Canadians who spend time fishing or swimming in lakes and rivers across the country may have encountered an unfamiliar creature lurking in the waters.

Reports of freshwater jellyfish — a species called Craspedacusta sowerbii — seem to be on the rise in parts of the country, sparking questions about how they got there.

Boaters and swimmers have described seeing the jellyfish in waters across Ontario, including the Bay of Quinte and Belmont Lake, according to the Toronto Star.

Eva Pip, a biologist at the University of Winnipeg, says that the rising frequency of jellyfish sightings over the last few years — especially in eastern Manitoba, where they are even rarer  — seems to be an indicator of an ecological change. But she says it's hard to tell whether that change is positive or negative.

"A number of other species, too, are either increasing or decreasing," said Pip, adding that many factors may be responsible.

'There are jellyfish years where they bloom all over the place and then they're not seen again for several.'—Jellyfish expert Claudia Mills

"Obviously the climate is different now than it used to be, especially these last few years …We've also had more human activity in the form of disturbance and pollution, and under those sorts of conditions, you get community changes."

The umbrella-shaped C. sowerbii jellyfish are small compared to their saltwater counterparts, reaching a diameter of about 2.5 centimetres when fully grown. They feed mostly on plankton and are harmless to humans.

Despite their rarity in the past, there have now been reported jellyfish sightings in more than 100 lakes and rivers in Ontario and over 50 in Quebec, according to a website run by biologist Terry Peard. He told the Toronto Star that he received several hundred reports in 2012 alone.

Jellyfish cycles

Claudia Mills, a research scientist specializing in jellyfish at the University of Washington, says that fluctuations in jellyfish could just be part of a natural cycle.

She explains that jellyfish embryos, called polyps, form on the bottom of lakes, but don't always develop into an adult medusa form, unless conditions are favourable.

"It can be years between sightings," Mills said. "There are jellyfish years where they bloom all over the place and then they're not seen again for several."

While natural cycles may account for fluctuations in the jellyfish population, Pip says they have been having an especially productive peak recently. Based on observations of other animal species, she doesn't rule out the impact of recreational activity or rising water temperatures, but adds that until there is more research, she doesn't think it's much of a concern.

"If we have further increases in their population, then maybe it will be easier to see what kind of impact they're having on the ecosystem on the whole," Pip said. "Now they're still a minor component of the ecosystem."