Ships have been described as floating islands or mobile marine ecosystems. Along with cargo, they can transport animals from one part of the world to another. Sometimes these species latch onto the hull, rudder or propeller blades.

But more often, they're carried along in the ballast water that ships use for balance and positioning. A single tanker may carry more than 50 million gallons of water, picking it up in a port and dumping it thousands of miles away.

In our special interactive feature, see how species are able to spread across the globe via ballast water and the damage that some of the worst have done.