If you spot a jellyfish in British Columbia, chances are you are not looking at an ocean drifter. You might be standing at the edge of a lake, watching a thumbnail-sized freshwater, nearly transparent medusa pulse just beneath the surface, something that feels almost unreal the first time you see it. It is the kind of sight that feels like a mistake, until it happens again, and again, in more places.
That is the strange reality exposed by a recent study led by UBC Institute of Ocean and Fisheries researchers Dr. Florian Lüskow and Dr. Evgeny Pakhomov. Pulling together decades of sightings, the team examined how the invasive peach blossom jellyfish has been appearing across B.C., and why those appearances may become more common as summers warm. The species, originally from China, has been reported in B.C. since 1990, and it has now been documented in at least 34 locations in the province, making it the northernmost known range for this species in North America.
What makes this story more than just a curiosity and how easy it is to miss. “This is an introduced jellyfish species from China which has spread around the world. We know very little about how they affect ecosystems and biodiversity of these systems in Canada, because the research hasn’t been done yet,” Dr. Lüskow said. The concern is not about human safety; these jellyfish are not harmful to people, but about what they could do to freshwater ecosystems if they appear more often or in more lakes.
At first glance, the medusa stage is the easiest part to notice, when small jellyfish seen in late summer, floating and pulsing in the water column. However, the visible stage is only one chapter of the species’ life cycle. The other key stage is the polyp, a tiny bottom-dwelling form that can live undetected and produce medusae only when conditions are right.
“Polyps are very small, usually around a millimetre in size, and it is challenging to locate them,” Dr. Pakhomov explained. He added that polyps can inhabit shallow areas attached to rocks and submerged wood debris. The researchers and citizens often realize the species presence once medusae appear in warm waters. In other words, a lake can host the jellyfish long before anyone sees a single medusa.
Temperature appears to be the gatekeeper. In the team’s observations, the medusa form appears only when water temperatures exceed about 24°C, and in B.C. the medusae are typically seen between July and early October. This detail matters because climate change is pushing conditions toward warmer summers and milder winters, which may increase the number of lakes that cross the threshold for medusa production. Dr. Lüskow noted that B.C. sits at the northern edge of the species’ current range, and mild winters and high summer temperatures help determine whether the jellyfish can reproduce.
There is a twist that makes this invasion even stranger: every jellyfish examined in B.C. so far has been male and genetically identical. In other words, they are clones. This suggests the population may be spreading through asexual reproduction from a shared source, likely linked to polyps that persist and produce medusae when conditions allow. Dr. Pakhomov also notes that because only males have been found, the jellyfish may not be able to complete sexual reproduction in B.C., which could limit how well they adapt to new environments. However, clonal persistence, combined with warming and accidental transport, could still allow the species to keep appearing.
How they move between lakes is also not pinned to a single pathway. The researchers discuss likely routes such as polyps being transported on recreational boats or moved by birds. The team also observed a habitat pattern: jellyfish were found in ponds, quarries, and lakes, but not in creeks or rivers.
For the researchers, the next steps are practical and urgent. Dr. Pakhomov describes two priorities: mapping the true distribution and range of peach blossom jellyfish in B.C., and quantifying their impacts on freshwater ecosystems, including possible effects on young salmon. Dr. Lüskow points to environmental DNA as a promising tool, because it could detect presence even when medusae are not visible.
Freshwater jellyfish can feel like a curiosity, the kind of thing you mention because it sounds impossible. This study suggests that they are also a signal, a reminder that freshwater ecosystems are changing, sometimes quietly, and that the first step toward responding is knowing what is already there.
The study, Brazil’s Invisible Invaders: Are Craspedacusta jellyfish a ticking ecological bomb?, was conducted by an international team of researchers, including UBC Institute for the Oceans and Fisheries scientists Florian Lüskow and Evgeny Pakhomov.
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