Poorer diversity but tougher species in old ballast water: Biosecurity challenges explored from visual and molecular techniques

Highlights: • Planktonic species hitch-hike together with millions of tons of water inside ballast tank every day. • Through visual and molecular tools an increasing of potential nuisance taxa over travel duration were detected. • Resistant species dispersed by BW would be threating the global marine biodiversity. • These points must be taken into account for modeling and assessing the bioinvasions risk associated to BW. Millions of tons of water cross the oceans inside ships' ballast tanks every day. Planktonic species hitch-hike with water and some may pose risks to ecosystems and economies if get released and establish outside their native range. We monitored ballast water in different trans-equatorial travels, visually and using molecular techniques, and found significant increases of potential nuisance taxa over travel duration, despite evident diversity depletion. Thus, less diverse but more resistant and potentially more harmful communities persist in ballast water over long voyages. If we consider the enormous volume transported every day, the persistence of resistant species in ballast water would be threating the global marine biodiversity. This should be taken into account when modeling and assessing the bioinvasion risks associated with the ballast water and transfer considered in the future research.