Are Ponto‐Caspian species able to cross salinity barriers? A case study of the gammaridPontogammarus maeoticus

Abstract

Recently, Ponto‐Caspian species (i.e., area of Azov, Black, and Caspian Seas) have invaded brackish and freshwater habitats of the North and Baltic Seas and the Laurentian Great Lakes in much higher numbers than expected based on shipping frequency and environmental conditions among these regions. Therefore, it has been suggested that Ponto‐Caspian species may have inherent advantages over other species in colonizing new habitats, or that they are of freshwater origin. Artificial selection offers the possibility to investigate phenotypic plasticity, shifts in environmental tolerance, and heritability of environmentally sensitive traits; therefore, in this study, we conducted artificial selection experiments on Ponto‐Caspian amphipod Pontogammarus maeoticus collected from 10 PSU to evaluate adaptation capacity of this species to different salinities, and to shed additional light on a possible freshwater origin of Ponto‐Caspian invaders. Our results indicated that selection to lower salinity than that of the population's ambient salinity is possible within few generations due to a likely existence of standing polymorphic variation for selection to act on. In contrast, selection to higher salinity was unsuccessful because the phenotypic variation was mainly caused by environmental variance and therefore might depend on new mutations. Consequently, the results of our study suggest that the tested species might be of freshwater origin and lacks necessary genetic background for adaptation to fully marine conditions. Further selection studies using more species and populations, as well as molecular techniques, should be conducted to elucidate if other Ponto‐Caspian invaders are of freshwater origin as well.

Are juveniles as tolerant to salinity stress as adults? A case study of Northern European, Ponto‐Caspian and North American species

“…Recently, Pauli and Briski (2018) conducted an extensive literature search on the salinity range of Ponto‐Caspian NIS in their native and non‐native habitats and determined that though Ponto‐Caspian species occupy wide ranges of salinity, more than 67% of the species were recorded in freshwater habitats in their native region, with a tendency of a decreasing number of species as salinity increased. The similar evidence was provided by Pauli, Paiva, and Briski (2018) demonstrating that artificial selection of one Ponto‐Caspian gammarid, originating from a salinity of 10 g/kg, is possible to lower salinities and freshwater conditions, but not to higher salinities. Finally, a comparative salinity assessment, using adults of 22 populations of eight gammarid species originating from the Ponto‐Caspian, Northern European and Great Lakes–St.…”

“…The salinity of the control treatments was identical to ambient water of the collection site for each species: for G. salinus, 16 g/kg; for P. maeoticus , 10 g/kg; and for G. tigrinus , 10 g/kg. The high and low salinity treatments began at the ambient salinity of the species collection site, which was then increased/decreased by 2 g/kg every two days, until reaching 40 g/kg and 0 g/Kg, respectively (Delgado, Guerao, & Ribera, 2011; Paiva et al., 2018; Pauli et al., 2018). Therefore, we emphasize here that due to the different ambient salinities of the three species, the high and low treatments did not start from the same salinities for all three species.…”

Aquatic invasion patterns across the North Atlantic

“…This indicates that their invasion to Chesapeake Bay may be secondary in source from other ‘stepping stone’ regions, or vice versa. Whilst Ponto‐Caspian taxa are highly tolerant to freshwater and brackish conditions (Paiva et al, 2018; Pauli et al, 2018), this lack of invasion to the extensively brackish Chesapeake Bay suggests that other ecological or socioeconomic (e.g. trade and transport patterns) context dependencies are at play in mediating invasion success, and these require further elucidation.…”

“…Similarly, studies have highlighted the disproportionate contributions from the Ponto-Caspian region globally when considering invasion dynamics of key taxonomic groups, such as gammarid crustaceans (Cuthbert et al, 2020). The evolution of tolerance to harsh salinity-temperature heterogeneities may predispose Ponto-Caspian species to invade wide-ranging aquatic conditions worldwide (Paiva et al, 2018;Pauli et al, 2018).…”

Sea freshening may drive the ecological impacts of emerging and existing invasive non‐native species

“…The Ponto‐Caspian region is a donor hotspot for INNS (Cuthbert et al., 2020), with many euryhaline species deemed pre‐adapted to invade and establish in new environments (Casties et al., 2016; Paiva et al., 2018; Pauli et al., 2018). Indeed, many Ponto‐Caspian INNS can thrive in areas of anthropogenic alteration, including pollution and large salinity ranges, at the expense of natives (Den Hartog et al., 1992).…”