Plankton is not a species. It's a collective name for all the tiny marine and freshwater life that exist in a floating state. Too small or weak to fight the currents, they drift where the water takes them.
Apart from bacteria, these plants, animals, and other types of organisms are the most abundant life form on Earth and play a critical role in the marine food chain.
Above: Aglaura hemistoma, a very small jellyfish that's no more than about 4 millimeters in diameter.