Tiny freshwater jellyfish spread through Swiss lakes

A tiny freshwater jellyfish from China is spreading in Swiss lakes, with warmer water favouring its appearance, SRF reports. Craspedacusta sowerbii is harmless to humans and is probably more widespread than direct sightings suggest.

The nearly transparent jellyfish usually measures one to two centimetres across and is difficult to distinguish from the surrounding water. Its stinging cells help it feed on tiny plankton organisms but do not cause noticeable effects in humans.

The species was first described in London in 1880, after being found in a botanical greenhouse. It has since spread across almost every continent. It has lived for years in lakes in Italian-speaking Switzerland and northern Italy.

In Ticino, it was first spotted in 1962 in the River Ticino on the Magadino Plain. It has also been documented in Lake Lugano, where it appears sporadically during the warmer months. Its presence has been confirmed in Lake Geneva as well.

Human activities have helped transport the species between bodies of water. Aquatic ornamental plants, aquariums and the movement of fish have all contributed to its spread. The jellyfish is visible only during a short phase of its life cycle. For most of its existence, it lives as a tiny polyp attached to the lakebed or aquatic vegetation.

The polyps are only a few millimetres in size and can go unnoticed while being transported to new environments. The jellyfish usually appears between summer and early autumn, when water temperatures rise above 25 degrees Celsius. These occurrences are irregular, lasting a few weeks in some lakes and not returning for years.

A study published in 2024 recorded the species at least 61 locations in Italy, including Lake Garda, Lake Maggiore, Lake Como and Lake Lugano. Experts believe it is present in more places than direct observations show. Climate models suggest that warming waters could make the jellyfish increasingly common in temperate lakes.

So far, its effects on lake ecosystems have been negligible. Researchers say it remains difficult to predict whether that will continue, and they will need to monitor how the species becomes incorporated into lake food chains.