Non-indigenous tanaid Sinelobus vanhaareni Bamber, 2014 in the Polish coastal waters – an example of a successful invader

The paper reports on the first record of Sinelobus vanhaareni, a non-native tanaid, in the Polish coastal waters (Gulf of Gdansk, southern Baltic Sea). The species was found in the port of Gdynia in 2014, while in 2015–2017 it already colonized the western part of the Gulf of Gdansk, inhabiting mainly hard substrates, including both natural (e.g. boulders) and anthropogenic ones (e.g. vertical concrete piles or walls of offshore structures and breakwaters, horizontal PVC plates and oyster shells used as filling in habitat collectors). During the survey period, S. vanhaareni was found in different seasons of the year (from winter and early spring to autumn), which, combined with the presence of ovigerous females as well as high abundance (up to tens of thousands of individuals per square meter), allows us to assume that the species has already established a population in the Gulf of Gdansk.