Patterns and mechanisms of aquatic invertebrate introductions in the Ponto-Caspian region

The Black, Azov, and Caspian sea drainages (i.e., Ponto-Caspian region) have an extensive and long history of species introductions. Here we review patterns and mechanisms of introductions of aquatic invertebrate species into these ecosystems. Since the late 1800s, 136 free-living and 27 parasitic invertebrate species have been introduced outside their native ranges and have established reproducing populations in the Ponto-Caspian region. The bulk of these introductions are represented by crustaceans (53%), flatworms (15%), and molluscs (13%). Most of the introduced species are native to other areas within the Ponto-Caspian region (37%), with other sizable contributions from the Atlantic-Mediterranean (15%) and boreal European-Siberian (14%) geographic regions. Mechanisms of introductions were dominated by deliberate releases (29%) and shipping activities (22%), with the former occurring principally in freshwater habitats and the latter in marine and estuarine ones. Other introductions resulted from unintentional release (21%) and hydrotechnical development (14%), notably the construction of reservoirs and canals. Global and regional trade, particularly that mediated by commercial ships, provides dispersal opportunities for nonindigenous invertebrates to and within the Ponto-Caspian region, rapidly changing the composition of its endemic fauna.