Plankton are organisms that live in the water column and drift with the currents. Bacteria, fungi, algae, protozoans, invertebrates, and some vertebrates are represented, some organisms spending only parts of their lives (e.g., larval stages) as members of the plankton. Plankton is a relative term, since many planktonic organisms possess some means by which they may control their horizontal and/or vertical positions. For example, organisms may possess paddlelike flagella for propulsion over short distances, or they may regulate their vertical distributions in the water column by producing oil droplets or gas bubbles. Plankton comprise a major item in aquatic food chains.