Aquatic invertebrate species that have broad salinity tolerances may be pre-adapted for invasion success and biogeographic distributional range expansions, facilitated by human-mediated dispersal (HMD), leading to a trend to become neocosmopolitan across many regions of the world. This pattern appears to characterize many Ponto-Caspian (P-C) aquatic invertebrates, which have a > 100-year history as aquatic invasive species (AIS), spreading throughout much of Eurasia and for some, in North America and beyond. Our study compiles comparative salinity conditions and distributional data for AIS invertebrate species globally versus those originating from the P-C region, to test whether they statistically differ.

Our investigation discerns that a total of 1861 invertebrate AIS taxa have been recorded worldwide, with (A) 70.5% exclusively living in the saline adaptive zone of brackish (0.5–30 ppt; A1) and/or marine waters (> 30 ppt; A2), (B) 20% in the freshwater adaptive zone alone (0–0.5 ppt), (C) 7.5% being euryhaline (across both A and B), and (D) 2% being semi-aquatic in either (D1) freshwater/terrestrial or (D2) saline/terrestrial environments. In contrast, our results indicate the following proportions for AIS invertebrates of P-C origins: (A) 27% exclusively inhabit the saline adaptive zone, (B) 25% are entirely freshwater, (C) 45% are euryhaline, and (D) 3% are semi-aquatic, significantly differing from the global pattern. Euryhaline AIS native to the P-C region thus markedly outnumber (45%) those originating from other regions (7.5%), likely pre-adapting them for widespread establishment in harbors, estuaries, and coastal areas. Moreover, most P-C invertebrate AIS (70%) contain freshwater-tolerant populations (B + C), rendering them very successful invaders of inland water bodies. These broad salinity tolerances of P-C AIS underlie their tremendous invasion successes and growing neocosmopolitan distributions with HMD.

An evolutionary and recent history of broad salinity tolerances of a large proportion of P-C invertebrates appears to enhance their ability to invade, establish, and spread in new regions, especially harbors, estuaries, and freshwaters, leading to their increasing neocosmopolitan distributions. This trend likely will continue—accelerating with climate change and increased global transportation—meriting worldwide conservation agency focus and cooperation, along with public education programs aimed to rapidly identify and circumvent new introductions and spread.