Hyytelösammaleläin returns to Lake Vanajavesi in Hämeenlinna, Finland

A familiar invasive species, the jellyfish-like organism known as the “hyytelösammaleläin,” has made a notable return to Lake Vanajavesi in Hämeenlinna, Finland. Residents of the area have reported encountering swarms of these creatures, with social media groups noting clusters of over 20 individuals. “There clearly seem to be more than last year at this time,” says Suvi Mäkelä, a water specialist at the Vanajavesikeskus.

Mäkelä has received numerous reports about the organisms from lakeside residents, which align with insights from Kirsi Kuoppamäki, an environmental expert from the Kokemäenjoki Water Protection Association. She confirmed an increase in reports submitted to a database tracking invasive species, noting a significant rise compared to last year.

Since early June alone, 87 sightings of the hyytelösammaleläin have been reported in Hämeenlinna. The summer’s warm weather has contributed to their population growth, as the organisms thrive in waters exceeding 20 degrees Celsius. Initially identified in Finland about twenty years ago, the origins of this species remain somewhat unclear, but it is suspected they arrived via ballast water from ships.

While completely eradicating invasive species from waterways is challenging, efforts can be made to control them. If individuals spot hyytelösammaleläin, they are encouraged to remove them from the water and bury them to help prevent further growth. To avoid unintentional spread, transporting vessels between different water bodies should be minimized.

The jellyfish-like organisms can be a nuisance, tangling fishing gear and cluttering docks. Despite their non-toxic nature, water experts like Suvi Mäkelä suggest that if they were to disappear completely, few would miss them.