Evidence for non-indigenous freshwater jellyfish Craspedacusta sowerbii spreading in Finland

Cnidarians are among the most successful aquatic invaders, and the freshwater jellyfish Craspedacusta sowerbii is a prime example from non-marine systems. The hydrozoan has spread through northern temperate latitudes worldwide, and in the 1990s, the first records from Finland marked its northernmost distribution in Europe. Since then, no systematic monitoring of C. sowerbii’s distribution and genetic identity has been made. We compiled and analysed occurrence data that had mainly accumulated through community science websites to the FinBIF data warehouse. By 2024, over 65 reports of freshwater jellyfish from 19 locations across southern Finland were received. Numerically, most reports were from a few artificial swimming ponds in the vicinity of population centres, whereas most locations were shallow lakes or bays scattered through rural southern Finland. The number of reports has sharply increased in recent years, along with overall increases in community science observations and an upward trend in average surface water temperature of lakes and coastal sea. Most of the observations were from late summer (mid-July to September). The northernmost record (in 2020) was from 62.82°N, 250 km north of the initial public record in 1999. We also assessed the phylogeographic relationship of Finnish samples from the COI gene. The global mitochondrial C. sowerbii diversity comprises three deep lineages all distributed on multiple continents. The Finnish specimens represent a branch within the most widespread lineage, with a sequence identical or nearly identical to ones from Czechia, Greece, Singapore, and China. As spreading is predicted to continue with global warming, monitoring efforts in freshwater systems are advised.