‘Plankton’ is a catchall name for a diverse group of marine or freshwater organisms that are so small and/or weak that they can’t swim against a current. Indeed, this inability alone is what classifies an organism as planktonic, with bacteria, algae, molluscs, crustaceans and more all falling under this label.
Despite their minuscule size, plankton species number in the hundreds of thousands and are a critical component of food chains. Fish and marine mammals – including those as massive as whales – feed extensively on plankton (some exclusively) and without them many ecosystems in the ocean would simply collapse.
Plankton are subdivided according to size, with those larger than 20 millimetres (0.8 inches) – such as jellyfish – referred to as megaplankton, while at the other end of the scale, organisms less than 0.2 micrometres – such as marine viruses – are known as femtoplankton. In between these two extremes there are several other categories, containing a wide array of organisms ranging from cephalopoda (like octopus hatchlings) through to flagellates.