Alessandro Saccà

ABSTRACT: Alien species, defined as those species that are introduced to an area outside their natural range, are recognized as a major threat to the conservation of biodiversity and functioning of marine ecosystems. Coastal marine habitats are among the most heavily invaded systems on earth, which is derived, in part, by human-mediated transport of potentially invasive species in ships’ ballast as well as intentional introductions for aquaculture and fisheries enhancement. Transfers of cultured fish and shellfish are also responsible for the accidental introduction of ’hitch-hiking‘ species, which could become invasive and severely affect the recipient ecosystem. In addition, man-induced climate change and eutrophication are expected to increase the susceptibility of ecosystems to invasion by alien species.
Although microorganisms are by far the most abundant and diverse of aquatic life forms, research on microbial alien species introductions has lagged behind that on metazoans and macroalgae. It is now evident, however, that due to human activities, thousands of species of marine viruses, prokaryotes and protists are being transported within and between oceans, irrespective of any natural barriers. Because of their metabolic plasticity, short generation times, wide tolerance to environmental conditions, and ability to form resting stages, microorganisms are likely the most successful invaders of coastal ecosystems.
It is well known that invasive microalgae capable of forming massive blooms can cause economic losses to aquaculture, fisheries and tourism activities and have important environmental and human health impacts. There is also growing evidence that, besides free-living species, pathogens and parasites are being frequently introduced into coastal systems, with potentially enormous consequences for economic activities and human health. Regulations at national and above-national levels are thus being implemented to face these emergent issues.