of snails or slugs will do. A lucky larva enters its new host by burrowing in through the foot tissue.
The larva develops through two more stages inside the new host.
"The third stage inside the slug/snail is the typical source of infection," says Kutz.
The muskox lungworm's third stage larvae leave the slugs and sit curled up on vegetation, protected by slug slime, waiting for an muskox to accidentally eat them, starting the process all over again. Kutz and Jenkins hypothesize that this behaviour is an important adaptation to life in the far north. By leaving the slugs, which burrow down into the soil when it gets cold, the larvae have longer to meet up with muskoxen.
The muscleworms that infect sheep further south behave a bit differently, says Jenkins. "At most, about half the muscle worm larvae will leave the slug. They don't seem to have the same pressure to leave as the more northern muskox lungworm."
The length of time it takes the parasites to reach their third stage of development varies, and it depends on temperature.
If summer temperatures are around the long-term normal for arctic Canada, the muskox lungworm usually takes two summers. However, a warm summer can compress that development into one season.
"Even a very small temperature difference can make a big difference in time frame," says Jenkins. "Warming seems to be good for worms."
With climate change bringing warmer summers to the arctic, what's good for worms might be bad for their hosts. More worms could more parasites and more disease for muskoxen, caribou, and Dall's sheep.
"Climate projections for the Mackenzie Mountains in the NWT and the Yukon are warmer and wetter -- perfect slug weather!" says Jenkins.
How a changing climate might affect the parasites and their hosts is still a puzzle -- and that's where last summer's slug hunt on Herschel Island comes into the picture.
"Because of the long-term environmental monitoring on Herschel Island, it might be a good place to look at climate change impacts on the important slug intermediate hosts," Kutz explains.
She asked some Yukon colleagues to set out slug traps on the Herschel Island tundra and see what they could find. They found slugs, but not a lot of them.
Kutz is not sure yet whether there are enough slugs to use Herschel Island as a study site, but simply finding the slugs is exciting.
"It's amazing how little we actually know about slugs."
- For more information about wildlife parasites and their hosts, contact Susan Kutz (susan.kutz@usask.ca) or Emily Jenkins (emily.jenkins@usask.ca) at the Research Group for Arctic Parasitology, University of Saskatchewan.
