Plankton aren’t a particular kind of plant or animal—rather, “plankton” is a category for organisms that aren’t capable of swimming against a current. In fact, the word plankton comes from the Greek word “planktos,” which means “wanderer” or “drifter.”
Plant, animal and more
Plankton are generally divided into three groups: phytoplankton, zooplankton and bacterioplankton. Phytoplankton are plants; they live near the water’s surface where there’s enough light to support photosynthesis. Zooplankton are animals; sometimes the eggs or larvae of larger animals. And bacterioplankton are, simply, bacteria: one-celled organisms that live their whole lives as plankton.
From very tiny to very big
Some species of plankton are microscopic. Others are easily visible to the naked eye. And others are hard to miss, such as jellyfish! It’s true: jellyfish are plankton, unable to swim against a current. The largest known species of jellyfish is the lion’s mane, and the largest recorded specimen had a bell (or body) that was seven feet across and tentacles that were 120 feet long!
Just about everywhere! Plankton are found in all natural, fresh- or saltwater environments. The largest numbers of phytoplankton live relatively close to the water’s surface, where there is enough sunlight to support the photosynthesis they need to survive. Because zooplankton feed on phytoplankton, their numbers are largest in areas close to the surface as well.
Because phytoplankton are plants, they gain energy through photosynthesis. Zooplankton feed on other plankton and single-celled organisms. Bacterioplankton primarily decompose the remains of other organisms.
All filter feeders eat plankton, including clams, krill, sponges, baleen whales and many different species of fish. There are even some bird species that are filter feeders. What’s a filter feeder? An animal that feeds by straining suspended matter from water, thus “filtering out” the food.