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The native area of gammarids from the so-called ‘Caspian complex’,Pontogammarus robustoides(G.O. Sars, 1894),Obesogammarus crassus(G.O. Sars, 1894),Dikerogammarus haemobaphes(Eichwald, 1841) andD. villosus(Sowinsky, 1894), is associated with brackish waters. Over the last several decades they have colonized the European inland waters and part of the brackish Baltic Sea. It is believed that anthropogenic increase in the salinity of inland waters facilitated their expansion. However, the influence of salinity on the dispersal of gammarid species outside their native area is not fully understood. We tested the hypothesis that salinity was a major factor in determining distribution, based on the abundance of Gammaridae in three coastal areas of low salinity (brackish Baltic), i.e. 0.3, 3.4 and 7.3 PSU, successfully inhabited by them. Additionally, for the first time, the effect of water salinity on the osmoregulatory capacity ofO. crassuswas examined under laboratory conditions, for the salinities given above. The experiments showed that similarly as in the case of other Caspian complex species, salinity values of about 7 PSU create better conditions for osmoregulation inO. crassusthan lower salinities (i.e. 0.3 and 3.4 PSU). In the environmental part of the study, we observed that onlyD. villosusachieved a significantly higher abundance in the area of 7.3 PSU. Thus, we concluded that in the range of 0.3–7.3 PSU, salinity is not a key factor governing the distribution of Ponto-Caspian gammarids. [ABSTRACT FROM PUBLISHER]