Researchers look to ghost fleas 'haunting' Sask. lake to solve scientific mystery

People on a boat, in the middle of the night, trying to find out what’s haunting the lake. This sounds like a classic ghost story, but it was actually part of a research project at the University of Regina. 

U of R study aimed to find out why fish in Katepwa Lake have high levels of neurotoxin methylmercury

Leptodora, or 'ghost fleas,' are the reason for high levels of methylmercury in fish in Saskatchewan’s Katepwa Lake, University of Regina researchers have found. (D. Drew/Yale Peabody Museum of Natural History)

People on a boat in the middle of the night, trying to find out what's haunting the lake.

What sounds like a classic ghost story was actually part of a research project at the University of Regina. 

The "ghost hunters" were scientists sampling the water, and the "ghosts" were invertebrates called Leptodora, or ghost fleas, which are part of the zooplankton community. 

Dr. Britt Hall, a professor in the department of biology at the U of R, has led a study trying to find out why fish in Katepwa Lake, southeast of Fort Qu'Appelle, have shown high levels of the neurotoxin methylmercury. 

"Ghost fleas in particular are interesting creatures," said Hall.

"They are transparent so you can see through them. And that means that they can avoid being eaten by fish during the day…. But they are still big, so fish really do like to eat them."

Besides being almost completely clear — like a ghost — the roughly 1.7-centimetre-long creatures also use another strategy to avoid being eaten. According to Hall, they spend most of the day at the bottom of the lake, near the mercury-rich sediments. At night they function like an elevator, migrating to the top of the lake and taking the methylmercury with them.

Katepwa Lake is considered a productive lake with a lot of nutrients. That results in more algae growth, Hall says, usually diluting the methylmercury and resulting in lower levels in fish.

"We didn't really understand the mechanism for how this happened, because it kind of was opposite to what we thought was happening in productive lakes here and elsewhere," said Hall.

A mystery solved

To find out why the fish had high levels of methylmercury, the researchers had to find out how fish could actually hunt the ghostly fleas in the dark. A former post-doctoral student at the university finally found the answer, Hall said.

"The fish actually eat the Leptodora by sensing the waves that they make when they are swimming through the water. So this gives the fish the ability to eat the zooplankton at night, and allows that mercury to get into the fish because they're bringing it up from the bottom of the lake."

They're just the most bizarre things you've ever seen.​​​​​​- Peter Leavitt, U of R professor

For this project, Hall teamed up with Peter Leavitt, a professor of limnology (the study of inland aquatic ecosystems) at the University of Regina. He began collecting data 23 years ago and gave the creatures their nickname.

"They are huge by invertebrate standards," said Leavitt.

"If you had a glass of water and you had one in it, the only thing that you'd be able to see would be its eye. It's got this one weird swiveling eye on top of its pointy little head, and they're just the most bizarre things you've ever seen."

The ghost fleas can set off a chain reaction when transferring the methylmercury to smaller fish, which in turn are eaten by larger fish that might eventually end up on our plates.

"The province does monitoring for mercury content in the fish and regularly issues advisories in the years when it's a problem," said Leavitt.

According to the University of Regina, pregnant women, infants and young children are most at risk of methylmercury poisoning, which can damage brain activity in humans. 

Researchers from the university will continue their long-term studies of Qu'Appelle lakes in the future.

The Qu'Appelle Valley Long-Term Ecological Research program has been running for 27 years, making it one of the longest-running lake programs in the country.


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