Major die-off of Banded Mystery Snails piques curiosity of experts worldwide

A major die-off of freshwater snails in an eastern Ontario lake has stirred up attention from zoologists and environmental scientists.

Banded Mystery Snails, native to the southeastern U.S., were first reported in the Great Lakes in the1860s

Katie Ohlke's photograph captured hundreds of dead Banded Mystery Snails floating in a bay on Lake Kashawakamak, Ont. (Katie Ohlke)

A major die-off of freshwater snails in an eastern Ontario lake has stirred up attention from zoologists and environmental scientists.

In mid-May, high school art teacher Katie Ohlke was paddling her kayak along a protected bay on Lake Kashwakamak, about 120 kilometres northwest of Kingston, Ont., when she discovered hundreds of dead Banded Mystery Snails.

A carpet of small shells, each about 2.5 centimetres diameter, covered the surface of the water.

"It struck me as odd," recalled Ohlke. 

The odour of decay was powerful and pungent, so Ohlke took a quick photograph and paddled away.

Ohlke shared the image with her cousin, lake ecologist Liz Favot, who in turn tweeted it to her followers, sparking response from the international aquatic science community.

It was commented upon by oceanographers, zoologists, aquatic ecologists, biology professors and PhD students across North America.

"I thought, wow, this is amazing, I've never seen snails in such high density," reflected Josh Kurek, a Mount Allison University professor of freshwater ecosystems, who said the presence of so many invasive snails in one place had set off "alarm bells" for him. 

'It is just a stunning picture'

Eventually, the snail image landed on the desk of the Museum of Nature's Andre Martel, one of Canada's leading mollusc experts.

"When I saw it I have to say, I did not see anything like this before. It is just a stunning picture," said Martel, who worried about what it said about the scale of the snail's invasion.

The snail, once only found in the southeastern United States, has been in Canada since the 1860s. It is sometimes mistaken for the Chinese Mystery Snail, a more recent arrival likely introduced through the aquarium trade.

Martel said like most snails, the Banded Mystery Snails are subject to senescence, a coordinated die-off after spring reproduction.

But to see so many snails gathered in one place still demanded an explanation.

Another snail die-off in Norway Bay

Martel compared Ohlke's photograph with a second one showing a smaller die-off of mystery snails along with Banded Chinese Snails discovered in May by Jennifer Haughton along the Ottawa River in Norway Bay, Que. 

Martel shared the image with his friend, retired University of Guelph professor Gerry Mackie, who is preparing to publish a book titled, Mystery Snails: Facts and Myths.

"The curious thing to me is, that's a massive number of snails to be in one area at one time," said Mackie who suggested that gas released by decomposing flesh inside the shells caused them to be temporarily buoyant.

Wind or currents had probably gathered the shells into a single bay in the window between decomposition and their eventual sinking.

Both Martel and Mackie suggested the Lake Kashwakamak and Norway Bay die-offs may have been caused by a May heat wave combined with low water levels. 

They said the two images were powerful reminders of the importance of taking care to wash boats and trailers to prevent transferring snails or other invasive molluscs like zebra mussels between water bodies.