Invasive Jelly Bryozoan Spreads via Fish and Boating in Finland
A recent study in Pirkanmaa and Häme has uncovered new details about the spread of the invasive jelly bryozoan (Pectinatella magnifica), revealing that fish both consume and disperse the species. The research, conducted by the Kokemäenjoki Water Protection Association (KVVY) and the Vanajavesi Centre, found that the bryozoan’s statoblasts—tiny, dormant dispersal structures—remain intact after passing through fish digestive systems.
Yle reports that the project’s lead researcher, KVVY environmental expert Kirsi Kuoppamäki, initiated the study after anglers reported finding jelly-like substances in fish stomachs. Over two summers, researchers examined at least 500 fish, including those from Hattulanselkä and Kirkkojärvi in Lempäälä. In one fish, thousands of statoblasts, roughly one millimeter in size, were discovered. While traces of jelly were also detected, Kuoppamäki noted that further research is needed to confirm this.
The findings suggest that fish may limit the bryozoan’s presence in some areas while simultaneously aiding its spread, particularly upstream. Yle explains that this mechanism partly explains the species’ rapid expansion in Finnish waterways.
Boating and Equipment Play Key Role in Spread
The study also highlighted how human activity contributes to the bryozoan’s dispersal. Researchers found that statoblasts adhered efficiently to artificial substrates, including the attachment ropes of collection plates. Ilta-Sanomat (IS) adds that anchor ropes and docks are common sites for large, gelatinous colonies—some growing up to half a meter in size—to become entangled.
IS reports that active boating appears to facilitate the species’ spread, with harbors in sheltered bays providing ideal conditions for its growth. The project, funded by the Ministry of the Environment, mapped the bryozoan’s distribution in Pirkanmaa, where it has been observed in water systems such as Pyhäjärvi near Lempäälä and Vanajavesi in Häme.
Seasonal Patterns and Citizen Reporting
The jelly bryozoan is most visible in late summer as waters warm, when statoblasts hatch into gelatinous colonies composed of thousands of individual bryozoans. During cooler months, the species exists in its dormant statoblast form. Yle notes that while eradication is difficult, populations can be managed through targeted measures.
KVVY encourages citizens to report sightings of the species via the vieraslajit.fi website, including photographs. To prevent further spread, boaters and anglers are advised to inspect and clean equipment before moving between water bodies.
The species has been present in Finland since at least the early 2000s, with confirmed sightings in the Vuoksi water system (including areas near Lappeenranta, Varkaus, and Joensuu) and the Kokemäenjoki system. IS highlights that while the bryozoan poses no direct harm to humans, it can impair recreational water use by forming large, slimy masses.
