Scientists usually divide plankton into three groups that align with major divisions of life. The plant-like organisms are phytoplankton (from phytos, Greek for plant); small animal plankton are zooplankton (from zoios, Greek for animal); and bacteria and archaea, minuscule cells that decompose material and recycle chemicals in the ocean, make up the bacterioplankton. But these categories are far from perfect.
For example, phytoplankton are not true plants. The vast majority are photosynthetic bacteria, including cyanobacteria, which are believed to be among the oldest organisms on Earth and the origin of the photosynthetic organelles in plant cells known as chloroplasts. Larger phytoplankton are single-celled algae also known as protists—tiny organisms that also contain chloroplasts. Many photosynthetic protists are capable of movement and some also hunt and eat other single-celled organisms.
Many creatures called zooplankton are also tiny protists, but the category simultaneously includes animals on the other end of the size scale. Jellyfish are among the simplest animals on Earth and are considered plankton, but some individuals have been measured at 130 feet long, longer than a blue whale. Other zooplankton, even though they are transparent and soft-bodied, are chordates, complicated animals related to fish and humans. Finally, although “plankton” comes from the Greek word planktos, meaning “drifting,” some zooplankton can swim up to 25 feet a minute and up to a mile a day, migrating from near the sea surface during the day to half a mile down during the day and back again.
Because of these complications, scientists find a scheme based on size a better way to divide plankton, with categories ranging from femtoplankton, for marine viruses, to megaplankton, for large gelatinous animals like jellyfish.