When people sail the sea, marine organisms tag along. If carried long distances, these hitchhikers can invade and disrupt ecosystems far from their natural homes, pushing out the local species. Some invaders catch a ride by attaching themselves to the sides or bottoms of boats. But many more stow away in ships' "ballast tanks."

Scientists from the Smithsonian's Environmental Research Center and elsewhere are studying techniques such as filtering and treating ballast water in an effort to reduce the risk of introducing invasive species to new habitats. Here, a ship flushes and refills its ballast task in mid-ocean to prevent marine organisms from moving from one port to another.

Read about 5 Invasive Species You Should Know on the Ocean Portal.