Interesting small creatures, which (as a side note) can have enormous ecological impacts as the Wikipedia article mentions:
"On the other hand, in the late 1980s the Western Atlantic ctenophore Mnemiopsis leidyi was accidentally introduced into the Black Sea and Sea of Azov via the ballast tanks of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish."
What would be a solution to carrying lifeforms elsewhere in the ballast tanks? Turning the water over regularily to reduce the distances that these creatures can travel aboard?
"On the other hand, in the late 1980s the Western Atlantic ctenophore Mnemiopsis leidyi was accidentally introduced into the Black Sea and Sea of Azov via the ballast tanks of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish."
https://en.wikipedia.org/wiki/Ctenophora