The crew of the research ship Tara sent two and a half years and 70,000 miles tracking plankton, microscopic organisms that drift on the oceans' currents.
The longest animal is a type of plankton called siphonophores, which is a relative of jellyfish and corals.
The siphonophores are huge colonies of tiny organisms that gang together and specialise so that they appear to be a single complex animal.
The best known is the Portugese Man of War and some of the colonies can grow to over 50 metres long.
During their voyage, the scientists logged 1.35 million previously unknown species.