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They came from the Danube River - History and present status of Dreissena polymorpha and Orconectes limosus in Croatia

The freshwater mussel Dreissena polymorpha has become one of the most dominant species in many lakes and rivers of Europe, since it started spreading from the Caspian area at the beginning of the 19th century. The first literature data on the appearance of D. polymorpha in the Danube River appeared during that same century. During 1980s it was observed that D. polymorpha spread from the Danube River into the Drava River. Since then, it has been spreading upstream to the town of Varaždin and is still progressing westwards. In the year 2002 it has also been found in the Jarun Lake in Croatian capital, Zagreb. At the same time the most western location of this invasive species in the Sava River is below the mouth of rivers Drina and Bosut. This invading success can be attributed to (1) the ability of the adults to adhere to hard surfaces with their byssus, (2) development by free-swimming veliger larvae, unique among freshwater bivalves, and (3) the extraordinary high fecundity. Therefore, the biology and ecology of this species (with special emphasis on the reproductive cycle, development and settlement of larvae) has been more thoroughly studied on the two populations inhabiting lotic and lenthic ecosystems (artificial lake Dubrava and river Drava) near town Prelog. In the year 2003, another invasive species was discovered entering Croatian freshwater habitats from the Danube River. American freshwater crayfish Orconectes limosus was found appearing in the waters of the Park of nature Kopački rit. It is a very aggressive species with higher fecundity than the autochthonous crayfish and a known carrier of a crayfish plague. Its spreading westwards to the other rivers and lakes is to be expected.

Dreissena polymorpha; Orconectes limosus; Danube River

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