Two species of gammarids (Amphipoda, Gammaridae) have been found in Bulgaria outside of their native range. The two species - Pontogammarus robustoides and Dikerogammarus villosus are Ponto-Caspian species common in the Black Sea coastal waters and the lower part of the Danube River. Recently, the two species have been recorded in the inland waters of Bulgaria, P. robustoides being translocated to the Aegean Sea basin.

Pontogammarus robustoides Sars, 1894

Pontogammarus robustoides was first recorded in the Aegean Sea basin in 2006. It was found in the Mochuritsa River, about 200 m upstream of its confluence with the Tundzha River. Since then, the species spread along the whole middle and lower reaches of the Tundzha River. The native perecarid species in the Tundzha River basin (all the Bulgarian sector) are Gammarus arduus G. Karaman, 1975, and G. komareki Schäferna, 1922. At present, these two species retreat from their natural habitats, while P. robustoides has become the dominant macrozoobenthos species. P. robustoides has been also found in the Ovcharitsa Reservoir, within the Maritsa River catchment, Aegean Sea basin.

Dikerogammarus villosus Sowinsky, 1894

This species was recorded in 2011, and then again in 2016 in Gorni Dabnik Reservoir (near the town of Pleven), which is located about 30 km far from the Danube River. This reservoir is part of a large canal irrigation system and receives water from the nearest Danube tributary - the Vit River. The native amphipod species found in the river are Gammarus dacicus Dobreanu & Manolache, 1942 (old syn. G. balcanicus) and G. arduus Karaman, 1975. The first species has been registered as an active migrant through the Vit River canal system, including Gorni Dabnik Reservoir. On the other hand, D. villosus has not yet been found out of the reservoir. Therefore, it is assumed that the introduction of D. villosus to the reservoir did not happen by natural movement of the species upstream of the Vit River. Most probably D. villosus was transferred directly to the reservoir with the help of humans and their activities, such as fish stocking, fishing, and transfer of recreational fishing and boat equipment.

Pontogammarus robustoides - general view. Male individual from the Tundzha River near Yambol Town (Aegean Sea basin)		Dikerogammarus villosus - general view. Male individual from Gorni Dabnik Reservoir near Pleven Town

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