3. The Apparently Relentless Spread of the Major Decapod Alien Species in the Mediterranean Basin and European Inland Waters

Among the many alien crustacean species so far recorded in the Mediterranean basin are numerous decapods—crayfish, crabs and prawns—that have in common the reputation of being both aggressive and adaptable to various environments. Their entrance to the Mediterranean basin has often been fostered by human actions, such as the transfer of new species for commercial purposes or accidentally, as bycatch or because they were present in discarded ships’ ballast water. Marine alien species enter the Mediterranean basin from two main directions: from the Atlantic and beyond, and from the Indian Ocean via the Suez Canal. For freshwaters, the extensive European canal network is an important aid to migration, but human discards are also significant. Protection of native fauna and ecosystems is at best partial. Freshwater ecosystems particularly at risk include lagoons, river estuaries, ponds and marshes, while comparable marine systems are shallow muddy coastal regions, rocky reefs, estuaries/bays and salt marshes. Local communities, principally anglers and people working with aquatic resources, usually are curious at the appearance of a new species and the possibility of establishing a new business is often the first thought. However, freshwater decapod alien species cause major damage to agriculture production, infrastructures, riverbanks, irrigation systems and fish production, leading to consistent economic losses. In particular, they are known to be injurious to plant production, causing increasing losses to agriculture, consuming young seedlings and seeds and causing substantial water losses from field areas. Less information is available for seawater species to date, but their increasing number and growing size of populations requires the attention of the scientific community to evaluate and predict their influence on the marine environment and/or on local fish and bivalve production. Both freshwater and marine alien decapods thus represent a serious threat to native European species and have a negative impact on species richness and habitats. The economic losses caused by these species require reliable and cost-effective options for managing their abundance and if possible for their eradication.