Our study revealed that all tested populations tolerate wide ranges of salinity, however, different patterns arose among species from different regions. Ponto-Caspian taxa showed lower mortality in fresh water, while Northern European taxa showed lower mortality in fully marine conditions. Genetic analyses showed evolutionary divergence among species from different regions. Due to the geological history of the two regions, as well as high tolerance of Ponto-Caspian species to fresh water, whereas Northern European species are more tolerant of fully marine conditions, we suggest that species originating from the Ponto-Caspian and Northern European regions may be adapted to freshwater and marine environments, respectively. Consequently, the perception that Ponto-Caspian species are more successful colonizers might be biased by the fact that areas with highest introduction frequency of NIS (i.e., shipping ports) are environmentally variable habitats which often include freshwater conditions that cannot be tolerated by euryhaline taxa of marine origin. K E Y W O R D Sfreshwater origin, Gammaroidea, marine origin, nonindigenous species, Ponto-Caspian species, salinity tolerance --

No abstract

Biological invasions are largely considered to be a “numbers game”, wherein the larger the introduction effort, the greater the probability that an introduced population will become established. However, conditions during transport – an early stage of the invasion – can be particularly harsh, thereby greatly reducing the size of a population available to establish in a new region. Some successful non‐indigenous species are more tolerant of environmental and anthropogenic stressors than related native species, possibly stemming from selection (ie survival of only pre‐adapted individuals for particular environmental conditions) during the invasion process. By reviewing current literature concerning population genetics and consequences of selection on population fitness, we propose that selection acting on transported populations can facilitate local adaptation, which may result in a greater likelihood of invasion than predicted by propagule pressure alone. Specifically, we suggest that detailed surveys should be conducted to determine interactions between molecular mechanisms and demographic factors, given that current management strategies may underestimate invasion risk.

Macrozoobenthic communities within and outside of the drift algal mats were compared in Ko˜iguste Bay, NE Baltic Sea. The patches of the drift algae were on average 0.5-1 km wide in diameter covering about 25% of the total bottom area of the bay. Thickness of the mat did not exceed 6 cm. The biomass of the mat varied between 35 and 1391 g dw m )2 . The drift algal mats had no clear negative effect on macrozoobenthos except for a few infaunal species. The drift algae favoured several detrivorous, herbivorous and carnivorous species. Among the studied variables, the thickness of algal mat and oxygen concentration at near-bottom layer explained the best the structure of macrozoobenthos. Total number of invertebrate species increased curvilinearly with the thickness of algal mat having the peak value at 3-5 cm thick algal mat. To conclude, moderate drift algal mats increased habitat complexity and, thus, the diversity of benthic faunal assemblages in otherwise poorly vegetated coastal areas.