Jaera istri spotted for the first time in the Scheldt estuary
During biological monitoring of the Scheldt estuary (MONEOS program), INBO staff discovered a new invasive species: Jaera istri.
We found this tiny, white freshwater isopod, barely 2 to 4 millimeters in size, in the Rupel River near Terhagen in June 2025. The discovery confirms that the Scheldt is a true hotspot for exotic species. Jaera istri arrived in Flanders via an impressive journey: the creature spread via the Danube and the Main-Danube Canal to rivers such as the Rhine and the Elbe. It has been found in the Netherlands since the 1990s and in Flanders since 2000 (mainly along the Meuse and in a few canals). The species, which lives primarily on hard, man-made substrates, may already be widespread but is difficult to find. It scavenges dead organic matter and feeds on the algal biofilm that grows on hard substrates. As a result, it plays a role in recycling nutrients in rivers. The impact of this on the ecosystem still needs to be investigated.
The Convention on Biological Diversity aims to significantly reduce the introduction and impact of invasive species. It all starts with a clear overview of which exotic species are present in Flanders. Jaera istri joins a long list of Ponto-Caspian newcomers in the Scheldt, including fish such as the round goby and the infamous Dikerogammarus villosus, also known as the “killer shrimp,” which preys on other small amphipods. Representatives of that region in Southeast Europe are also found among other groups of organisms, such as bristle worms and aquatic annelids, amphipods, and bivalves. Ponto-Caspian species thus account for one-fifth of the exotic fauna of the Scheldt estuary.
Jan Soors, Ada Coudenys, Gunther Van Ryckegem, Carl Van Colen, Merlijn Jocqué