Plankton are a group of organisms that lack the ability to move by themselves, but drift with the currents instead. They include representatives from all domains of life, spanning six orders of magnitude: there are submicron members like diatoms as well as jellyfish that measure a couple of centimeters. Plankton play a vital role in the marine food chain, since phytoplankton are responsible for half of global organic material production. That in turn is consumed by zooplankton, which is a crucial food source for other marine species higher up in the food chain.
Depicted above: Acartia tonsa (shrimp-like creatures at the front), Daphnia pulex (big oval creatures with antlers)
Length of Limanella Dictyoceras: ca 200 µm