Seen a jellyfish off the dock? You’re not imagining things — they’re increasingly in our lakes

Montreal: As a lake researcher, I have in recent years had quite a few people ask me whether jellyfish really do live in lakes.

Some people think they are seeing things, or that their friends or family are. I’ve even heard of marital spats over the issue! So, are jellyfish present in Canadian freshwater lakes or not? Yes, jellyfish do exist in lakes here in Canada. These organisms are related to saltwater jellyfish (Cnidaria), only much smaller. The reasons why more people are suddenly becoming aware of these creatures is simply due to their growing numbers — numbers that are on the rise due to climate change.

The bottom line is that freshwater jellyfish are real, and Canadians need to get used to their growing presence in our lakes as the planet warms.

Craspedacusta on the rise

Currently, the most observed genus in many parts of the world, including Canada, is Craspedacusta, with the most common species being Craspedacusta sowerbii. Some have given it the common name of Peach Blossom Jellyfish. In French it is simply known as the “meduse d'eau douce”.

This species of freshwater jellyfish was first described and scientifically named in 1880 at the Royal Botanical Gardens in London. They are an introduced species outside of their native China and have been observed on all continents except Antarctica.

These jellyfish are not able to sting humans – so no need to worry about encountering them while swimming. They are somewhere between one to two-and-a-half centimetres in diameter and mostly transparent. They can be very abundant when their “blooms” occur, appearing in the hundreds to thousands. However, these blooms typically only last about a week or two — just long enough for them to carry out sexual reproduction.

Freshwater jellyfish have been around some lakes in southern Canada for a long time. The earliest reported detection that we know of was in a Québec lake in 1938. So why are we only hearing more about them now? There are a couple of main reasons.

Unseen presence

The Peach Blossom Jellyfish spends most of its life cycle as a tiny polyp about one to two millimetres in length. A population of polyps can survive for years in this asexual phase, reproducing by budding.

In this form, they live attached to vegetation, rocks and other surfaces near the bottom of lakes and are hard to see. They feed on other passing organisms; mainly small crustaceans called zooplankton and particularly those that live near the edges of lakes. During periods when the environment is colder with less abundant food, they enter a dormant phase called a podocyst.

But it is sightings of the larger bell-shaped gelatinous forms, typical of the marine jellyfish we all know (and try to avoid when swimming), that are being increasingly reported. What is the connection? The mature form of the Peach Blossom Jellyfish is only produced under the right environmental conditions. A critical trigger appears to be warm temperatures. When water temperatures exceed 25 C, fully grown jellyfish can suddenly appear in large quantities.

The species has already been living there as polyp, maybe for years, and suddenly ghost-like jellyfish appear as if out of nowhere, forming abundant blooms in a lake.

Invisible invader

It is thought that the first observation of the most common freshwater jellyfish resulted from imported Chinese plants in the Royal Botanical Gardens; plants to which polyps or their dormant forms could easily have been attached. This is a typical way in which aquatic invasive species get moved around the planet.

Aquatic bird movement and migrations may also play a part in how they are moved between lakes or continents. More locally, they can be dispersed by recreational lake users who inadequately clean their equipment between water bodies.

Natural processes are also at work, with polyps and hardy podocysts being transported by animals and flowing water from one lake to another. As more lakes are colonized over time, more reports of jellyfish sightings will occur.

As mentioned, warm water temperatures appear to be the main trigger for the mature form of the freshwater jellyfish. The original habitat of these organisms is tropical-subtropical Asia. Thus, as climate change increasingly warms the waters of northern temperate lakes, we will see more Peach Blossom Jellyfish blooms. Climate change is pushing habitable zones for these creatures further and further north, as recent modelling has shown.

Consequences

What will happen to lake food webs as the jellyfish appears more often in its mature form, feeding on plankton in the water column and not just at lake edges? This is still largely unknown. We have a lot to learn about these organisms and their effects because they are hard to study. They’re difficult to culture in the lab and we can’t yet predict when a bloom will occur in nature.

Some lab work has shown that the mature form can consume 16 times more plankton prey than do polyps. Whether this leads to significant loss of lake zooplankton is still an open question given that the blooms are often short and zooplankton reproductive rates are relatively high when waters are warm.

Zooplankton are important prey for some fish species and especially juvenile fish. But for the moment, the occurrence of waters warm enough to produce mature jellyfish (usually mid to late summer) and the occurrence of young fish (usually early summer) do not overlap. Whether the timing of species interactions will overlap more as the climate warms remains to be seen as species will likely adapt at different rates.

As the waters of northern temperate lakes remain warmer longer, we do risk seeing freshwater jellyfish becoming more permanent members of the summer food web. This could reduce prey availability for fish species like cisco (Coregonus species), that rely on zooplankton prey as their main meal.

Thus, with climate change, not only is human reporting of sightings expected to increase, but so is the effect of these freshwater jellyfish on lake food webs. (The Conversation)