Plankton comes from the Greek word planktos, which means wandering or drifting. Any living creature carried along by ocean currents is classified as plankton. It ranges in size from the tiniest virus to siphonophores, the longest animals in the world, and also includes microscopic algae, krill or fish larvae.
Some plankton, like these jellyfish, salps, or sea gooseberries, drift all their lives. Others like pteropod molluscs and fish are only planktonic during their embryonic or larval life. When they reach adulthood, they settle or swim freely.
Planktonic organisms play important roles in human life. Many microscopic species get their energy from photosynthesis. They absorb carbon dioxide and produce oxygen. Thus, they constantly renew the air we breathe.
Plankton has also been a great provider of fossil energy. When planktonic organisms die, they sink to the sea-bed creating a layer of sediment. Over millions of years, this sediment fossilized, producing our precious oil.
Finally, plankton nourishes us: it is the basis of the food chain in which the large eat the small. Without plankton, there would be no fish!