New research reveals how invasive jelly-like species spreads in Nordic lakes

Fish consume the invasive freshwater bryozoan, but their digestive systems also help disperse it, according to a study in southern Finland.

The finding comes from a joint project by the Kokemäenjoki Water Protection Association (KVVY) and the Vanajavesi Centre, which examined how the species—known locally as hyytelösammaleläin—moves through aquatic ecosystems.

Researchers found that the bryozoan’s dormant statoblasts pass intact through fish digestive tracts. One fish sampled contained thousands of millimetre-sized statoblasts, suggesting a key pathway for upstream spread.

“This helps explain why the species has spread so rapidly against water currents,” said Kirsi Kuoppamäki, an environmental expert at KVVY who led the study.

The idea for the fish study originated with local anglers who had noticed jelly-like material in fish stomachs. In summer 2025, the team collected and examined nearly 500 fish from lakes in Hattula and Lempäälä.

Statoblasts can also attach to boat ropes and anchor lines, making them easy to transport between water bodies. Researchers observed this during field trials using artificial substrates, where statoblasts adhered more effectively to nylon lines than to standard collection plates.

The freshwater bryozoan is typically observed in late summer when water temperatures rise. During cooler months, it exists in its dormant statoblast form. When waters warm, statoblasts hatch into larvae that form gelatinous colonies—sometimes reaching half a metre in size.

Eradication is difficult, but authorities recommend burying or composting removed colonies to limit spread.